551

The C anad.i an M ine ralo gi stVol. 31, pp. 551-582 (1993)

THE CRYSTAL CHEMISTRY OF STAUROLITE.I. CRYSTAL STRUCTURE AND SITE POPULATIONS

FRANK C. HAWTHORNE-" LUCIANO TINGARETTI, ROBERTA OBERTI, FRANCACAUCIAATpATHOS CALLEGARI

CNR Centro di Sndio per la Cristallochimica e la Cristallografiq vin Bassi 4, I-27100 Pavia' Italy

AssrRAcr

The crystal structures of 42 samples of staurotte have been refined to R indices of I-2Vo using single-crystal MoKc X-ray

6u1a. 5amFles were selected to cover as wide a range of compositions as possible, and site populations were assigned by site-

scattering-refinement and vja the development of mean bond-length - cation-radius relationships for the staurolite structure.The cation site-nomenclature is revised to give a scheme that is more convenient and compatible with schemes of site nomen-clature in other rock-forming silicate minerals. The chemical composition that best approximates most examples of staurolite is

FeuVe)Jil(A)l /J.2q;.MA)l NrctM(l)Jule\ si8tr(l)l oqwz_twith the occupancies indicated. Saurolite is monoclinic,

lC?il, observed p betrieen 90 and SO.aS"l, the p angle conehting with the differential occupancy and sizes of the M(3A) andtt4(:f) oceh"dtu. Positional disorder was observed at the I(2) site, which is occupied byFe,Zn, Li, Co, Mg' Al and tr (vacan-

cy); there is some indication that the disorder in (2) correlates with variation in the p angle. Site-scattering refinement shows

r.all bot significant occupancy ot M(l) t= M(t-l) + M(lB)], M(2) nd MQ) t= M(3A) + MQB)lby transition-metal cations.

Detailed conrid"-tion of ltreniicat compositions and mean bond-lengths shows sipificant Mg occupancy of M(l)' M(2) nd

M(3).Ia most staurolite samples, -40% of the Mg occupies the octahedrally coordinated M(I), M(2) afi M(3) sites; therernainder occupies the tetahedrally coordinated I(2) site. There is a considerable range of M(4) scattering (0.0-l 1.5 electronspfu); several arilments show M(4) to be occupied by Fe (+ n), except for one very Mg-rich sample of staurolite (Mg = 3 apfu),io *ni"n M(4) is occupied by Mg. There is also significant variation in the refined scattering at the M(3) site, spanning &e

range 1.75-2.09 cations (Af pfu. no*r the high occupancy of M(4) and the low occupancy of M(3) are correlated with

increased H contenL Local order involving cations and vacancy at he T(2) nd M(4) sites is a very importaat feature of the

staurolite structure; stereochemical arguments allow quantitative assignment of vacancies to the Z(2) site from the refined M(4)

site-scattering.

Keywords: staurolite, crystal-structure refinement, electron-microprobe analysis, ion-microprobe analysis' site populations,

order-disorder.

Sou:uaru

l,a structure cristalline de monocristaux de staurotide provenant de qumante-deux dchantillons a 6t6 affinde jusqu'a un

r6siduR deld,2Voenutilisantdesdonn&sobtenuesavecrayonnementMoKa.les6chantillonsont6tdchoisispourcouvrir,dans la mesure du possible, la ga:nme complOte de compositions de cette espbce. L'occupation des sites a 6t6 ddtermin6e par

affinement du pouvoir de dispersion i chaque site, et par la relation nouvelle entre longueur de liaison moygnne dtns cetle

structure et rayin cationique. Nous proposons une r6vision de la nonenclature de chaque site, pour la rendre plus commode et

compatible avec le protocole de nomenclature d'autres silicates Ftrologiquement ilnportants. Ia formule chimique qui r6pondle deux i la composition de la plupart des 6chantillons serait Fe3a["(2) M@)] NzqIry@l NrcW()YQ)1,!i9!f(t)J Oat

H24, avec l'occupation des sites ielle qu'indiqu€e. Ia staurotide esi monoclinique, C2lm, p otserv6 e4r9 90 e!:If.4f:' l'anglephbnmnt une con6lation avec l'occupation et la dimension diff6rentielles des sites octa6driques M(3A) et M(3B)._Nous met-

tons en 6vidence un d6sordre de position au site Z(2), qu'occupent Fe, Zn,Li, Co, Mg, Al et tr (lacune); ce ddsordre pourait

avoir un lien avec la variation dans I'angle B. L'affinenent du pouvoir de dispersion montre une occupation faible mais impor-rante des sites M(1) l= M(IA) + M(lB)1, M(2) et M(3) l= M(3A) + M(38)l par des ions des m6taux de transition. Une consi-

d€ration d6taill6e des compositions chimiques et des longueus de liaison moyennes d6montre un nivsalr imFortant de Mg dansles sites M(1), M(2) et MQ). Dans la plupart des &hantillons, environ 407o drtMg occupe les sites b coordinence ocla6driqueM(1), M(2) et MQ). k reste se trouve dans le site (2), h coordinence t6tra6drique. 11 y a une variation consid6rable dans la

dispeision associ6e au site M(4) (entre 0 et I 1.5 6lectrons par unit6 formulaire); nous proposons, plusieurs-arguments.i fappui,q6 V@) contient Fe (+ tr), sauf tlans un cas de staurotide fortement magndsienne (environ 3 atomes de Mg par unit6 formu-

ti,:r"1, iri c'est le Mg qui occupe M(4). Nous trouvons aussi une grande variation dans la dispersion associ6e au site M(3)'6quivalente d une variation entri | .7 5 et 2.09 cations (Al) par unit6 formulaire. l,e taux 6lev6 d'occupati on de M(4) et le faible

*Permanent address: Deparrnent of Geological Sciences, University of Manitob4 Winnipeg, Manitoba R3T 2N2.

552 THE cANADTAN MINERAL@rSr

taux d'occupation de M(3) sont associ6s avec une teneur dlev6e,en \y4rogbpe. Une mise en ordre locale impliquant cations etlacunes aux sircs f(2) et M(4) serait un aspect trds important de la squCture de 1a staurotide; des arguments i'ordre st6r6o-chimique permettent une attribution quantitative des lacunes au site Z(2) d partir de l'affinement de la diipersion associ6e au siteM(4).

Clraduit pax la R6daction)

Mots'cl6s: staurotide, affinement de la structure cristalline, analyse d la microsonde dlectronique, analyse i la microsondeionique, occupation des sites, ordre-d6sordre.

I]vrnolucuoN

Staurolite is a common mineral in metapeliticrocks, and is diagnostic of medium-grade metamor-phism. Its widespread occurrence and sensitivity toconditions of equilibration suggest that it should be agood indicator of metamorphic conditions. However,problems a$sociated with most a$pects of its crystal-lography, crystal chemistry and phase equilibria haveprevented its effective use in petrogenetic work, andconflicting results from experimental (Pigage &Greenwood 1982) and field studies (e.g.,Holdaway eta/. 1988) are common.

Staurolite is monoclinic with p = 90o, and is also

(a)

pseudo-orthorhombic. Its composition is not particu-larly complicated, but recent confirmation that both H(Lonker 1983, Holdaway et al. 7986a) and Li @utrowet al. L986, Holdaway et al. 1986b) are significantlyvariable components shows that earlier efforts tounderstand the chemistry of staurolite were hinderedby incomplete information. The principal difficultyarises in trying to decipher the site populations. Thisinvolves assigning A1 and Mg, and Fe2+ and Fe3* (+min61 fi, Cr, Zno with sometimes major Zn, Co andnow Li) to several crystallographically distinct sites inthe sffucture, including sites that are pseudosymmetri-cally related by the pseudo-orthorhombic symmetry.Although this is in principle no more difficult than in

Ftc. 1. The crystal structure ofstaurolite, showing the kyan-ite-like part of the structure.T1ne M(IA) and M(LB) octa-hedra are regularly dot-shad-ed; the M(2) octahedron isirregularly dot-shaded, andthe Z(1) tetrahedron is shadedwith crosses; (a) the structureviewed down [001]; (b) thestructure viewed 15o away(in the (001) plane) from[010]; note the senated octa-hedral edge-sharing chainsthat are cross-linked by ?'(1)tetrahedra; (c) an obliqueview of the structure. show-ing the interpolyhedral link-age.

t

IIa

TIIE CRYSTAL CHEMISTRY OF STAUROLTTE 553

554 TI{E CANADIAN MINERALOGIST

the case of other semFlex rock-forming silicates (e.g.,amphiboles, pyroxenes), the problem is exacerbatedby the fact that staurolite, unlike most other commonferromagnesian sil icates, does not show wide(absolute) ranges of chemical composition.Consequently, the response of the structure to compo-sitional variations is very subtle, and it is dfficult toconvincingly derive site populations by the usualmethods of X-ray or neutron scatiering and absorptionspectroscopy.

The lack of accurate information on site popula-tions means that it is impossible to derive thermody-namic data on adequately characterized material. Inaddition, it is also not feasible to develop reliableactivity models for thermodynamic treatment.Significant progress has recently been made in thechemical characterization of staurolite @vfrow et al.1986, Holdaway et al. 1986a, b, Lonker 1983), mak-ing the question of site populations more pressing thanever. Here we attack this problem by crystal-structurerefinement of a large number of staurolite samplescovering the full compositional range of the naturalminerals. A preliminary account of this work wasgiven by Ungaretti et al. (1987).

Sns NoNmNcleruns

The site nomenclature currently used for stauroliteis not satisfactory. Sites are identified by element sym-bols, a situation that is not very convenient where sub-stitution of various types of cation occurs at the samesite. The situation becomes absurd when the 'oFe site"contains no Fe whatsoever" but Zn or Co instead.Consequendy, we have relabeled the sites in stauroliteaccording to the scheme shown in Table 1.

Tetrahedrally coordinated cations are labeled by theletter 7, and octahedrally coordinated cations arelabeled by the letter M. Anion sites retain the oldlabels, as these are compatible with the new scheme oflabeling of the cations. The hydrogen positions arelabeled H, but the subsequent letter designation differsfrom previous studies; these have been changed suchthat O(1A) is the donor oxygen for H(lA) and H(2A.),and O(18) is the donor oxygen for H(B) and H(28).The A and B designations for atoms have been

TABTE t. REVISED tl0l'IENCU|nRE FOR CATI0N SITES It'l STAUROITE

N€r Mult. old New nuit. old

retained, as it is often convenient to consider pairs ofatoms, bonds or sites together, and these can then bejointly labeled by the atom number alone le.g., M(l)1.There is an additional site, Z(3), not found in previousstudies; this occurs in oxidized staurolite (Cauciaet al.1994), and. has been preliminarily described byUngaretti et al. (1988); it is included here for com-pleteness.

REvEw AND SyxrrnsrsOFPREVIOUS WORK

There has been much significant work done in thepast few years, and a detailed review and synthesis ofthe current situation are waranted, both to highlightthe principal problems in the crystal chemistry of stau-rolite, and to identify those factors that are currentlythe subject of controversy.

General structure

The structure of staurolite was essentially deter-mined by N6ray-Szab6 (1929), uging the orthorhombicspace-group Ccmm and a chemical formula (Z = l)FeoAl,6SirO*Hs. In their description of lusakite, thecobalt analogue of staurolite, Skerl & Bannister (1934)noted both the difference between the formula used byN6ray-Szab6 (L929) and the previous formula pro-posed by Homer (1915), and also the similarity of thelatter to the formula unit of lusakite: [Co1.e3Fe\.3,Mgr.orNio.roTis.l 1l>a.q3[Allu.ruFe3+1.e6Jzrz.szISiz.zsAh.z5l26oa3H2.ru. A careful morphological and X-raystudy (Hurst et al. 1956, recounted in detail byDonnay & Donnay 1983) confirmed the C-centeredlattice for staurolite, but also showed that staurolile is66a6glinis with space-group symmetry C2lm (assvm-ing the presence ofa center ofsymmetry, as suggestedby a negative piezoelectric test). At the same time,Juurinen (1956) reported what are now regarded(Holdaway et al. 1986b) as results of very high qualitychemical analyses of staurolite, and gave a new ideal-ized formula: FenAllsSlO*Ho (note that this has a netpositive charge of 2+). These new data promp0ed a re-investigation of the staurolite structure by Nfuay-Szab6 & Sasvdri (1958), using the space grotp C2lmand the formula unit Fe4AllssirOarHz. They showedthat the original structural arrangement of Ndray-Szab6 (1929) was essentially correct, but that theCcmm syl'l:.mstry was only a pseudo-symmetry, andthat the correct symmety is A,lm with B : 90', laterconfirmed in detail by Smith (1968).

The staurolite structure (Fig. 1) is traditionallydescribed as slabs of kyanite [Al2SiO{] and Fe-Al-oxide-hydroxide [- to]d1o.rt+tp"rOiOmt alternatingalong [010]. As noted by Ndray-Szab6 (1"929), this canalso be described as a close-packed array of anionswfth t6lAl talFe and talSi occupying the interstices. Thekyanite layer is ideally [All6Si8O40], but extensive

u(rA)r'r( 1B)H(2)r,r(3A)r'r(38)r{(4A)[(48)

Ar (lA)Ar ( lB)Ar (2)Ar (3A)Ar (38)u ( l )u(2t

I (1 ) 8

r(2) 4r(3) 4H(lA) 4

H( lB) 4

H(zA) 4

H(28) 4

82

z

J I

t e

,,roi-r,r'P(18) ,H(14)

H(28)

H(2A)

chemical data show significant Al = Si and possiblyFe = Al substitutions; however, the stereochemicaldetails of these substitutions are not known. Theoxide-hydroxide layer is far more complicated; thislayer is illustrated in Figure 2. A (rutileJike) chain ofedge-sharing octahedra extends along [001]. Thischain is fianked by (FeOJ letrahe&a, sharing verticeswith adjacent octahedra and assuming a staggeredarrangemetrt on either side of the chain; the resultinglM(TOlOr) sfructural unit is a common one in manychemical classes of minerals (Hawthorne 1990). It hasgenerally been assumed that the principal substitutionsin the structure occur in the oxide-hydroxide layer;although small amounts of o'cross-substitution" havebeen proposed by various investigators, crystal-chemical details are currently lacking.

Compositionnl range of staarolite

For such a complex mineral, staurolite has a sur-

555

prisingly small range of composition. More recent for-mulations have been variations of the type

(Fe,IvIgZn,Co):-+(Al,Fe) 17-1 6(Si,Al)sOasH3-a

in which the cation associations 'pithin parentheses donot necessarily reshict their substitution to the site(s)implied by the groupings.

First we will summarize the observed composition-al ranges irrespective of the siles occupied by the par-ticular cation. It should be noted here that antipatheticbehavior of cation pain (e.g., Fe2+, Mg) should not betaken as inferring direct 1:1 substitution at a specificsite, as some authors have proposed more complexcoupled substifutions involving cations of differentvalence at more than one type of site. Griffen & Ribbe(1973), Ribbe (1982) and Griffen et al. (1982) haveconsidered the formula and observed compositionalrange of staurolite, assuming 4 H apfu (atoms per for-mula unit). The observed ranges (in apfu) are fistedbelow:

a

TIIE CRYSTAL CHEMIS'TRY OF STAIJROLTTE

C-+

Ftc. 2. Polyhedral representation of the oxide-hydroxide layer in staumlite. T\e M(34)arrd M(38) octahedra are regularly dot-shade{ the M(4A) nd M(48) octahedra areirregularly dot-shade4 and tle I(2) tetrahedron is shaded with crosses. Octahedraadjacent along [001] share edges, and TQ) tetrahedra share faces with M(4A) andM(48) octahedra

556

Si 7.18-8.09 (7.09: crew & Sandiford 1984)Al 1656-19.24 (19.91: von Knorring et a1. 1979)Fe 1.86-3.66 (0.16: Chopin, pers.comm.)Mg 0-1..44 (3.01: Chopin, pers.comm.)Zn 0-1.54 (2.13: Chopin, pers,comm.)Ti 0_o.17 (0.34: Schreyer et al. 1984)Mn 0-0.15

Some of these ranges have been extended by morerecent studies (values given in parentheses), and addi-tional components have been recorded in significantamounts:

Cr 0-0.45Co U1.94Ni H20Li 0-1.57

(Ward 1984a)(Skerl & Bannister 1934)(Skerl & Bannister 1934)(Holdaway & Goodge 1990)

For at least some of these cations" it seems that theconstraints on their range are geochemical ratlier thanstructural, as synthetic Mg (Schreyer & Seifert 1969),Zn (Griffen 1981) and Co (Phillips & Griffen 1986)end-members have been synthesized, albeit at highpressures.

Obviously, the compositional ranges given aboveare not independent, as overall electroneutrality mustbe maintained, and there may be further additionalconstraints due to local crystal-chemical factors suchas bond-valence requirements or steric restrictions. Astrong correlation (Fig. 3a) befween tAl - (8 - Si)l and[Fe + Mg + Zn] was observed by Griffen et aL (1982),leading them to write a more general formula as(F e,Mg,Zn) rr.o_r.zsy'l r.s,_a.25i I 6.2_0.5,O48Ha, where16.58 < -r s 18.61. This does not incorporate theexamples of Co-rich staurolite that have been reporled(Skerl & Bannister L%4, eech et at. 1981., nringhurst& Griffen 1986); renormalization of tlese composi-tions on the same basis as that used by Giffer- et al.(1982) shows them to correspond to the trend of(Fe,Mg,Zn)-bearing staurolite shown in Figure 3a.

(FelMg+Zn) pfu

Ftc. 3. Variation in [Al - (8 - Si)] with [Fe + Mg + I[n] instaurolite: (a) compilation of Iiterature values normalizedon the basis of 44 02- + 4 OH-. from Griffen & Ribbs(1973); (b) the staurotte compositions of.Holdaway et al.(1986b) normalized on the basis of 44 02- + 4 OH- andexcluding Li2O; (c) the staurolite compositions ofHoldaway et al. (1986b) normalized on the basis of 481Oz-, OH-, F) and including Li2O; the line in (c) is thesame as in (b). In (b) and (c), circles represent staurolitesamples with H : 3 apfu; squares represent staurolitesampleswithH*4apfu.

=o.

?, 'n.qT@

I

5-S ,r"

CL

I9 1 7

I

oa

17,2

doabe:5g.oto.

osYo

a

tf-1

.Oaa\

\

\.\

(Fe+Mg+Zn) pfu

(c)

aa

" aa

a '

a

wIT

(Fe+Mg+Zn) ptu

TIIE CRYSTAL CHEMIS'TRY OF STAUROLIIE 557

However. the basis for the calculation of the formulaunit used by Griffen et aI. (1982) is now known to beinappropriate. They calculated their cell contents onthe basis of 46 atoms of oxygen l= 44 O + 4 OHl,assuming a constant H content of 4 apfu. Lonker(1983) and Holdaway et al. (1986a) have shown thatH is a variable component in staurolite, with a rangeot2.H.2 aptu. In addition, Dutrow et al. (1986)haveshown that Li can be a significant component in somesamples of staurolite. The effects of ignoring thesetwo factors (variability of H, the presence of Li) areshown in Figures 3b and 3c, which show the data ofHoldaway et al. (1,986b) plotted in the same fashion,recalculated on the bases of [44 O + 4 OH] and 48 (O,OH, D, respectively. It is immediately obvious fromFigure 3 that the correlation observed between [Al'' -(8 - Stl and [Fe + Mg + Mn] is induced primarily bythe inconect procedure for the unit-formula renormal-ization.

Thus the formula given above is based on the incor-rect assumption that H - 4 apfu. In addition, the workof Dutrow et al. (1986) and Holdaway et al. (I986b)shows that the presence of Li is sn imFortant factor insome parageneses. Holdaway et al. (1986b) showedthat the substituent species in staurolite fall into twowell-defined groups:

(r) (P* +Ii+ Il2H) range: 5.69-6.28 o = 0.15

(ii) (si + Al) runge: 25.IO-25.61 o = 0.13

in which the cation sums indicated show a veryrestricted range. Neither Li nor (H + Li) show any cor-relation with Al, but (R2+ + Li) shows a good correla-tion with (H + D, with a slope of approximately 0.57(Fig. 4a). This correlation suggests that the mostimportant substitution involving hydrogen is

2H=N+ (l)

[not 2 H = (n2+ + Li) as written rnHoldaway et aI.1986b1. Note that (1) is charge-balance4 whereas thelatter is not; Li must be included in the plot of Figure4a, as it also affects the fi2+ contelott via the substitution

3Li+Al = 3R2+ A)

(Dutrow et aI. 1986), as illustrated in Figures 4b and4c. It is notable that these "hidden substitutions"involving first-row elements (H, Li, Be, B) and vacan-cies have been a principal factor in inhibiting ourunderstanding of the chemistry of some of the morecomplex common silicate minerals (cf., Groat &Hawthorne 1989, Groat et al. 1989, 1992, Hawthomeet al. 1993).

7CLd

f,{+t

:tctd

f o.5

(b)

lao

!, t'

I61:6

t52.0

3x Al62.6 6g0

3.6

(H+R apfu

Frc. 4. Chemical correlations involving Li in staurolite [afterDtrtrow et al. (1986) and Holdaway et al. (l986ub)l: (a)(fiz+ + Li) as a fimction of (H + D; (b) Li as a function of3 Al; (c) U as a tunction of (R2+ + 0.5 H).

(c)

\3o\'&^..

1f*+0.5 il aptu

lbao

CL(!

f, o.o

(a)

558 TIIE CANADIAN MINERALOGIST

Hydrogen in staurolite

As is apparent from the previous discussions, ttreproposed H-content of staurolite has been very vari-able, from 2 apfu (Horner 1915) to 8 apfu (Ndray-Szab6 7929). Results of most chemical analyses sug-gest that the H content is variable between 2 and4 apfu (Tak6uchi et al. L972), but as noted byHoldaway et al. (1,986a), the concentration of H is par-ticularly difficult to determine in staurolile because ofttre mineral's refractory nature, the almost ubiquitouspresence of inclusions, and the probable evolution ofH in different forms (H2O, H2, CH/ on heating.Charge-balance calculations on unit-cell contents (i.e.,a6raalizsd on the basis of measured densrty) suggesta mean value of 4 H apfu (Griffen & Ribbe 1973), butthis is obviously not a reliable method of renormaliza-tion for individual mineral formulae, given the diffi-culty of measuring an accurate density. A sipificantstep toward the resolution of this problem was takenby Lonker (1983) and Holdaway et al. (1986a),bothof whom reported careful determinations of H in stau-rolite samples from a variety of different localities.Both studies show that H varies significantly in stauro-lite, but they differ somewhat in the ranges observed.The data of Lonker (1983), measured on a modifiedDupont moisture analyzer, vary from 1.8 to 3.6 apfu(0.95-7.92 wt%o H2O); the data of Holdaway et al.(1986a), measured on a H-isotope extaction-line andan ion microprobe, vary ftom 2.7 to 4.2 apfu (1.4-2.3 wtVo H2O). Holdaway et al. (1986a) concludedthat the former data are systematically low by-0.3-0.6 wtvo H2Q. Holdaway et al. (I986a) alsonoted that staurolite samples fall into two groups:(i) staurolite coexisting with gamet or biotite (or both)contain 2.71.4 H apfu (i.e., a me,an value of 3.06 Hapfu); (ii) staurolite that does not coexist with eithergarnet or biotite contains 4.09-4.16 H apfir (i.e., amean value of 4.14 H apfu).

On the basis oflocal charge-balance (bond-valence)requirements, Hanisch (1966) proposed that the Hatoms are attached to tle O(1A) and O(18) anions; thisarrangement was confi.rmed by the two-dimensionalrefinement of a staurolite crystal by Tak$uchi et al.(1972) using neutron-diffraction data. More accuratepositions of the protons were subsequently providedby the three-dimensional neutron-diffraction refine-ment of the structure by Tagai & Joswig (1985); theO-H distances, 1.08(1) and 0.97(1) A observed forO(1A)-H(14) and O(IB)-H(18), are each typical foran (OID group (with significant hydrogen-bond inter-action). Howevero site-scattering refinement showedneither hydrogen position to be completely occupied.The observed H occupancies of 0.38(1) and 0.48(2),respectively, for H(1A) and H(18) indicate a cell con-tent of 3,44 H apfu , within the range of 2.7 -3.4 H apfuobserved by Holdaway et al. (1986a) for staurolitecoexist''rg with biotite. However, Stdhl er a/. (1988)

have recently identified trro additional hydrogen posi-tions in a neufron-diffraction refinement of the struc-ture of a staurolite crystal from Pizzo Forno, St.Gotthard, Switzerland. All hydrogen positions areqhown diagrammatically in Figure 5. They all are -1A away from the O(lA) and O(18) oxygen atoms, asexpected for hydrgxyl groups. The positions H(lA)and H(%.) are -1 A apart, too close for simultaneousoccupancy; H(18) and H(28) are likewise arranged,also too close for simultaneous occupancy [whichwould^result in an (H2O) groupl. The position H(1A)is -2 A from both H(18) and H(28), a short distancefor simultaneous occupancy of H(lA) and eitherH(18) or H(28); the local arrangement mound H(lB)is similar. All of the H positions are less than half-occupied. Occupancy of specific H positions exertsconstraints on the occupancies of adjacent siles, as dis-cussed in detail by Stlhl er al. (1988). The latler alsonoted that the occurrence of the H(2A) and H(28) sitesin staurolite is concordant with the third protonnuclear magnetic resonance reported by Tak6uchi eral. (1972).

Disorder of tetrahedrally coordinated cationsin the oxide-hydroide layer

In his refinement of the St. Gotthard staurolite,Snith (1968) found that within the oxide*hydroxidelayer, letrahedmlly coordinated Fd+ [at t]re (2) srtelshows significant positional disorder. For a structuremodel with a single Z(2) position, difference-Fouriermaps showed three maxima of residual density in the

- cFto. 5. The hydrogen positions in staurolite [coordinates from

Stehl et aL (1988), site nomenclafire from Table l, thisstudyl. Hydrogen bonds are indicated by dashed lines;note that not all the positions indicated can be occupiedsimultaneouslv.

a

TI{E CRYSTAL CHEMISTRY OF STAUROLITE 559

mirror plane parallel to (010) at ) = Q, indicating thatthe Fe is actually positionally disordered into threenonequivalent sites within its tetahedron of coordinat-ing anions. Similar difference-Fourier maps also werepresented by Bringhurst & Griffen (1986) and StAhl eral. (1988). Alexander (1989) and St&hl & Legros(1990) refined models with Fe occupying tbree closelyspaced sites within the f(2) tetrahedron. However, theauthors of both studies were not satisfied witl theresults. Alexander (1989) noted the very high correla-tions and insignificant decrease in the R index, togeth-er with residual density in the difference-Fourier that"may indicate the presence of other (mino| occupiedpositions in the Fe tetrahedron in addition to the threepreviously observed subsites". StAhl & Legros (1990)refined a very constrained three-site model (atoms atthe apices of an equilateral ftiangle) and also refinedan anharmonic displacement model for a central site;from model calculations, they concluded that the sub-site separations are in the range 0.2C-{.25 A. Neitherset of autlors used the results of the subsite refine-ments in any quantitative crystal-chemical discussions,and it should perhaps be emphasized that this is quitecorrect; because ofthe obvious problem ofresolution,it would be wrong to attach too much quantitative sig-nificance to the results of the split-site refinements.

Dyu et aL (199I) have assigned three doublets toF&+ at three different (2) subsites in the Miissbauerspectra of a wide variety of staurolite samples. In par-ticular, these doublets were observed in synthetic Fe-rich staurolite, showing that the disorder cannot beentirely the result of solid-solution effects.

There have been various reasons suggested for thispositional disorder, but there have also been insuffi-cient crystallographic data on a wide range of compo-sitions to see if the proposed mechanisms are of gener-al applicability.

Order4isorder relations at M(3A) and. M(38)

In the staurolite structure, within the oxide-hydrox-ide layer, adjacent octahedral sites in the M(3) chainare symmefrically distinct, and are denoted M(3A) andM(38), respectively. N6ray-Szab6 & Sasv6ri (1958)assigned atl available Al to the M(3A) site; thus M(34)was fully occupied, and M(38) was empty.Conversely, Hanisch (1966) refined the stucture of asample of zincian staurolite in the space grovp Ccmm;with this syrnmetry, M(3A) amid M(38) are symmetri-cally equivalent, and both sites were considered ashalf occupied. ln perhaps the classic structural studyof staurolite, Smith (1968) showed that (Al + Fe) ispartly ordered between M(3A) afi M(38) n C2lm.Similar partial ordering over these sites has since beenobserved by Tagai & Joswig (1985), Bringhurst &Griffen (1986), St6hl et al. (1988) and Alexander(1989), with the scattering power and assigned site-populations at M(3A) always exceeding those at

M(38).Stehl & Legros (1990) illustrated a relationship

between'ooccupancy" and mean bond-length at theM(3A) and M(38) sites, two distinct correlations (bysample, not by site) being shown. They suggested thedifferent correlations to be due to unspecified compo-sitional differences between the samples. However, wenote that they "normalized" the 'ooccupancy" factorsfor each staurolite to oopure Fe' at the Z(2) site, and to(0.67 Al + 0.33 Mg) at the M(3A) and M(38) sites.This procedure is unjustified by general stoichiometryof stiurolite, and results in a range ofoonormalizedoccupancies" that is totally incompatible with theresults of the preseDt study.

Mechnnism of substintion anddisorder among cations

The chemical variations discussed above concernvariations in cation proportions, and only in a fewcases did the discussion focus on details of cationoccupancy of specific sites. However, there have beenmany (commonly contradictory) proposals as todetails of site populations in staurolite; these will bebriefly outlinednext.Divalent catiow: The principai substitutions proposedfor staurolite are the homovalent substitutions Mg;=F&*,ZI=F&*, and Co =Fe2+. It has been shown byall strucfure-refinement studies that Fe2+ primarily(but not completely) occupies the tetrahedrally coordi-rdlted T(2) site (albeit with some positional disorder),with subsidiary occupancy ofthe octahedrally coordi-nated M(4A) and M(48) sites; Dyar et al. (199L) alsosuggested M(3A) and M(38) as possible sites forl6lFe2+. It was generally assumed that Mg, Zn and Cosubstitute for Fe2+ at the T(2) site, and certainly thestructural studies of Zn-bearing staurolite (Hanisch1966) and Co-bearing staurotte @ringhurst & Griffen1986) show this to be dominantly the case. Howeverothere have been many proposals for small amounts ofFe (commonly specified as Fe3*) and Mg occupyingtJLe M(I,2,3) sites, and small amounts of Al occurringat the T(2) site. Indeed, from a statistical study ofstaurolite compositions, Griffen & Ribbe (1973) con-cluded that 'the generally accepted view,that Mg isthe primary substituent for Fe at the Fert site is notsupported by the statistical analysis. It appears that Aland Zn (probably in that order) are more importantsubstituents at the FeN site". This conclusion was re-emfhasized by Griffen et al. (1982), who made thefoliowing (admittedly simplified) assignments: allavailable Mg occupies the M(3) sites; M(1) nd MQ)are fully occupied by At; the (2) site is fully occupiedby (Fe + Zn + Al). However, it is apparent fromFigure 3 that some degree of spurious correlation wasintroduced into these studies by the assumption thatthe formula unit contains 4 H atoms. Bringhurst &Griffen (1986) assigned allMg (1.23 aptu) to the M(1)

560 THE CANADIAN MINERALOGIST

and M(3) sites in their refinement of a Co-bearingstaurolite. Enami & Zang (1989) emphasized theimportance of the preferential replacement of t6lAl byMg, although they noted that t4lMg ._- t4lFe also seemsto occur in some samples of high-Mg staurolite. Onthe other hand, Holdaway et al. (1986b) concludedthat very little Mg replaces Al, but that a smalt amounrof Al substituting for R2+ [presumably at the T(2) orM(4) sitesl o'cannot be excluded on the basis of ouranalytical results".Trivalent and tetravalent cations: Staurolite containssmall amounts of trivalent and tetravalent transitionmetals (Fe3+, Ti4+, Cf+, V3+); their distribution overthe cation sites is totally uncharacterized. It has gener-ally been assumed that in staurolite, Fe3+ is octahedral-ly coordinated, although Tagai & Joswig (1985) didassign 0.24 Fe3+ apfu to the f(l) site in tleir neutron-diffraction refinement of the structure. Stihl et al.(1988) assigned 0.48 Fe3+ apfu to the M(1) sites,whereas Alexander (1989) assigned 0.14 Fe3+ apfu tothe M(3) sites. Holdaway et al. (1986b) proposed sig-nificanr Fe3+ Fr Al substitution 10.:M.:S apfu) inthree of their staurolite samples; on the basis of size ofcations, in octahedral coordination, they suggested thatthis occurs at M(3A), but this argument is incorrectfrom a predictive point of view (polyhedral size isaffected by the identity of the constituent cations, norvice versa). Bringhurst & Griffen (1986) assigned 0.20Fe to M(3) but did not specif its valence state.

Dyar et al. (1991) have thoroughly reviewed the lit-erature on Mijssbauer spectroscopy of staurolite. Aswith the structural and chemical studies reviewedabove, tlere have been a number of different interpre-tations of essentially very similar data.'Ihey alsoexamined a wide variety of samples of staurolite byMiissbauer spectroscopy, including several of thoseexamined b-y us. With regard to Fe3+, they concludedthat the Fer+ contents are usually low (-3Vo of totalFe) but can go up to L2Vo [in sample EH-6, suspectedby Holdaway et al. (1986b) of having a high Fe3+ con-tentl; they also suggested that Fe3+ is tetrahedrallycoordinated, occurring at the (2) site.

With regard to Ti4, it is usually assumed to beoctahedrally coordinated. However, on the basis oftheobserved pleochroism in staurolite, Ward (1984b)argued that it must occupy the (2) site in order to par-ticipate with I4lFe in a Fe2+ = Ti4+ change-traniferry19tion polarized sftongly along the c axis. Normally,WCT (InterValence Charge-Transfer) reactions arepromoted by tle close approach of t}le constituentcations, and must have an orbital path along which topropagate. This can involve metal-metal interactionvia dbect orbital-overlap across shared faces or edgesof coordination polyhedra, or by metal - ligand -metal super-exchange. In both cases, the constituentcation polyhedra must share one or more ligands. Instaurolite, the T(2) tetrahedron does not link to neieh-boring I(2) tetrahedra, indicating that T(D-T:Q)

charge-transfer reactions are unlikely. Latero we pro-pose an alternative site for Ti.

Other minor cations (Cf+, V3+; are usually presentin the range 0.01-0.10 apfu. They have been assignedto tJtLe M(4) sites in several studies, but again there is nodirect evidence for this; in this regard note that the pop-ulations of the M(4) sites are, by and large, so low thatthe sizes of the coordination polyhedra reflect the"size" ofthe vacant site rather than the occupied site.

Dyar et al. (L991) and Dutrow (1991) suggestedthat Li occurs at the TQ) site because of the decreasein Fe with increasing Li in synthetic Li-Fe-bearingstaurolite. The conclusion may be correct, but the rea-soning is not; there could also be coupled tr._-Fe2+,Li ._- n substitutions at different structural sites. Thequestion of where Li occurs in the structure must stillbe considered as open.

To summarize, there have been a large number ofproposals for site-population patterns of differenttypes; however, except for the principal substitutionsAI = Si at the Z(1) site and 7n=F&+ and Cd+ =F&+ at the T(2) site, there is no systematic evidencefor the general validify ofthese schemes.

Eprr.nmnar Mg'r{oos

The provenance of the samples used in this work isindicated in Table 2. We tried to cover the entireobserved compositional range of staurotte; specificanalyzed samples were obtained from the authors ofvarious published and unpublished studies to ensurethis coverage. In particular, samples S(41) and S(42)are unique single crystals drilled from petrographicthin sections; the slightly lower quality of the dataobtained is compensated by their very unusual chem-ical composition (Chopin, pers. comm.). All theseparticular samples were supplemented by staurolitesamples of more usual compositions.

Collection of X-ray data

Crystals were selected for crystallographic mea-surement and collection of iniensity data on the basisofoptical clarity, freedom from inclusions, and equantshape. For the chemically well-characterized samplesof Holdaway et al. (7986b), several crystals wereexamined from each mineral separate to check forsample homogeneity. All crystals were mounted on aPhilips PW 1100 automated four-circle diffractometerand examined with graphite-monochromated MoKaX-radiation. Crystal quality was assessed rlla the pro-files and widths of Bragg diffraction peaks; on thisbasis, several crystals were discarded. Diffraction datawere measured only from those crystals judged to beof suffrciently good quality. Unit-cell dimensions werecalculated by least-squares refinement of the d valuesobtained for 40 rows of reciprocal space by measuringthe center of gravity of each reflection in the 0 range

TIIE CRYSTAL CHEMISTRY OF STAI]ROLITE

TABTE 2. PROVTTANCE OF STAUROLITE CRYSTALS USED IN THIS I{ORK

s61

crystal Sample code Local ity References

s(l) 7t-62R/rs(2) 7r-62R16s(3) 7r-6?R/5s(4) EH-616s(5) EH-614s(6) EH-6/2s(7) 3-3/rs(8) 3-312s(e) 106038/rs(r0) r06038/3s(rl) tt7189/6s(l?) 117189/7s(13) r17189/21s(14) u718e/ls(15) 6-3l ls(16) 6-312s(r7) !IARD/7s(18) t{O"t 12s(le) Mus/6-1s(20) r4us-AR/3s(21) HPtlzs(22) r4us/3s(23) Pro/us(24) PIo/13s(25) DV7/rs(26) 0r,,7/8IIs(27) DER/ro-Zs(2s) DER/zs(29) DrR/ls(30) DER/10-4s(3r) MARIFPTSs(32) MAR/Is(33) MAR/rss(34) MAR/r6s(35) l'lAR/sss(35) MR/zs(37) t'lAR/6-1s(38) !rAR/8s(3e) MAR/7rrs(40) r4AR/zrs(41) 85DH66bs(42) BARR888

BI ack l , l tn. , N.H.B lack M tn . , N .H .B lack l 4 t n , , N .H .Eno ry H l l l , N .Y .Ene ry H l l l , N .Y .E rne ry H l l l , N .Y .t,l€st Sidney, I'lainet,lest Sidney, MaineF rank l i n , N .C .F rank l i n , N . c .Strat ford, N.C.Strat ford, N.C.Strat ford, N.C.Strat ford, N.C.East l l in throp, MaineEast Ul lnthrop, MaineFjordland, New Zealandunknown local ltyPizzo Forno, s|{itzer'landPizzo Forno, Switzer landPizzo Forno, Switzer landPizzo Forno, swltzeriandBrazl IBrazi IAntarcti caAntarctl caDervio, ItalyDervio, I ta lyDervio, I ta lyDervio, I ta lyTlc ino r iver , I ta lyTic ino r lver , I ta lyTlc ino r iver , I ta lYTlc ino r lver , I ta lyTlc ino r iver , I ta lYTic ino r lver , l ta ' lyTic lno r lvsr , I ta lyTic ino r iver , I ta lyTlc ino r lver , I ta lyTlc lno r iver , l ta lyDora Maira, I ta lyBarrhorn, switzerland

l.l. Holdaway at al. (1986b)!

,

q

C.M. Uard (1984a)A. Mottana, pers. comt'tl.

This vrork

B. Lombardo, pers. comm.

This rork

!

C. Chopin, pers. corm.

2-35o, together with that of each "antireflection" atnegative 0 values. Refined cell-dimensions for thecrystals used in the collection of the intensity data negiven in Table 3. For the Zn-ich staurolite LL7L89from Holdaway et al. (1986b), cell dimensions weremeasured on L4 different srystals.

Intensity data were collected for the monoclinicequivalents hkl ard hEl n the mnge 2 < e < (3H5)",depending on crystal size, using the step-scan profiletechnique of Lehmann & Larsen (L974). For all tlecrystals but S(4), S(19), S(20), S(23), S(24), S(29),S(32), S(33), S(37), S(38) and S(42), only intensereflections were colected beyond 0 = 30o. Full detailsof the data-collection procedure are given in Ungaretti(1980) and Ungaretti et al. (1981). The intensity datawere corrected for absorption by the method of Northet al. (1968), corrected for Lorentz and polarization

effects, averaged and reduced to structure factors. Areflection is considered as observed if its intensityexceeds that of five standard deviations based oncounting statistics.

Structure refinement

Fully ionized scattering-factors were used for thenon-tetrahedrally coordinated cations, and both neutraland ionized scattering-factors were used for the teta-hedrally coordinated cations and the oxygen atoms(Ungaretti 1980). n indices are of the standard forruand are given as percentages.

All reflnements were done in the space_group Cllm,even where the equality of I(hkD alid (hk[) (togetherwiti p = 90" and the identity of the A and B sites) sug-gested orthorhombic Ccmm symmetry. In particular,

562

TABLE 3. CELL DII.IENSIONS (A, A" O) AND I4ISCELTANEOUS STRUCTURE REFINEIIIENT INFORI,IATION FOR STAUROLITE

s( l) s(2) s(3) s(4) s(5) s(6) s(7) s(8) s(e) s(r0) s( l r) s(12) s(13) s(14)ab

tsv0*.R._R.;.RarrNoo,Nr..

7.863 7 .863 7 .865 7 .879 7 ,882 7 .881 7 .874 7 .873 7 ,878 7 ,877 7 .868 7 .868 7 .868 7 .86916.613 16.612 16.608 16.624 t6.622 16.625 t6.625 t6.627 16.606 16.606 16.610 16.609 16.609 16.6065.650 5 .650 5 .651 5 .661 5 .660 5 .661 8 .662 5 .663 5 .657 5 .657 5 .658 5 .658 5 .659 5 .65990.09 90 .14 90 .23 90 .00 90 .01 90 .06 90 .02 90 .04 90 .08 90 .09 90 .03 90 .13 90 .27 90 .45738.1 738.1 738.1 741.5 741.6 741.7 74 t ,3 74 t .2 7q0.0 740.0 739.5 739.4 739.6 739.5

45 45 45 32 45 45 45 45 45 45 45 45 45 451 . 8 1 . 7 2 . 1 1 . 9 1 . 3 1 . 4 1 . s 2 . 5 2 . 4 1 . 2 1 . 4 2 . 2 1 . 9 2 . 2r . 7 t . 7 1 . 8 1 . 8 1 . 8 r . 7 t . 9 1 . 8 2 . ' 1 . 6 2 . 0 ? , 1 1 . 9 1 . 82 , 3 2 , A 2 , 0 3 . 5 2 . 2 2 , 1 2 , 4 2 . 3 2 . 8 1 . 9 2 . q ? . 6 2 , 2 2 . 1

I?67 1957 1744 992 ?037 t70? 1653 1661 1989 1977 1760 1735 l99t 18491454 2t62 1915 i339 2233 1904 1848 1847 2t53 2138 t947 1939 2162 2024

s(15) s(16) s(17) s(r8) s( le) s(20) s(21) s(22) s(23) s(24) s(25) s(26) s(27) s(28)ab

tsv0*"

Rtl

No"'f,l

7.671 7 .871 7 .BT? T ,g6s 7 .868 7 .s66 t .869 2 .869 7 ,a67 7 .96s 7 .875 7 .868 7 .868 7 .B lz16.587 16 .572 16 .612 16 .619 16 .615 16 .614 16 .614 16 .620 16 .622 16 .609 16 ,614 t6 .622 16 .619 16 .6235.655 s .651 5 .655 5 .6s8 5 .655 5 .655 5 .658 5 .657 5 .657 5 .656 5 .658 s .657 5 .660 5 .66290.02 90 .00 90 .12 90 .00 90 .02 90 .06 90 .07 90 .16 90 .03 90 ,03 90 .04 90 .10 90 .09 90 .11738.3 737.0 739.4 739.8 739.3 739.0 739.8 739,8 739,7 738.8 740.3 739.9 740.1 740.9

45 45 45 50 40 40 45 50 40 66 40 45 66 401 . 8 2 . 4 2 , 0 2 . 3 t . 7 t . 2 1 . 9 1 . 1 1 . 4 1 . 8 1 . 2 1 . 6 1 . 1 1 . 71 . 8 z . t 2 . 0 2 . 2 1 . 9 2 . 0 2 . 3 t . 7 2 . 0 2 . 8 2 . 1 2 . 6 2 . 6 1 . 82 . 5 2 , 8 2 . 4 2 . 5 2 . 9 2 , 8 2 . 7 2 , 1 2 . 9 4 . 3 2 . 4 2 . 7 2 . 7 2 . 1

1320 1445 1628 2359 1840 1983 t94l 1556 1904 5234 1580 2020 3782 16841534 1687 1799 2500 2367 2364 2t04 1706 2373 6722 1742 2187 3858 1855

s(2e) s(30) s(31) s(32) s(33) s(34) s(35) s(36) s(37) s(38) s(3e) s(40) s(41) s(42)

b

Bv0_,D

K .

Ratt

N.ot

7.871 7.869 7.869 7.868 7.867 7.866 7.868 7.864 7.869 7.869 7.868 7.863 7.871 7.85316.624 16.62? 16.608 16.620 16.620 16.625 16.6 l t 16.619 16.620 16.607 16.601 16.619 16.541 16.5345.660 5.660 5.656 5.661 5.658 5.660 5.654 5.659 5.658 5.658 5.654 5.658 5.632 5.63990.12 90.12 90.00 90.05 90.09 90.10 90.16 90.17 90.17 90.22 90.26 90.29 90.01 90.00740.7 740.3 739.2 740.3 73s.7 740.2 739.0 739.5 740.0 739.4 738.5 739.4 733.3 732.2

40 66 45 40 40 40 40 45 40 40 50 45 30 351 . 9 l . l 1 . 4 2 . 4 1 . 3 3 . 4 1 . 6 1 . 9 1 . 6 2 . 3 2 . 9 2 . 8 5 . 8 3 . 32 . 0 1 . 7 1 . 9 2 . t 1 . 9 2 . 2 l . s 1 . 9 2 . 1 1 . 8 1 . 9 t . 8 4 . 7 * 2 . 2? . 2 1 . 9 2 , 2 2 , 9 2 , 9 2 , 5 1 . 9 2 , 2 3 . 0 2 . 8 ? , 1 2 , t 9 . 1 5 . 7

1922 2734 l94t 1949 t880 t724 1404 2tt2 1897 t922 2328 1663 652 9192382 2881 2105 2377 2376 1899 1582 2272 2374 2362 2435 t8l5 t1l3 1665

Note: e.s.d. < 0.002 A for a and c, < 0.004 A for b and < 0.020 for Br reflnenent done on the reflectlons lrith I a 3 o(I)

rhe crystals s(4), s(5), s(16), s(18), s(19) and s(31)were treated as monoclinic for both unit-cell and crvs-tal-structure refinement in order to facilitate comp;ri-son across the whole series reported here and in theliterature. For the structure refinements, the startingparameters were those of Smith (1968); the startingoccupancies represetrted an average composition forstaurolite. The refinements converged to R indices of-2Vo for fall-maaix refinement of all atomic positions,those site-occupancies considered as variable (seebelow, Table 5), and anisotropic displacement factors.At this stage, difference-Fourier maps showed the pre-viously noted (Smith 1968, Alexander 1989, Sten &Legros 1990) three-lobed residual density around rheZ(2) site. We attempted to model this feature in ourrefinement; the details are given in the next section.Modeling the disorder at the T (2) position: Althoughdifference-Fourier rnaps show the characteristic three-lobed structure, Fourier maps calculated with a central

T(2) atom were not very informative. Consequently,refinement was initiated by displacing the three off-center sites a small arnount from the central position,and dividing the aggregate scattering power at the cen-tral site equally among the tbree off-center sites. Thestarting R-index was high, but converged to a valueslightly less than that for the central-site model.Howevel suspicious of the possibility of false mini-ma, we repeated the procedure with a slightly differentstarting model; the refinement converged to a differentgeometrical solution with approximately the same R-index. Repeating this procedure $everal times pro-duced a series of different solutions. all of which hadabout the same R-index. It is apparent that the solutionobtained is dependent on the starting model, whichindicates that most (if not all) of the solutions resultfrom convergelsg 16 fal5s minima.

In an effort to overcome this problem, we collecteddata on one crystal to very high resolution

TIIE CRYSTAL CHEMISTRY OF STAIIROLITE

IAELE 4. SETTCTED IIITERAIOIIIC OISTAilCES (A} FOR STAWOLITI

563

s{l) 5(?) s(3) s(1) s(5) s(6) s(7) s(8) s(e) s(10) s(l t) s( lz) s(13} s(14) s(15) s(16) s(17) s(18) s(re) s(20) s(2r)

I ( t ) -0?A 1 .634 1 .634 1 .635 1 .635 1 .636 1 .635 1 .637 1 .637 1 .636 1 .637 r . f f i5 1 .634 1 .634 1 .635 1 .633 1 .631 1 .633 1 .635 1 .633 1 .634 1 .6 : |3-028 1 .634 1 .634 1 .635 1 .534 1 .636 1 ,636 1 .636 1 .636 1 .636 1 .636 1 .634 1 .634 1 .633 1 .631 1 .630 r .631 1 .632 1 .635 1 .633 1 .633 1 .6 : i3-03 1 .648 1 .650 1 .653 1 .655 1 .655 1 .655 1 .656 1 .655 1 .658 t .65S 1 .653 r .652 1 .654 1 .651 1 .654 1 .655 1 .652 1 .655 1 .551 1 .651 1 .651-04 1 ,644 t .644 1 .616 1 .642 1 .642 1 .641 1 .644 1 .643 1 .643 1 .644 t .641 1 .641 1 .643 1 .643 1 .642 1 .641 1 .640 1 .64? 1 .642 1 .642 1 .641

<T(1) -0> 1 .640 1 .640 1 .642 t .642 l .U2 1 .642 1 .643 1 .613 1 .643 1 .644 1 .64 t 1 .640 1 .64 t 1 .641 1 .640 1 .639 1 .639 1 .642 1 .639 1 ,640 1 .640

r(21 -0lA-018-05

<r(2)-0>

r,r(tA)-024 1.937 1.937 r.935 1.940-04 1.898 1.898 1.895 1.899'05 1.892 1.891 1.892 1.907

<n( tA) -0> 1 .909 1 .909 1 .907 1 .918

u(18) -028 1 .938 1 .940 1 .91? 1 .949-04 t.90t 1.902 1.903 1.899-0s 1 .89? 1 .891 1 .891 1 .905

< i t ( lB) -0> 1 .910 1 .911 1 .912 1 .918

tf(z)-olA 1.927 1.927 t.926 1.924-0 lB 1 .925 1 .9?1 t .9?4 1 .926-024 1 .924 1 .927 1 .930 1 .923-028 1 .917 1 .915 1 .911 1 .923-03 1 .869 1 ,869 t .873 t .8 t8-05 1.068 1.068 1.869 1.869

<n(2) -0> 1 .905 1 .905 t .905 1 .907

il(34)-01A 1.845 1.844 t.$7 r.87?-03 2.037 2.031 2.016 2.047

4l(34)-0> 1.913 1.969 1.957 1.909

u(38)-0tB !.854 1.859 1.864 1.8t0-03 ?.050 2.462 2.07t ?.047

<it(38)-o> 1.990 1.995 ?.002 1.988

if(44)-0rA ?;10'l 2.7t9 2.118 ?.087-05 2 .191 2 ,190 ? ,188 ? .167

<n(4Al -0> 2 .163 2 .163 2 .164 ? . r53

lf(48)-0lB 2.097 2.092 e.086 ?.088-05 2 .195 2 .195 2 .196 2 .165

<f'f(48)-0> 2.t62 z.ri l ?.159 2.153

<n(1) -0> 1 .909 1 .910 t .9 t0 1 ,9184l(3)-0> 1.901 1.98U t.979 1.9€0<[ (11-0> 2 .162 2 .162 2 .161 2 .153

2,055 2.055 e.057 2.016 2.016 2.019 2.029 2.032 2.02e 2.922 2.016 2.013 2.015 2.011 2.034 2.040 2.028 2.035 2.035 2.033 2.0342.051 2.049 ?.04,1 2.015 2.016 2.02A 2.030 ?.030 2.02r 2.023 2.016 2.017 2.021 2.022 2.033 2.040 2.021 2.0X2 2.036 2.033 2.0361.961 1 .960 1 .960 1 .969 1 .968 1 .970 1 .979 1 .979 1 .971 1 .971 1 .976 1 .976 1 .975 1 .975 1 .971 1 .965 1 .971 t .977 1 .97? 1 .971 1 .9742.007 2 .006 2 .005 1 .992 1 .992 1 .995 2 .004 ? .00s 1 .996 1 .997 1 .996 1 .996 1 .996 1 .996 2 .002 2 .003 1 .999 a .005 2 .004 2 .002 2 .005

949 1 .946 1 .945 1 .943 1 .943 1 .942 1 .942 1 .941 1 .936 1 .932 1 .943 1 .915 1 .941 1 .942 1 .944 1 .941 1 .943899 1 .897 1 .098 1 .899 1 .890 1 .897 1 .898 1 .897 1 .895 1 .893 1 .901 1 .903 1 .899 1 .898 1 .900 1 .890 1 .899900 1 .905 1 .899 1 .898 1 .900 1 .901 1 .899 1 .899 1 .897 1 .8 ! ,7 1 .889 1 .886 1 .896 1 .69 t 1 .895 1 .896 1 .896919 1 .916 1 .914 1 .913 1 .914 1 .913 1 .913 1 .912 1 .910 1 .907 1 .911 t .9u 1 .912 1 .912 1 .913 1 .912 1 .913

1.918 1 .950 1 .915 r .915 r .946 1 .946 1 .944 1 .944 1 .919 1 .954 1 .946 1 .946 1 .948 1 .942 1 .943 1 .943 1 .9441.E99 1 .901 1 .899 1 .900 1 .899 1 .900 1 .900 1 .901 1 .903 1 .905 1 .903 1 .903 1 .902 1 .899 1 .899 1 .900 1 .901r .90 t 1 .908 1 .899 1 .899 1 .905 1 .903 1 .902 1 .90 t 1 .903 r .903 1 .889 1 .884 1 .899 1 .896 1 .895 1 .096 1 .8991 . 9 1 8 1 . 9 2 0 1 . 9 1 5 1 . 9 1 s 1 . 9 1 7 1 . 9 1 6 1 . 9 1 5 1 . 9 1 5 1 . 9 1 8 1 . 9 2 1 1 . 9 1 3 1 . 9 1 1 1 . 9 1 6 1 . 9 1 2 ! . 9 1 2 1 . 9 1 3 1 . 9 1 5

t .921 1 .9?4 1 .922 r .922 1 .921 1 .922 t .922 1 .922 1 .920 1 .920 1 .919 1 .919 ! .919 1 .920 1 .920 1 .921 1 .9201.925 1 .923 1 .922 t .922 r .9?3 1 ,923 1 .922 1 .923 1 .925 1 .930 1 .921 1 .920 1 .921 1 .922 1 .920 1 .921 1 .9191.922 1 .925 1 .924 1 .926 1 .923 1 .922 1 .926 1 .930 1 .936 1 .941 1 .926 1 .923 1 .928 1 .924 1 .925 1 .926 1 .9281.922 1 .919 1 .924 1 .924 1 .917 1 .918 1 .924 1 .924 1 .917 1 .908 1 .92s t .922 1 .920 1 .925 1 .925 1 .922 1 .9231.080 1 .879 1 .8 t8 1 .878 1 ,800 X,880 1 .873 1 .874 1 .875 1 .879 1 .887 1 .889 1 .877 1 .614 ! .874 1 .875 1 .0751.869 1 .868 1 .867 1 .868 1 .867 1 .867 1 .867 1 .868 1 .869 1 .869 1 .859 1 .856 1 .867 1 .857 1 .868 1 .868 1 .8661.907 r .90? 1 .906 1 .906 1 .905 1 .905 1 .906 1 .906 1 .907 1 .907 1 .906 1 .905 1 .906 1 .905 1 .905 1 .905 1 .905

1.870 r ,E6t 1 .859 1 .857 1 .855 1 .856 1 .861 1 ,E57 1 .E49 1 .844 1 .852 1 .849 1 .856 1 .855 1 .859 1 .857 1 .0562.041 2.042 2.044 2.A11 2,035 ?.037 2.046 2.035 2.020 2.006 2.031 2.036 2.035 2.043 2.046 2.040 2.0391.988 1 .984 1 .982 1 .980 1 .975 1 .976 1 .984 1 .976 1 .963 1 .95? 1 .975 1 .971 1 .975 1 .980 1 .984 1 .979 1 .978

l ,€69 1 .0 t3 r .061 1 .862 1 .86 / i 1 .862 r .S63 1 .865 1 .869 1 .875 1 .853 1 .850 r .S67 l .Ssa 1 .860 1 ,862 1 .8622,049 ?.055 2.047 2.051 2.051 2.050 ?.0s0 2.059 2.072 2.A89 2.042 2,037 2.059 e.046 ?.048 ?,052 2.0531.989 1 .995 1 .995 1 .988 r .989 1 .987 1 .987 1 .995 2 .004 2 .018 1 .979 1 .975 1 .995 1 .983 1 .985 1 .908 1 .990

?.090 ?.094 2.099 2.100 2.105 2.103 2.093 2.@8 2.108 2.1162.183 2.185 2, t96 2. t97 2.181 2.183 ?.187 2.106 2.1A5 2.184?.15e 2.155 2.1f i a.164 2.156 2.156 ?.156 2.r57 2.159 ?.161

?.091 2.m6 ?.096 2.094 2.094 2.096 ?.090 2.006 2.081 2.0732.183 2.197 2.196 2. t98 2.185 2.185 2.188 2.189 2.190 2.191?.152 e. t53 2.163 2.163 2.15s 2.155 2.156 2, t5r l 2.153 2.152

" ?, t01 2.100 2.096 ?.097 2.100" 2.188 2.196 ?.194 2.192 2.192" 2, t59 ?. t64 2.161 2.161 ?.161

. 2.@'t 2.098 2.095 ?.090 2.@2

. 2.191 2.195 2.194 2.193 2.195

. 2.156 2.163 2.161 2.159 2.161

t .918 1 .918 1 .914 1 .914 1 .915 X.914 1 .914 1 ,913 1 .914 1 .9 t1 1 .9 t2 1 .911 1 .911 1 .912 1 .912 l .9 le 1 .9141.90€ 1.989 1.983 1.984 1.982 1.981 1.985 1.985 1.985 1.985 1.977 1.974 !.985 1.981 1.984 1.98'1 1.9042.L52 2 .154 2 .163 2 .163 ? .155 2 .155 2 .156 2 .155 z . r f i 2 .157 2.157 2.163 2.161 2.160 2.161

l loto: s.8.d. ars 3 0.002 l.

20 = 135",5234 unique reflections). The problem offalse minima still persisted, and we also concludedthat the T(2)-T(2) separations obtained from the three-site models depend on the resolution of the X-ray dataused in the refinement. The minimurn possible inter-planar spacing allowed by the very high-resolutiondata is dro =1tJ2 srn 0* = 0,38 A, and the minimumseparation that can be reliably imaged in a map ofelectron density is 0.715 d^o= 0.27 A. Thus we con-clude that the real T(2)--T(2) separations arc less thanA.27 A, and hence they can only be adequatelyresolved with a radiation of considerably shorterwavelength than MoKcr. This is in accord with theseparations estimated by Stihl & kgros (1990) fromthe magnitudes of the central-site isotropic displace-ment factors.

The following procedure was used to cope with thisproblem. A central I(2) site was used, and the occu-pancy and thermal parameters were refined un-

constrained. Upon convergence, small amounts ofscatlering were added at the three subsidiary positionscorresponding to the familiar lobes in the difference-Fourier, and the model was refined to convergence.We emphasize that the three subsidiary positions haveno stereochemical significance except to indicate thatone (or more) disordered positions of the cations liesalong each vector joining the position of the centralcation to the subsidiary positions, as assumed in allprevious work. However, we found that the total scat-tering from the T(2) site (or aggegate scattering fromthe central site and subsites) was the same in the one-site model, the three-site model (without the centralsite), and the four-site model; it is apparent that theadditional density added at the three subsidiary posi-tions was accompanied by a decrease in the displace-ment factor at the central site and a correspondingdecrease in the density at the cental site, \r.ith a netchange in scattering of zero. For simplicity, we give

5&

IABLE 4. COI{TINUED.

T1IE CANADIAN MINERALOGIST

s(2?) s(?3) s(24) s(?5! S(26) s(2r) S(?s) S(2e) s(30) S{3t) s(32) S(33} s(34) s(35) S(36) s(37) S(38) S(3e) s(10) s(41) s(42)

l (1 , -02A 1 .634 1 .633 1 .633 1 .633 1 .634 1 .634 1 .636 1 .636 1 .635 1 .635 1 .634 1 .633 1 .635 1 .635 1 .632 1 .533 1 .633 1 .634 1 .634 1 .631 1 .630-028 1 .633 1 .634 1 .633 1 .633 r .633 1 .634 1 .635 r . f f i4 1 .634 1 .635 1 .635 1 .633 1 .633 1 .634 1 .633 1 .635 1 .632 ! .634 1 .634 1 .630 1 .631-03 1 .651 1 .654 1 .653 r .650 t .65 t t .654 1 .655 1 .655 1 .655 1 .6s5 1 .653 1 .651 1 .652 1 .6s2 1 .651 1 .652 1 .651 1 .65s 1 .652 1 .652 1 .649-04 1 .641 1 .643 1 .64? 1 .641 r .642 1 .641 1 .613 1 .6{4 1 .641 1 .6 /$ 1 .642 1 .641 1 .642 1 .643 1 .641 1 .643 1 .641 1 .645 1 .6112 1 .644 1 .637

<I ( l ) -0> 1 .641 1 ,641 1 .640 1 .640 1 .640 1 ,641 1 .642 r .642 1 .641 1 .642 1 .641 1 .640 1 .640 1 .641 1 .639 1 .641 1 .639 1 .642 1 .641 1 .639 1 .6 i17

I ( 2 ) - 0 t A ? . 0 3 5 2 . 0 2 8 2 . A 3 2 ? . 0 4 0 2 . 0 3 9 ? . 0 3 6 2 . 0 3 7 2 . 0 3 4 2 . 0 3 8 2 . 0 2 9 2 . 0 3 0 2 . 0 3 5 2 . 0 3 3 2 . 0 4 0 2 . 0 3 5 2 . 0 3 1 2 . ' 0 ? 8 2 . 0 2 5 2 . 0 3 4 2 . 0 1 6 2 . 0 0 7-0r8 2.037 2.029 2.03t 2.040 2.040 2.035 2.03? 2.031 2.036 ?.026 2.029 2.035 2.031 2.038 ?.033 2.031 2.032 2.029 2.029 2.025 2.005-05 1 .975 1 .977 1 .976 1 .971 1 .972 t .977 1 .976 1 .978 1 .976 L974 1 .979 1 .975 1 .978 1 .968 1 .978 1 .975 1 .972 1 .971 1 .977 1 .943 1 .961

<T(2) -0> 2 .005 2 .003 2 .001 2 .005 ? .006 ? .006 2 .006 2 .005 2 .006 2 .001 2 .004 2 .005 2 .005 2 .004 2 .006 2 .003 2 .001 1 .999 2 .004 1 .98? 1 .983

l . i ( lA ) -o?A 1 .940 1 .943 1 .942 1 .944 r .940 t .941 1 .942 1 .911 t .942 t .942 1 .942 r .941 1 .939 1 .937 1 .938 1 .939 1 .938 1 .936 1 .935 1 .940 1 .935-04 1 . .E96 1 .899 1 .898 1 .898 1 .896 1 .898 1 .898 1 .S97 1 .099 1 .898 1 .09S 1 .900 1 .900 1 .897 1 ,097 1 .897 1 .897 1 .895 1 .896 1 .904 1 .903-05 1 .896 1 .896 r .896 1 .897 1 .895 1 .895 1 .S97 1 .89 t 1 .896 1 .897 1 .896 1 .896 1 .895 1 .894 1 .894 1 .895 1 .894 1 .894 1 .893 1 .897 1 .888

< i l0A) -0> r .9u 1 .913 1 .912 1 .913 1 .911 1 .91 t 1 .912 1 .911 1 .912 1 .9 t3 1 .912 I .912 t .g t t 1 .910 1 .910 1 .910 1 .910 1 .909 1 ,900 1 .914 1 .909

[ ( rB) -028 1 ,948 1 .913 r ,94? 1 .946 1 .945 1 .947 1 ,947 1 .947 1 .917 1 .943 1 .945 1 .944 1 .944 1 .944 1 .947 1 .946 1 .949 1 .946 1 .950 1 .943 1 .935-04 1 .900 1 .899 1 .89S 1 .900 1 .903 1 .901 1 .901 1 .900 1 .901 1 .897 r .900 1 .90r t .90r 1 .902 1 .903 1 .902 1 .905 1 .902 1 .901 1 .896 1 .902-05 1 .898 1 .897 1 .896 1 .898 1 .096 1 .897 1 .89s 1 .Se8 1 .898 1 .898 1 .899 1 .896 1 .899 1 .8e6 r .897 r .899 1 .900 1 .898 1 .900 1 .890 1 .885

< i r ( tB) -o> 1 .9 t6 1 .9 t3 1 .912 1 .9r5 1 .915 1 .9r5 1 .915 1 .915 1 .915 1 .913 1 .9 t5 1 .913 l .g ls 1 .914 r .916 1 .916 1 .918 1 .915 l .9 lS 1 .910 1 .900

r . t (z ) -o lA 1 .9?2 1 .921 1 .920 1 .923 1 .923 1 .921 1 .923 t .922t .9?2 1 .919 1 .920 1 .921 1 .918 1 .923 1 .919 1 .921 1 .9?0 1 .920 1 .919 1 .9201.922-0 lB 1 .923 1 .921 1 .92 ! 1 .923 1 .923 1 .923 t .9?4 1 .925 1 .923 1 .921 1 .920 1 .922 1 .918 1 .923 1 .918 1 .923 1 .924 1 .923 1 .918 1 .916 1 .924-0?A 1 .928 1 .926 1 .925 1 .926 1 .929 ! .928r .927 1 .928! .927 1 .923 1 .?8 1 .929 1 .932 1 .928 1 .935 1 .933 1 .933 1 .933 1 .938 1 .912 1 .919-o2B 1 .916 1 .9?4 1 .9?3 1 .922 ! .919 1 .92r 1 .920 1 .919 1 .9? l 1 .922 1 .925 t .923 1 .926 r .917 1 .921 1 .918 1 .917 1 .915 1 .914 1 .917 1 .918-03 1 .6 i6 1 .876 1 .875 1 .873 t .872t .876 1 .S7? 1 .877 1 .876 1 .878 1 .876 r .873 1 .814 1 .874 1 .0?5 1 .875 1 .871 1 .877 1 .877 1 '077 1 .873-05 1 .867 1 .867 1 .066 t .S69 1 .869 1 .666 1 .868 1 .867 1 .86s 1 .867 1 .868 1 .868 t .860 1 .860 1 .868 1 .869 1 .866 1 .868 1 .867 1 .865 1 .861

<t r (? ) -o> t .905 1 .906 r .905 1 .906 1 .906 1 .906 1 .906 1 .906 1 .906 1 .905 1 .906 1 .906 1 .906 1 .905 1 .906 1 .906 1 .906 1 .906 1 .905 1 .901 1 .903

t{(3A)-01A 1.849 1.857 1.851 1.8s7 1.853 1.850 1.8s3 !.8s? 1.84S 1.857 X.857 1.853 1.853 1.847 1.848 1.850 1.849 1.843 1.840 1.815 l.8il5- 0 3 2 . 0 2 8 2 . 0 4 3 2 . 0 4 0 2 . 0 4 3 e . 0 3 6 2 . 0 3 4 2 . 0 3 5 2 . 0 3 2 2 . A 3 2 2 , 0 1 2 2 . 0 1 t 2 . 0 3 8 2 . 0 3 6 2 , 0 2 9 2 . 0 2 7 2 . 0 2 9 2 . 0 2 5 ? . 0 1 5 ? . 0 1 3 2 . 0 3 5 2 . 0 i 1 5

<t { (3A) -0> r .968 1 .981 1 .977 t .98 t 1 .975 t .g tz t . s r4 1 .972 1 .971 1 .980 1 .980 1 .976 1 .975 1 .968 1 .968 1 .969 1 .967 1 .958 1 .955 1 .90? 1 .972

it(38)-o1B t.865 1.859 1.854 r.863 1.862 1.660 1.863 1.865 r.861 1.859 1.862 !.861 1.864 1,S63 1.867 1.86S 1.868 1.866 1.874 1.856 1.8,48- 0 3 2 . 0 6 0 2 . 0 4 7 2 . A 4 5 ? . 0 5 3 ? . 0 6 1 2 . 0 5 6 2 . 0 5 5 2 . 0 5 7 2 . 0 5 7 2 . 0 4 4 2 . 0 5 1 2 . 0 5 6 2 . 0 5 0 2 . 0 6 2 2 . 0 6 5 2 . 0 6 6 ? . 0 6 7 2 . 0 6 9 2 . 0 7 7 2 . 0 3 5 2 . 0 4 1

<t f (3B l -D 1 .995 1 .984 1 .983 1 .989 1 .995 1 .991 1 .991 1 .993 1 .992 L .sEz 1 .988 1 .991 1 .993 r .996 1 .999 2 .000 2 .001 2 .001 e .009 1 .975 1 .976

i l ( ! tA) -O lA 2 . l l l 2 .og l 2 .102 2 . lo l 2 .103 2 . tOO 2 .105 2 .105 2 .108 ? .098 Z ,os l 2 .102 2 .10r 2 .108 2 .106 2 .106 z ,LgS 2 .A4 2 .115 2 .079 ? '099- 0 5 2 . 1 9 5 2 . 1 9 5 2 . 1 9 3 2 . 1 9 3 2 . 1 9 3 2 . 1 9 6 2 . 1 9 6 ? . 1 9 6 2 . 1 9 6 2 . 1 9 1 2 . 1 9 6 ? . 1 9 6 2 . 1 9 5 ? . 1 8 9 ? . 1 9 6 2 . 1 9 2 2 . 1 8 8 2 , 1 8 4 2 . 1 9 2 2 . 1 6 2 2 . 1 7 0

<t { (4A) -0> 2 . t65 2 .162 2 . t63 2 , t62 e .163 ? .166 ? .166 2 .166 2 ,166 2 .160 2 .163 2 .164 2 .164 ? .162 2 .166 ? .163 2 .161 2 .160 2 .166 2 ,134 2-116

lf(48,-0lB ?.oss 2.094 2.09s 2.095 2,09t 2.A92 2.0s2 2.089 2.0s2 2.0s5 2.092 2.092 2.089 2.089 2.0U 2.0U ?.083 2.084 ?.075 2.100 2.096- 0 5 2 . 1 9 8 2 . 1 9 5 ? . 1 9 4 2 , t 9 5 2 . t 9 1 2 . 1 9 9 2 . 1 9 0 2 . 1 9 9 2 . 1 9 S 2 . 1 9 t 2 . 1 9 8 2 . t 9 7 2 . t 9 9 ? . 1 9 5 ? . 2 0 0 2 . 1 9 7 2 . 1 9 4 2 . 1 9 1 2 . 2 A 0 2 . 1 5 5 2 . 1 6 8

< l f ( / tB) -D 2 .162 2 . tOL 2 .162 2 .161 2 .16? 2 .163 2 .163 2 .15? a .163 e .159 2 .163 2 .162 2 .162 2 .159 2 .161 2 .159 2 .157 2 .155 2 .18 2 .137 2 .144

<r (1 , -0> 1 .913 1 .913 1 .912 1 .914 r .913 t .913 t .913 1 .913 1 .913 1 .913 r .913 1 .913 1 .913 1 .91? 1 .913 1 .913 1 .914 1 .912 1 .9 t3 1 .912 1 .908<ilati-o: t.gst 1.982 t.980 r.989 1.995 t.g8l t.g8? t.ga? r.g8t t.ggl r.904 1.98i 1.984 1.982 1.983 1.984 1.984 1.s79 1.982 1.978 1.9t4< i ta4 i -D ? .163 2 .161 2 .162 2 . t6 t 2 . t62 2 .164 ? .164 2 .164 2 .164 2 .159 2 .163 2 .163 2 .163 2 .160 2 .163 2 .161 2 .159 2 ,15 '1 2 .162 2 .135 2 .145

the central-site results only.Full-matrix refinement of all variables converged to

final R-indices in the range l-2Vo. Anmic coordinatesand displacement paraaeters may be obtained fromthe Depository of Unpublished Data CISTI, NationalResearch Council of Canada, Ottawa, Ontario KIA0S2, selected interatomic distances are given inTable 4, and site-scattering data are listed in Table 5.

Elec tron-mic rop ro b e armly s i s

Subsequent to the experimental crystallographicwork, some of the crystals used in the collection of theX-ray-diffraction data were analyzed by electron- andion-microprobe techniques. The crystals were mount-ed in piccolite in small holes in 254 cm diameter cop-per discs; each disc also contained several crystals ofstaurolite 7142R and EH-6 (Holdaway et al. L986b)as a check on mount-to-mount compatibility. Thediscs were polished and subsequently carbon-coated.

Electron-microprobe analyses were done on a flrllyautomated Cameca SX-50 operating in the wave-length-dispersion mode with the following conditions:excitation voltage: 15 kV; specimen current 20 nA;peak count time: 20 s; background count time: L0 s.The following standards and crystals were used for KcX-ray lines: Al: Al2O3, TAP; Fe: fayalite, LiF; Si:diopside, PET; Mg: MgO, TAP; Zn:. gahnite, LiF; F:fluor-riebeckite, TAP; Mn: tephroite, LiF; Ti: titanite,LiF; Cr: chromite, LiF; Co: cobaltite, LiF. Each grainwas analyzed at a minimum of 12 points (commonlytwice this number, depending on the size of theexposed surface) to check for compositional zoningand to get a representative composition for the wholecrystal used in the diffraction experiment. In addition,samples 71,42k and EH-6 were analyzed on all fiveprobe mounts to check for consistency of analyticalconditions; no significant differences from mount tomount were observed. Data reduction was done withthe Q(pZ) method @ouchou & Pichoir 1984, 1985),

TIIE CRYSTAL CHEMISTRY OF STAIJROLITE

TABLE 5. REFINTD SCATTERING PO!'ERS* (PFU) IN STAUROLITE CRYSTALS

565

N** s( l ) s(2) s(3) s(4) s(5) s(6) s(7) s(8) s(e) s(ro) s ( r l ) s ( r2 )s(13) s(14)r'r(rA)r'r( 1B)H ( 2 )l,l(34)t'l(38)fi(44)r4(48)T(2)

44I22224

52.7 52.55 2 . 8 5 2 . 5

106.5 105.81 3 . 9 1 4 . 99 . 8 8 . 87 . ? 7 . 53 . 9 3 . 0

71.3 70 .6

5 2 . 8 5 3 . 1 5 2 . 352.5 53 .1 82 .2

1 0 5 . 1 1 0 6 . 7 1 0 5 . 2t 7 . 4 1 3 . 4 1 3 . 77 . t 1 3 , 4 1 3 . 49 . 0 1 . 5 1 . 51 . 7 1 . 6 1 . 4

71.9 86 .4 84 .9

52.7 52 .852.7 52,7

106.0 106.11 4 . 6 1 3 . 912.3 t2 .71 . 5 I . 31 . 3 l . l

8 6 . 0 9 2 . 3

52.6 51 .8 52 .152.4 52.2 52.1

105.3 106.0 104.914. I 14 .6 14 .41 2 , 3 1 1 . 5 1 1 . 91 . 5 1 . 8 2 , 0r . t 1 . 6 1 . 5

9 1 . 3 8 9 . 8 9 0 . 7

52.3 5?.452.2 5?.2

105.4 105.2t2,9 ls . lt2,2 10.31 . 4 1 . 91 . 3 0 . 8

103.4 102.6

52.6 52.75 2 . 3 5 2 . 5

106.4 107.217.8 2t.57 . 8 4 . 32 , 0 2 . s0 . 3 0 . t

103.9 102.9N** s(15) s(16) S(17) s( t8) s( te) s(20) s(21) s(22) s(23) s(24) s(2s) s(26) s(27) s(28)

lr(lA)r'r( lB)l'| (2)r'r(34)l,r(38)!r(44)r{(48)T (2 )

4 52.8 53.4+ 52.8 53.4I 106.4 106.8z t3,4 13.32 12.5 13.02 -2 -4 78.1 7?. ,6

52.9 52.852.7 52.9

1 0 7 . 0 1 0 5 . 31 5 . 2 1 3 . 1I t .0 t2 .93 . 0 1 . 7l o I o

7 7 . 0 9 0 . 1

52.2 52.252. r 52 .4

105.3 104.01 3 . 0 1 3 . 912.6 l t .82 . 8 2 . 52 . 5 2 . 0

8 2 . 5 8 3 . 2

5 2 . 3 5 2 . 05 2 . 3 5 2 . 0

108.8 105.21 4 . I 1 6 . Iu . 4 9 . 72 . 8 2 . 62 . 3 1 . 0

85.9 90 .2

52.3 52.452.3 52.3

106.9 105.21 3 . 4 1 3 . 51 2 . 5 1 2 . 51 . 9 2 . 0l . o l . o

88.2 87 .7

5 2 . 5 5 3 . 052.4 53.0

106.3 107.91 3 . 6 1 4 . 91 t . 7 1 0 . 2? , 9 4 . I2 . 4 2 . 1

86.5 84 .3

52.? 52.55 2 . 0 5 2 . 5

104.4 105.51 4 . 9 1 s . 0l l . 0 l l . l2 . 0 2 . 5t . 2 1 . 5

91.4 90 .0

M( lA)14( lB)l,l(2)l,l(34)r,r{3B)r{(44)t't(48)T(2)

N** s(2e) s(30) s(31) s(32) s(33) s(34) S(35) S(36) S(37) s(38) S(3e) S(40) S(41) S(42)51 .9 52 .551.9 52 .6

104.8 106.91 3 . l 1 3 . 91 2 . 8 1 2 . I1 . 8 1 . 61 . 7 t . 4

88. I 9L .7

4 5 2 . 5 5 2 . 1q 52.2 51.8I 106.5 104.6z 1 5 . 4 1 5 . 22 t0 ,7 10 .82 ? . ,2 2 .32 I . l 1 , . 24 9t.2 90.7

52.8 52 .752.6 52.8

107. I 107.01 4 . 5 1 4 . 8l t . l 1 1 . 13 . 0 2 . 7z , t 2 . 0

8s .9 89 .3

52.5 52 .552.q 52.3

106.6 106.416.0 t6 ,29 . 4 9 . 44 . 4 2 . 8t o 1 I

79.8 88. I

53.1 53 .0 5? .253.0 53 .0 52 .3

105.9 106.6 105.316.6 17 .? 18 . I9 . 2 8 . 8 7 . 82 . 9 2 . 5 3 . 9l . t 0 . 8 1 . 0

86.9 85 . I 87 .1

53.3 5?.9 52.453.2 53.5 52.6

107.4 105.4 106.019.0 12.8 t2.67 . t 1 3 . 0 l t . 53 . 2 6 . 2 2 . 80 . 5 6 . 2 2 . 5

47.7 25 .2 8 l . l

* e.s.d. are 0.1-0.4 elect,rons** N ls the nunber of times thls slte occurs in the structural formula

and the resultant mean composition for each grain isgiven in Table 7. Crystals S(41) and S(42) were ana-lyzed by electron- and ion-microprobe methods inParis by C. Chopin (pers. comm.); results are given inTable 6.

Several of the samples examined here werepreviously atalyzed by Holdaway et al. (1986b).Table 7 provides a comparison of the compositionsof grains from the same mineral separates; theagreement is excellent, and as both different instru-ments" sets of standards. and standardization andcorrection procedures were used (Bence-Albeemethod by Holdaway et al. 1986b; Q(pZ) method inthis work), the agreement is indicative of accurateanalyses.

Ion-microprob e analy s i s

Ion-microprobe analyses were done on a CamecaIMS 4F according to the method of. Ottoliai et al.(1992, 1,993). Both ion-microprobe and crystal-chemi-cal results suggested that the Li20 content of sample6-3 given by Holdaway et al. (I986b) was overesti-mated by at least 20Vo. This sample was re-analyzedby Holdaway (pers. comm., 1991), with a resultantvalue of 0.99 ttt%o Li2O that agrees exactly with theion-microprobe result Clable 7).

Normalizgtion of theformula unit

The correct way of calculating the formula unit ofstaurolite is on the basis of 48 (O,OH,F), includingLi2O contents. Such unit formulae are available for thestaurolite samples obtained from Holdaway et aI.(1986b), but not for the remainder of the samples, asthe paucity of material in most cases precluded analy-sis for H2O (F was not a significant component in anyof the staurolite samples analyzed). In the absence ofsuch analytical dat4 Holdaway et al. (L986b) suggest-ed following schemes of renormalization: (1) a basisof 46.47 O (i.e.,H = 3.06 aptu), with Li = 0.2 aptu forstaurolite that coexists with garnet or biotite; (2) abasis of 45.93 O (i.e., 4.1,4 H apfu) for staurolite thatdoes not coexist with gamet or biotite; (3) a basis of25.53 (Al + Si) for staurolite forming under veryreducing conditions, and coexisting with graphite andhematite-free ilmenite. For those cases for which wedo not have H concentrations, we have renormalizedthe unit formulae on the basis of 46.5 O (= 3.0 H aptu)with the determined Li2O values, similar to the frstmethod suggested by Holdaway et al. (L986b). We canbe sure that none of these samples have H - 4 apfubecause of our site-scattering refinement results, asthere are several features of H-rich stauro[te [highM(4) scattsrrrng,low M(3) scatteringl that differ signif-

s66 TTIE CANADIAN MINERALOGIST

TABLE 6. RESULTS OF ELECTRON AND ION MICROPROBE AMLYSEST AND UNIT FORI,ITJLAE* OF STAUROLITE CRYSTALS

s(l) s(2) s(3) s(4) s(5) s(6) s(7) s(8)'t231 t25l t23l t25l t23l 1241 I24l l2rls(e) s(10) s(r2) s(13) s( ls) s(15)tz4l t23l 126l t12l tz3l t24l

s(17) s(18)124\ trzl

Sl0a wttt 27.57 27.8341,0, 54.85 55.12Tl0" 0.42 0.35Cr,o, 0.03 0,03Fe.0, 0.43 0,12Feo 12.40 12.?1l . lg0 0.67 0.66l{no 0.22 0.22Coo 0 .01 0 .02Zno 0.13 0.20Llao 0.36 0.36llo (2.261 (2.26')

27.82 27.90 27.64 27.78 27.25 27.05 26.75 26.48 27.16 27,19 28,16 28.40 27.48 26.2l54.91 53.20 53.36 53. Is 53.99 53.82 54.74 54.66 52.96 52.64 54.99 55.36 54.23 54.300 .32 0 .37 0 .37 0 .39 0 .41 0 .41 0 .33 0 .27 ' s . 48 0 .41 0 .4s 0 .43 0 .54 0 .470 .01 0 .03 0 .05 0 .04 0 .04 0 .03 0 .01 0 .01 0 .03 0 .03 0 .05 0 .06 0 .03 0 .020 .43 l . 7 l 1 . 70 1 .71 0 .46 0 .47 0 .35 0 .35 0 .42 0 .42 0 .61 0 ,62 0 .39 0 . i 18

12 .39 11 .31 l t . l 8 11 .26 13 ,46 13 .67 10 .05 10 ,24 9 .03 8 .99 10 .39 10 .55 11 .33 13 .890 .62 3 .92 4 .07 3 .98 1 .85 1 .69 2 .61 2 .55 1 .77 1 .88 0 .87 0 .91 3 .41 1 .650 .?2 0 .16 0 .13 0 .14 0 .44 0 .50 0 .36 0 .34 0 .13 0 . l l 0 . 50 0 .49 0 .09 0 .180 .02 0 .04 0 .04 0 .03 0 .02 0 .02 0 .01 0 .02 0 .02 0 .01 0 .03 0 .03 0 .01 0 .010 .19 0 .30 0 .28 0 .26 0 .20 0 .21 3 . l l 3 . 13 6 .39 6 .71 0 .53 0 .43 0 .13 0 .140 .36 0 .02 0 .01 0 .02 0 .08 0 .14 0 .01 0 .01 0 .05 0 .05 0 ,92 1 .04 0 .02 0 .07

(? .26 ' . ) 11 .52 ) ( 1 .52 ) ( r . 53 ) ( 1 .43 ) ( 1 .43 ) ( 1 .62 ) ( 1 .62 ) ( r . 65 ) ( 1 .65 ) ( 1 .80 ) ( 1 .80 )

99.58 99.55 100.48 100.35 100.29 99.63 99.44 i00,01 99.68 100.09 100.09 99.30 lo0. l2 97.66 97,42

7.679 7.693 7.733 7.669 7.773 7.666 7.636 7.483 7.44L 7.677 7.698 7,784 7.784 7,656 7.4240.321 0.307 0.267 0,331 0.287 0.334 0.364 0.517 0.559 0.323 0.302 0.216 0.216 0.344 0.5768.000 8.000 8.000 8,000 8.000 8.000 8.000 8.000 0.000 8.000 8.000 8.000 8.000 8.000 8.000

Sun 99.35

st 7.641Al 0.359sum 8.000

Al 17.559 17.605 17.589 l7. l l2 t7. l l8 17.105 17.568 17.543 17.530 17.545 11.320 17.262 17.700 17.668 17.463 17.551Cr 0.007 0.007 0.002 0.007 0,011 0.009 0.009 0.007 0.002 0.002 0.007 0.007 0.011 0.013 0.007 0.004Tt 0.088 0.073 0.067 0.077 0,077 0.081 0.087 0.087 0.069 0.057 0.102 0.087 0.094 0.089 0.1t3 0.100Fe" 0.090 0.087 0.089 0.357 0.3s5 0.357 0.097 0.100 0.074 0.074 0.089 0.089 0.127 0.128 0.082 0.102Feo 2.874 2.81A 2.865 2.622 2.594 2.615 3.167 3,227 2,351 2.407 2.135 2.72A 2.402 2.418 2,640 3.290l , fg 0.277 0.272 0.256 1.620 1.683 1.647 0.776 0.711 1.u3 1.068 0.746 0.793 0.359 0.372 1.416 0.697t i ln 0.052 0.051 0.052 0.038 0.031 0.033 0.105 0.120 0.085 0.081 0.031 0.026 0.117 0.114 0,021 0.043Zn 0.027 0.041. 0.039 0.061 0.057 0.053 0.042 0.044 0.642 0.649 1.334 1.403 0.108 0.087 0,027 0.029C0 0.002 0.004 0.004 0.009 0.009 0.007 0.005 0.005 0.002 0.005 0.005 0.00? 0.007 0.007 0.002 0.002t i 0.40r 0.400 0.400 0.022 0.01t 0.022 0.091 0.159 0.011 0.011 0.057 0.057 1.02t 1.146 0.022 0.0m

Catsum 29.377 29.358 29.363 29.925 29.946 29.929 29.946 30.003 29.879 29.899 29.826 29.854 29,948 30.042 29,793 29.898

H 4.178 4. r60 4.169 2.810 2.813 2.815 2.684 2.693 3.023 3.037 3. l l l 3.116 3.3le 3.291 (3) (3)

r Fe"Q values are taken from Dyar et al.(1991) rhero aval'lablei all others are estJnated as 3,{ of the total tq as foundby Dyar et al.(1991) for mosi staurollte crlistals; H,0 values are taken fron Holdawey et at. (1986a).

* staurol i te crystals 'S(t ) -s(16) nonnal lzed on the basis of 48(0,0H,F); staurol l te crystals S(17)-S(40) normal ized on46 .5 0 .'values ln [] are the nmb€r of analyses on each crystal.

icantly from those of normal stauro[te. Unit formulaeaxe given in Table 6.

Renormalization on the basis of 46.5 O obviouslyintroduces some error into the formula unit.Recalculation of the analytical data of Holdaway et al.(1986b) on the basis of 46.5 O gives an idea of tlemagnitude of the errors so infioduced. These are negli-gible except for Al and Si, by far the most abundantcations in the structurs. Consequently, for correlationsinvolving Si and Al, only the staurolite samplesobtained from Holdaway et al. (L986b) generally wereusd whereas for correlations involving otler cations,all data were used.

SrIE PoPULATToNS:Gmmar CoxsrosRAnoNs

The assignment of site populations is a very com-plicard problem in staurolite, and here we define theprocedure that we use. During structure refinement,the occupancy of each cation site was set as x M +(l-r) N, wherc M and N are the principal (possible)scattering species at that site le.g., M = Al, N = Fe forM(2), M = Al, N = n for M(3A) afi M(38)1. However,in complex solid-solutions, the resultant site-occupan-

cies do not necessarily represent exactly what atomsare at the si!e; in fact, they only give the amount of X-ray scattering from the site, and we prefer to refer tothis procedure as site-scattering refinement. We havefound it clearer and more convenient to express theresults of the site-scattering refinement in terms of thesite-scattering power, tJte number of electrons associ-ated with the atoms at that site in the stuctural formu-la. These electrons may then be assigned to (or associ-ated with) specific chemical species on the basis ofknown chemistry and via stereochemical analysis.These scattering powers were converted into epfu(electrons per formula unit) by taking into account Zand the site multiplicities for tle C2,lm space group.When the site-scattering powers are converted intochemical species, this gives the site populations,which do not necessarily sum to unity, but sum to thenumber of equivalent sites in the corresponding struc-tural formula.

The dominant X-ray scattering species in staurolite(besides O) are A1, Si, Mg and Fe. As Al, Si and Mgall scatter X rays in a very similar fashion, they cannotbe differentiated directly by their X-ray scatteringpower. At some stages in the assignment of site popu-lations, we will group species of similar scatterinq

THE CRYSTAL CHEMISTRY OF STAI]ROLITE 567

TABLE 6. CONTINUED.

s(20) s(zz) s(23) s(24) s(zs) s(26) s(27) s(30) s(33) s(3s) s(35) s(3e) s(40) 's(4r) 's (42)t12l t lz l t12 l t rz l [12 j 124] l24 l t l2 l t lz l t lz l t12 l t241 t12l

st0?Al?03Tt0?Cre03Fee03Fe0ilS0iln0Co0Zn0Lir0H.0Sum

5 l

AISum

AICrTiFe"Fet"i,lglilnZnCoL I

rtJ6 27 .01 26.93 26.59 26.64 27 .07 26.9355.02 54.04 54.{8 54.45 53.32 53.900.49 0 .52 0 .46 0 .49 0 .36 0 .360.02 0 .01 0 .02 0 .02 0 .02 0 .020.44 0 .44 0 .44 0 .45 0 .44 0 .42

12.87 72.77 12.94 13.01 12.78 12.362 . 3 2 2 . 1 9 1 . 9 6 2 . 0 3 1 . 9 4 1 . 7 70.20 0 .26 0 .35 0 .33 0 .22 0 .250.02 0 .02 0 .02 0 .03 0 .04 0 .030 . 2 9 0 . 5 5 0 . 3 7 0 . 3 7 0 . 9 4 r . 0 60.07 0 .09 0 .09 0 .08 0 .11 0 . I r

ga.t i gt.ai i l . t i et.go st.zi gt.zt q.ci

7.507 7.568 7.484 7 .4A7 7.667 7.6t7 7.5340.493 0 .432 0 .516 0 .513 0 .333 0 .383 0 .4668.000 8.000 8.000 8.000 8.000 8.000 8.000

26.59 26.79 26,98 27.32 27.03 27.84 30.66 28.4953.e7 53 .7s 54 .70 55 .49 54 .21 54 .32 57 .45 54 .120.34 0 .60 0 .52 ' 0 .55 0 .58 0 .49 0 .18 0 . l l0 .02 0 .02 0 .01 0 .04 0 .06 0 .030.49 0 .46 0 .41 0 .45 0 .36 0 .16

14.19 13 .28 11 .98 12 .98 10 .63 13 .32 0 .72 1 .401 . 6 6 1 . 7 4 1 . 7 6 1 . 6 0 1 . 7 3 1 . 6 8 7 . 1 7 0 . 4 50 . 1 5 0 . 1 5 0 . 3 7 0 . 1 6 0 . 3 8 0 . 1 8 - 0 . 0 10.02 0 .02 0 .02 0 .03 0 .01 0 .020.12 0 .43 0 .48 0 .21 2 .58 0 .14 0 .10 11 .820 . 1 0 0 . 1 0 0 . 1 2 - 0 . 1 4 0 . 1 7 0 . 9 0 0 . 4 5

97.6s 97.34 97.35 98.83 97.71 98.65 97.78 96.85

7.t18 7.577 7.581 7.576 7.607 7.740 7.958 7,970.482 0 .423 0 .419 0 .424 0 .393 0 .260 0 .042 0 ,038.000 s.000 8.000 8.000 8.000 8.000 8.000 8.00

26.5953.770.330.020.49

14.201 . 6 60. l40 .040 , t 20 . l0

17.530 17.468 17.556 17.523 17.466 17.586 17.491 17.503 17.494 17.696 17.713 17.588 17.538 17.534 17.8: l0.004 0.002 0.004 0.004 0.004 0.002 0.004 0.004 0.004 0.002 0.009 0.013 0.0070.102 0 .110 0 .097 0 .104 0 .a77 0 .077 0 .070 0 .072 0 .128 0 .110 0 .115 0 .123 0 .102 0 .035 0 .020.092 0.093 0.093 0.095 0.094 0.089 0.104 0.104 0.098 0.087 0.094 0.076 0.0962.991 3 .001 3 .046 3 .058 3 .027 2 .924 3 .365 3 .35s 3 .141 2 .815 3 .010 2 .502 3 .097 0 .156 0 .330.961 0.917 0.822 0.851 0.819 0.746 0.701 0.700 0.734 0.737 0.661 0.726 0.696 3.007 0.190.047 0.062 0.083 0.079 0.053 0.060 0.034 0.036 0.036 0.088 0.038 0.091 0.0420.060 0.114 0.0'17 0.077 0.t97 0.22t 0.025 0.025 0.090 0.100 0.043 0.536 0.029 0.019 2.450.004 0.005 0.005 0.007 0.009 0.007 0.009 0.005 0.005 0.005 0.007 0.002 0.0040.078 0 .102 0 .102 0 .090 0 .125 0 .125 0 .114 0 .114 0 .114 0 .136 0 .000 0 .158 0 .190 0 .940 0 .51

Catsum 29.869 29.874 29.85 29.888 29.871 ?9,837 29,917 29.918 29,844 29.776 29.690 29.815 29.801 29.691 29.33

H (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (4) (4)

I analyses from C. Chopin (pers. conun., l99l)

TABTT 7. CfiPARIS${ OF RESULTS OF ETTCTROII+ITCROPROBE AI{ALYSES OF STAUROLITE sAnPT$IN THIS SIUDY (TS) AXD HoIOAIAY et ol. (1986b) (lOS)

7l-62R EH-6 3-3 106038 117189

sjoe |ft? 27.74 27.79

Aleo! 54.96 54.95

?7.78 27. t3 27.r553.23 53.03 53.91

27.22 ?6.62 26.90 27.17 ?7.46 28.28 28.36

54. 14 54.70 54.56 52.86 52.94 55.18 55.04

Ti0,

Cr203

Co0

Fe0

ilsot{n0

0 . 3 8

0.04

0.04

0.47

0.06

0.03

0 . 4 1

0.04

0.02

0.54

0.09

0.02

0.30

0 . 0 1

0.02

0.38

0.05

0.02

10.37

2.63

0.36

3 .0s

0 .46

0.03

0 . 0 2

9.39

I .81

0 . t 2

6 .49

0.53

0.06

0.02

9. 13

1.93

0. 15

0.440 .060.03

0.400 . l 00 .02

0.36 0 .43

0.02 0 .05

0.02 0 .03

12.71 12.83 t2.79 12.79

0.65 0 .69 3 .99 4 .02

13.99 14 .06 10 ,46

t .77 1 .89 2 .61

0.35

10.99 10 .88

0.89 0 .91

0.430.480 , 4 70 . u0 . l 40.230 . 2 2

ZnO 0.17 0.24 0.28 0.30 0.21 0.18 3.12Li"o 0.36 0.29 0.01 0.01 0.11 - 0.01 0.01 0.05

H,0 (2.261 2.26 (1.52) 1.52 (1.43) r .43 (1.62) r .62 (1.65)

6.33 *0.48 0.99

0.05 0.98 **0.99

l .6s (1 .80) 1 .80

Sun 99.q7 99.85 100.20 99.51 99.51 100.10 59.82 99.99 100.05 100.32 99.61 99.92

* thls sanple is very lnhomogeneous (Holdaway, pers, cormn., 1992), and this difference ln ZnO contents mayrell reflect thls lnhonogeneity;

* Holdaway (pers. com., l99l).

568 THE CANADIAN MINERAI,OGIST

M(44)

M(48)' tr(u)r t - 1 r r \

0.138 Fe"

0.075 F6&

0.2r4(0.85)

0. r38(0.55)

TABLE 8. CALCUTAT1ON OF VACAflCIESAT T(2) Il.| s(1) FoR UNcouPLm (u)Al'10 C0UPLED (c) lt(4) 0CCUPANCIES

can also accommodate vacancies. [n such cases as this.assignment of site populations has to rely on crystal-chemical argument as well as site-scattering refine-ment. The problem with such arguments is that theyare usually difficult to test. The situation is complelelydifferent where a large amount of crystallographicdata is available. Then the argument is forced to workfor a much larger data set, and one expects to developquantitative crystal-chemical correlations as a functionof mineral composition. Consequently, we will exam-ine each site (or selected pairs of sites) in the staurolitestructure, and attempt to derive consistent site-popula-tions. Ionic radii (r) are taken from Shannon (1976).

Tnn Z(l) Srrr

The site scattering shows that no transition metalsoccur at this site. Details of the stereochemistry of thissite across the range of staurolite crystals are shown inTable 4. The <Z(1)-O> bond-lengths vary betweenL.637 and L.644 A. with a mean value of 1.641 A anda root-mean-square deviation of 0.001 A. atthoughthis is an extremely restricted range of variation, theelectron-microprobe data (Table 6) indicate that thereis significant variation in the t4lAl content of the (!)site, from 0.030 to 0.576 apfu. As t4lsi (r = 0.26 A)and t4lA1 @ = 0.39 A) aimer significantly in size, thereshould be a positive correlation between the I4lAl con-tent derived from the unit formula (= 8 - Si) and the

T(2) s l te-populat ion (apfu)

F6

7n

Mg

AtL I

tr

2 .30

0 .03

0 . 1 6

U . J I

0 .40

0.80

4 .00

value ln paranthoses ls perfonnula unlt

powers together; thus Al* = Al (Z = 13) + Mg (Z =L2), and Fe* = Fe2+ (Z = 26) + Fe3+ (Z = 26) + othertransition metals (commonly Ti, Cr, Co, Mn). Theassignment of species within these groups must relyon crystal-chemical analysis rather tlan site-scatteringrefinement. The situation is firrther complicated by thefact that some of the sites in the staurolite structure

)<

oAF.

1.640

(oal taptu)

Ftc. 6. The variation in <Z(l)-O> as a function of constituent site-populations (expressedas (1)Al) for the firlly analyzed stauolite crystals refined in this study (filled circles)and the silica-rich staurolite crystals S(41) and S(42) (hollow circles); the line tbroughthe data is not a least-squares line, but is drawn as a guide to the eye.

1.685

o.4o.2

THE CRYSTAL CHEMISTRY OF STAUROLTIE 569

6 1 a r 6.r=

a'/

/

t / 4

MS Gpru)

Ftc. 7. The variation in <M(l)-O> as a function of total Mgcontent of stauolite; solid circles represetrt staurolitesamples for which complete analyses (1.e., includingHrO) were done, stars represent staurolite samples with-out H2O determinations, and hollow circles denote S(41)and S(42). The straight line is the regression line for fhesolid circles only; the dashed line represents a hard-sphere model for Al=Mg repLacemenl

<f(1)-O> distance. As indicated in Figure 6, there is asignificant correlation, but because the range of thesubstitution is small, the correlation is not usefirl as apredictive indicator of the Al content of the T(1) site.The observed variations in <Z(1)-O> ard t4lAl contentare compatible with a hard-sphere model, giventhe size (-0.001 A) of the standard deviations of the<Z(l)'tQ> distances, and preclude the possibility ofany Mg occurring arT(l).

TwM(IA) aroM(18) Srrrs

As these two sites are equivalent in the orthorhom-bic shucture, initially they will be dealt with togetherand designated as the M(1) site. There are two princi-pal questions here: first what arc lhe mean site-popu-lations (1e., what is the average compositions of thesesites), and secondly, is there any ordering over the twosites?

M(1) site-populations

Unconsfrained site-scattering refinement (using Aland Fe scattering factors) shows that this site is domi-nated by cations with atomic number close to 13(Table 5). The grand mean site-scattering is 105.04epfu, indicating a small amount of substitution of tran-sition metals at these two sites; this point wifl beexamined in greater detail later on. Presuming thatI6lSi is not generally prcsent in staurolite, this leaves

Al and Mg as principal possible constituents of ttreM(IA) and M(18) sites. There are eight of these sitesin the unit cell, and the composition of staurolite indi-cates that tlte constituent cations must be dominatedby A1. This is confirmed by the mean bondJengths ofthe relevant coordination polyhedra: the grand<M(l)-O> distance is 1.913 A with a root-mean-square deviation of 0.001 A, within the range expectedfor an .{106 octahedron. Of course, this does notnegate the possibility of small amounts of Mg occur-ring at these sites, and tlis possibility is examinednexl.

Figure 7 shows the variation in <M(l)-O> as afunction of Mg apfu for the staurolite crystals exam-ined in this study. There is a well-developed positivelinear correlation, with one very significant outlier athigh Mg values. In the range G-2.0 Mg apfit there isan increase in <M(1)-O>; at low Mg values, theLi-Zn-ich S(42) crystal lies significantly below thegeneral trend. The value at 3.01 Mg aptu [S(41)] liesfar from the observed trend. and as will be shownlater, exhibits completely different behavior withrespect to Mg.

There is an interesting compositional observation tomake here. The grand <M(l)-O> value is 1.913 A;this corresponds to an Mg content of 0.78 Mg apfuaccording to the correlation observed. This should beclose to the mean Mg content of staurolite, and this isin fact the case. Griffen et al. (1982) gave the range ofMg values for 82 samples of staurolite as 0-1.44 apfu;the average value is 0.72 Mg apfu; close to the valueof 0.78 Mg aptu for the data of Table 6.

Thus the stereochemical data do indicate that someMg occurs at the M(74) and M(LB) sites. The nextquestion is oohow much?", as the trdnd of Figure 7 andthe outlier of Mg-rich staurolite S(41) indicate that notall Mg can occur at tfiese sites. We can get a verygood idea of the Mg content of the M(1) sitos from thebehdtior of the mean bond-lensths. The ionic radii oft6lAl (r = 0.535 A) and'rol14g 1i = 0.72 A) are signifi-cantly different and the mean bond-lengths should bea very sensitive indicator of the relative Al-Mg occu-pancy. The steep line drawn in Figure 7 shows theexpected variation in mean bond-length (for a hard-sphere model) if all the Mg in the forrnula unit were tooccupy the M(L) sites; the^ideal slope of this line is(0.72 - 0.535y8 = 0.023 (A per Mg aptu). The slopeof the observed trend is 0.0053, 23Vo of the ideal slopefor complete substitution. This slope suggests thatover this ratge, -23Vo of the Mg in the formula unit isordered at the M(1) sites. As the linearity of Figure 7persists essentially down to zero Mg content, the esti-manonof 23Vo Mg content atM(l) can be taken as anabsolute content. A different substitution obviouslyholds for the Mg-rich staurolite S(41). However, thisinference is not particularly surprising, as this specificsample of staurolite formed under coitditions ofunusually high pressnre.

oS(!rl)

TIIE CANADIAN MINERALOGIST

19oA

5Y

A

t

x 1e1so

'1.9t5

't"9'to

lrgF 136 _-.- gfi

rrgq o.77- 11718€

MgB 0.27 -71-@

€?E

=

9 o *

Flc. 8. The vmiation in <M(IA)-O> (a) and <M(lBFO> O)as a function of p angle in all the staurolite crystalsrefined here. Solid lines shorv variation within singlesamples; solid triangles: S(l-3), sample 71-62R; solidsquares: 5(4-6), sample EH-5; hollow circles: S(1 1-14;,sarnple 1 17189.

Any substitution of Fe3+, Tie and Cr3+ at M(l)could change the estimate of the Mg content, as tlesetransition metal cations also are larger than A1. Thelotal range of variation in Al-Fe occupancy as derivedfrom the site-scattering values of Table 5 is0.000-0.027 Fe3+. A substitution to this extent isexpected to affect tle mean bond-length by 0.02i7 x(0.645-{.535) = 0.003 A. This difference is substantialwith regard to the changes we are observing, but willonly be of significance f tbe substitution is correlatedin some fashion with Mg site-populations. As there isno correlation between the refined site-scattering fromM(1) and the Mg cont€nt, any transition-metal contentof M(I) is not contributing systematically to the varia-tion in mean bond-length shown in Figure 7.

-^.-^-

Fo...*

Ftc. 9. The variation in <M(IA)-O> (full symbols) and<M(1,8)4> (hollow symbols) as a function of p anglefor selected crystals of staurolits with different Mg con-tents.

M(lA)-M(IB) ordertng

Given that small amounts of Mg occur at the M(L)sites, the question now arises as to whettrer or notthere is any preferential ordering of Mg over theM(IA) nd M(18) sites. Again, we must evaluate thispossibility using variations in mean bond-lengthbecause of the insensitivity of X-ray scattering toAl-Mg ordering. However, we must also allow for thefact that variations in the mean bond-lengths of theM(LA) and M(18) octahedra may be induced bychanges in Al - tr order over the M(3A) and M(38)sites. This possibility is exarnined in Figure 8, whichshows the variations in <M(lAFO> and <M(18)-O>as a tunction of B. With increasing p, <M(lA)-O>decreases and <M(LB)-O> increases, such that<M(l)-O> remains approximately constant (except forthe compositional trend of Fig. 7). Fimt, the data indi-cate that at p^ = !Qo, <M(lA)4> x aid(1fi)--Q7 *1.910-1.918 A. Second, the data for the Zn-rich stau-rolite 117189 [crystals S(11), S(12), S(13) and S(14)]span the complete range of data; as these crystals allhave the same bulk composition, the general trend oftle data must be confiolled not by compositional vari-ations but by the degree of order associated with theAl--tl ordering ovet M(3A) nd M(38). The interceptsof the trends for S(1)-S(3) and 5(4)-5(6) (Fig. 8)show the expected range in <M(l)-O> as a function ofchemical composition atM(I). This behavior is furtherillustrated by the data for 71-62R, LL7789 and EH-6in Figure 9. Here it can be seen that the actual diver-gence of the sizes of the M(1A) afi M(IB) octahedraas a function of p occurs for all values of Mg conlenlwith only the <M(l)-O> value affected by the Mgcontent. Examinafiqa of Figure 9 suggests that the rel-ative divergence of the sizes of he M(IA) afi M(LB)octahedra as a function of p increases with increasing

I itf ----\e

. \.\^\ .

9o* '

TIIB CRYSTAL CHEMIS'TRY OF STAUROLITE 571

Mg content of the staurolite. The slope 1in fu"; of tlelines increases as a function of Mg content, from0.009 for 7L-62x* (0.27 Mg apfu) to 0.0155 forLl7l89 (0.77 Mg aptu) to 0.030 for EH-6 (1.65 Mgapfu); because of the scatter in the data as well as therestricted range of p shown by the EH-6 crystals, wedo not consider this point to be well established.Howevero it does suggest a different response ofM(IA) nd M(LB) to increasing p with differing Mgcontent. There are two possibilities here:(i) Mg is always totally disordered over M(lA) andM(lB), and the different response to changing p fordifferent Mg contents reflects the greater susceptibilityof M(IA) and M(IB) octahedra with lower aggregatechwge (i.e., higher Mg content) to inductive effectsfrom the rest ofthe sffuctureo specifically Al-{ order-ing overM(3A) ndM(38).(ii) Mg is disordered over M(lA) and M(IB) for B =90o, and increasing B correlates with Mg-Al orderingover M(IA) and M(lB). In this way, the differentresponse to Al-[] disorder for differing Mg contentsreflects the increased Mg content at M(IB) relative tothat at M(l.A) rvith increased total Mg in the crystal.

At the moment we have no way of distinguishingthese two possibilities because of the low Mg contentat the M(l) site; however, it is definitely establishedthat some Mg occupies the M(IA) or M(IB) sites (orboth).

THnM(2) Srrs

The observed mean bond-lengths (fable 4) and thesite-scatiering refinements (Iable 5) show this site tobe dominated by A1. The question now is whether ornot there is significant occupancy by Mg or transitionmetals such as Fe3+. Tie and Cf+.

Mg (aptu)

Ftc. 11. The variation in <M(2FO> as a function of total Mgcontent of staurolite; legend as in Figure 7.

The site-scattering is slightly in excess of 104 epfu(i.e., 8 x13 e), indicating very minor occupancy ofM(2) by ffansition metals of some sort; note that thesame result was obtained for the M(1) sites. The rela-tive behavior of the refined sile-scattering at M(l) andM(2) is examined in Figure 10. The diagonal line inthis figure indicates semFlete disorder of any transi-tion metals over M(1) afi M(2). Most of the dataoccur to the right of the "disordey'' line, indicating thatany transition metals that occur at these two (sets of)sites are preferentially (but not completely) ordered atthe M(2) site.

The dispersion of <M(2)4> distances in all crys-tals refined (Iable 4) is extremely limited. The grand<M(z)-O> distance is 1.906 A, with a root-mean-square deviation of 0.001 {, and the individual meanbond-lengths vary 0.002 A across the total range ofcomposition. There is no correlation between<IIQrc> and the M(2) site-scattering or the p angle.Figure ll shows the variation n<M(2)4> as a func-tion of total Mg content of the crystal. There is a weaklinear correlation, again with staurolite S(41) as anoutlier. The slope of the correlation is 0.0013, as com-pared with the ideal slope of 0.023 for complete sub-stitution of Mg for Al. This indicates that 6Vo of. theMg substitules at M(2), and the site populations wereso assigned.

Thus the variations in refined site-scattering andmean bond-lengths are compatible with only veryminor incorporation of components other than AI atrhe M(2) site.

TwM(34) ANDM(38) SnEs

The Al - n ordering over the M(3A) and M(38)sites is a feature of major importance in the sdurolite

3 tozEqt

E 1oooo

I rosdoo

3 roao

o s(+r) .,;' oa./' c' .

. ,";{';;:t,,"o o a' o l o o

104 106 106 107 108 109slts ecattorlng (ontu) at tu(e)

Frc. 10. Comparison of the refined scattering at the M(1) andM(2) sites in staurolite crystals. The dashed line corre-sponds to identical scattering from M(1) and M(2), hol-low circles are S(41) and S(a2).

572 TI{E CANADIAN MINERALOGIST

structure. With regard to site populations, there arethree principal problems:(r) is the total cation content of the M(3) sites fixedor variable?(ii) is there significant substitution of other speciesat M(3) (e.g.,Mg,FE*, etc.l in addition to A1 and n?(iii) if there are other cations at M(3) n addition toAl, do they show different patterns of order overM(3A) andM(3B)?

M( 3 ) site-populations : cartons versw yacancies

The site-scattering values of +he M(3A) nd M(38)sites are given in Table 5. The grand mean site-scatter-ine of M(34) and M(38) for the 42 crystals refinedherc is 25.74 epfu (=Al1.es + trz.oz pfu) with a root-mean-square deviation of 0.76. Thus there is a smallbut significant variation in the aggregate scatteringftom the M(3) sites, with a total range of 23.58-27.06epfu. Much of this variation is due to the H-rich stau-rolite crystals, which show low aggregate M(3) scat-tering (mean value = 23.78 epfu). Omitting these crys-tals [S(1-3), S(42)], the mean M(3) scattering for theremaining crystals is 25.94 epfu; nevertheless, there issti l l a significant range in M(3) scattering(25.124.06 epfu).

We need to consider three possible types of substi-tution at these sites: Fe* :* A1, Mg = Al and Li = Al.Only the first and third types of substitution signifi-cantly affect the scattering power at the M(3) sites, andso we will consider these substitutions first. The Fe3+-- Al substitution will increase the effective scatteringas Fe (26) scatters X rays more strongly than A1* (13);the second substitution will decrease the effectivescatiering as Li (3) scatters X rays less strongly thanAl* (13). The basic situation may thus be summarizedas follows [freating M(3A) nd M(38) as summed forttre moment]: there are four scattering species, Al*(= Al + Mg), Fe* (= Fe, Ti, Cr, Zn, Co), Li and u to beassigned fo one site. There is no unique solution to thisproblem if gnly the site-scattering values are con-sidered.

The first question to resolve is whether the Al* eon-lent of the M(3) sites is always exactly 2.0 apfu, withthe range of refined scatlering resulting from substitu-tion of other species (i.e.,Li, Fe) for Al at these sites.In tlis case, site-scattering less than 26 epfu couldonly be caused by substitution of atoms with lowerscattering power than Al (13); the only possible cationof significance here is Li, which Dutrow et al. (1986)have shown to be a significant component in somestaurolite samples. The H-rich staurolite always seemsto have significant Li conlento sufficient to account forthe low refined scattering at their M(3) site(s). This isa tempting hypotlesis, as it would account for thepresence of significant U in all the H-rich staurolitesamples analyzed (77 -55C, 7 l-608, 71-62R fromHoldaway et al. L986b; S(41) from Chopin, pers.

n,//

os{41)

Ftc. 12. The variation in <M(3lO> as a function of total Mgcontent of staurolite; legend as in Figure 7.

comm.). However, if such a substitution occurs, itshould significantly affect tle <M(3)-O> distance, ast6lAI (r jo.srs Aiis a lot smaller than I6\:i (r = 036A). es an example, for crystal S(1), the Li content ofM(3) would need to be 0.23 apfu; this would increasethe <M(3)-O> distance by -0.008 A (when taking intoaccount changes in the proportion of vacancies).Inspection of Table 4 shows that <M(3)-O> for S(1)(and all the other crystals of H-rich staurolite) is in thesame range as all the other staurolite crystals withM(3) scattering close to 26 epfu. This indicates thatthere is insignificant Li at M(3) in the H-rich stauro-lite. Thus the low refined scattering from M(3) in theH-rich staurolite crystals must be caused by additionalvacancies atM(3).

Now we need to consider the possibility of signifi-cant Fe* subntitution ,at M(3). The maximum refinedscattering at M(3) is for crystal S(5); assuming exacthalf-occupanpy of M(3), the refined scattering corre-sponds to n M(3) content of [0.08 Fe* + 1.92 Al(+Mg) + 2.00 nl. If Fe* is Fe3+ (or Cf* or Tial, tlre sitepopulation would lead to an increase of 0.005 A overzero Fe* occupancy; ifFel is Fe2*, the correspondingincrease would be 0.006 A. Both of these are reason-ably consistent with the observed <M(3)4> distancein crystal S(5). However, as we see in the next section,the variation in <M(3)-O> distance correlates wellwith the bulk Mg content of the crystal, indicating thatthe small variations observed in <M(3)-O> are dueprimarily to variable Mg content (rather than theincorporation of Fe2+). Thus if there is significant Fe*at M(3), it must be Fe3+ (or Cf+ or Tie) rather thanFe2+ or Mnz+. However, at the moment we have nobasis for assigning such populations, as we must con-clude from tle arguments given above that there is

Io

Ms (aPtu)

variable total occupancy of cations at the M(3) siteacross the range of stawolite compositions.

Ms at M(3)

The last point concerning M(3) is whether or notsignificant amounts of Mg occur at these sites. Figure12 shows the variation in <M(3)-O> as a function ofbulk Mg content of the crystal. Omitting the H-richstaurolite crystals for the reasons mentioned above,there is a fairly good linear correlation that can beinterpreted as being due to small amounts of Mgoccurring ar M(3). From the slope of the correlationand the relative sizes of A1 and Mg, the Mg content ofM(3) is calculated as -IUVo of the total Mg aptu; Mgpopulations were assigned accordingly.

T\rE M(44) llo M(48) Srrrs

These sites are partially occupied, and are octahe-drally coordinated; the octahedra each share one set ofopposing faces with adjacent (2) tetrahedra. It"isnotable that the M(44)aQ) and M(48)-(T2) distancesare both -1.65 A, and consequently there must besome sort of local ordering of cations and vacancies,as these distances arc too short for simultaneous occu-pancy of adjacent M(4) and Z(2) sites.

The refined scattering from the M(40 and M(aB)sites is given in Table 5; values range from 0 [com-pletely vacant S(15), 5(16)l to L2.4 epfa [S(41), theMg-rich staurolite of Chopin, pers. comm.l. However,the chemical identity of the cations occupying theM(4) sites is rather a difficult problem, primarilybecause of the very low scattering at these sites. Thebest chance of getting some idea of the type of cationsat M(4) is to consider those samples with high M(4)scattering tS(1-3) and 5(41)1. At first, there seems noobvious correlation with bulk composition; samples71.42k [crystals S(l-3)] and S(41) show few (if any)common compositional characteristics (Table 6).However, one can get some clue by comparing7l-62R, in which there is alarge M(4) scattering, with6-3, in which the M(4) scanering is zero. In a solidsolution, the size of the polyhedron coordinating thecation(s) changes in response to the different radii ofthe constituent cations: this observation is the case inboth cation;=cation substitutions and in tr;=cationsubstitutions, provided we realize that the radius of thevacancy is a function of the structure in which itoccurs. We may regard the M(4) sites as participatingin a cationi=D substitution; in the present case, stau-rolite 6-3 is the vacancy end-membero and staurolite7142Ii is an intermediate member with partial occu-pancy of the M(4) sites. However, both of these haveidentical <M(4)4> distances of 2.162 A. This indi-cates that we are substituting a cation at the M(4) sitethat is the same size as the vacancy at the M(4) site,and thus we can calculate the (mean) radius of the sub-

TIIE CRYSTAL CHEMIS'TRY OF STAUROLITE

Here it isM(44) s'te's

stituent cation(s) at M(4) by subtracting the radius ofoxygen from the o-bserved mean bond-length. Takingr(oxygen) = 1.38 A, r"n= 2.16-1.38 = 0.78 A, whichis the ionic radius of i6l-coordinate Fe2+. This indi-cates that Fe2+ occupies the M(4) sites in staurolite7L42R'.

This rather convoluted reasoning also is supportedby stoichiometric arguments involving the M(4) site-populations. T\e M(4) scattering in staurolite 7142Ris 10.8 eptu, which is equivalent to 0.90 Mg or 0.42 Feapfu. As discussed later, each occupied M(4) site isaccompanied by 2 n at adjaceqt Z(2) sites. If the M(4)site-populations were 0.90 Mg apfu, this would meanat least 1.80 n pfu and a maximum of 2.2 apfu of anykind at the T(2) site. This is not consistent with theobserved scattering atthe T(2) site, which indicates aminimurn content of -0.68 x 4 = 2.72 Fe apfu; thusMg does not ocdur at the M(4) sites in staurolite7142F, Also, staurolite 71,42k only has 0.284 Mgapfu [of which we have already assigned ha]f to theM(L,2,3) sitesl, cenfirming our stereochemical argu-ments. with 0.42F&+ aptux M(4), this indicates 0.86tr at the T(2) site;a value that is compatible with theobserved scattering. As we will see later, M(4A) andM(48) are usually occupied by Fe2+. However, in spe-cific cases of unusual composition or conditions offormation [e.g., high-pressure Mg-rich staurolite5(41)1, Mg may al$o occupy M(4A) and M(48}

I-ocal, CenoN - tr ORDERtrIcx T(2) aro M(4)

A crucial part of assigning site populations at the7(2) site is the asspssment of the vacancy content, andconsequently this must be considered before the I(2)site-populations cfn be reliably assigned. The M(4A)and M(48) octahe{ra share faces with the Z(2) tefrahe-dron. and the nding site-separations are

573

that occupancy of a specificto the occupancy of the adja-

eStremely short: VI(4A)-T(2) * M(48)-T(2) * 1.65A. Consequently, $ites that are locally adjacent acrossa shared poly face cannot be simultaneously

even if the effects of local relax-occupied byation associated \dith Z(2) positional disorder are con-

(short-range) ordering of cationsand vacancies mubt occur. Such models oflocal orderare easy rc but usuallv difficult to confirmexperimentally. , in the present case, some ofthe more compositions provide a sufficienttest of these

Incally M ( 4A) and M ( 48 ) o ccupancy

cent M(48) sites .e., fhete is complete local disorderof the M(4) l. Each occupied M(4A) siteforces vacancies the two adjacent Z(2) sites; similar-

M(48) site forces vacancies at thely, each

(a) (b)

574 THE CANADIAN MINERALOGIST

M(44)

T(e'M(48)Tf.2].

M(4A)

UNCOUPLED COUPLED

Ftc. 13. Models for uncoupled and coupled occupancy of theM(4) and (2) polyhedra: (a) uncoupled occupancy ofM(A$ nd M(48); occupancy of an M(4A) site is nor cou-pled to the occupancy of an adjacent M(48) site; (b) cou-pled occupancy of M(4A) afi M(48): occupancy of anM(4A) site is coupled to occupancy of an adjacent M(48)site; a totally ordered situation is shown here, althoughdisordered configurations [with lower aggegate M(4)occupanciesl are usually the case.

two adjacent (2) sites Gig. 13a). The total number ofvacancies atthe TQ) site is thus 12 M(4A) + 2 M(48)lptu.

Incally coupled M(4A) and. M(48) occupancy

Here it is assumed that occupancy of a specificM(44) site is coupled to ttre occupancy of the adjacentM(48) sites (i.e., there is maximum local order). Thisresults in an edge-sharing chain of occupied M(4A)and M(48) octahedra (Fig. 13b). Within this chain,each occupied M(4A) site forces vacancies at the twoadjacent (2) sites, as before. However, the associated

occupied M(48) sites do not have a similar effect, astheir locally associated Z(2) sites are already vacant.As the occupancy of M(4A) is always greater than theoccupancy of M(48), the total number of I(2) sitevacancies is 2 pW(44)) ptu.

Testing the models

These two models can be regarded as extremummodels representing total M(4A)-M(48) local disorderand total M(44)*(48) local order, respectively, withassociated populations of vacancies at the Z(2) site of2 [M(44) + M(48)] and 2 lM(4A)1, respectively. Forthose crystals of staurolte in which the (cation) sitepopulations of the M(4) sites are considerable, thesetwo models have significantly different compositionalimplications, differences that may be sufficient to dis-tinguish between the two "end-member" models,There are two ways in which we may test thesemodels:(i) to consider the effect of the resulting M(4) site-populations on the vacancy populations of T(2);(ii) to consider the effects of M(4A)-M(48) order-ineonT(2) site-populations with varying p angle.

M(4) occupancy and T(2) vacancies; consider firststaurolite 71.42R [crystals S(1), S(2) and S(3)]. Thissample has a large M(4) site-scattering of -10.7 elec-trons that we have previously shown to be due to Fe2+.The details of the vacancy calculation are shown inTable 8. The amounts of T(2) vacancies from theuncoupled and coupled models are 0.85 and 0.55 npfu, respectively. Comparison with the site-popula-tions assigned to the T(2) site shows the uncoupledmodel to agree with the assigned site-populations.Thus the arguments of this section indicate that theuncoupled model for M(4) site occupancy gives thecorrect proportion of vacancies at the T(2) site. Formost of the other samFles, the M(4) site-populationsare not suffrciently large for the differences in the cal-culated vacancies to be significant.

M(44)-M(48) ordering and T(2) occupancyi nowwe consider the effect of varying M(4A)-M(48) orderon the site population of Z(2) with varying B for crys-tals of the same composition. By far the most impor-tant samples in this regard are staurolite 117189 and7l-62F., both of which show significant ranges of Bfor the different crystals examined. The key argumenthere is this: rf the M(4) occupancies are locally uncou-pled, the I(2) vacancies are 2 W@A) + M(48)l ptu,which is independent of the degree ot M(4A)-M(48)order; ifthe M(4) occupaacies are locally coupled, the(2) vacancies are 2 IM(aQl pfu, and thus will varywith the degree of M(4A)-M(48) order as a functionof p. These models are examined graphically for stau-rolite 117189 [crystals S(11-14)] and 71-62R [crys-tals S(1-3)l in Figure L4.T\e lowest B value for eachsample is taken as the "starting value", and thechanges n fQ) populations are examined as a func-

Tc

L

TI{E CRYSTAL CHEMISTRY OF STAUROLITE 575

As; 2g

9 ro

$o{ - r

d o & & & t d oT(2) eleatrom trom sfucttrs r€frnement

Frc. 15. The total number of elecuons at fte T(2) site fromthe site-scattering refinement and from the availablecations as indicated by the results of the electron- andion-microprobe analyses; only selected samples areshown for clarity. Symbols denote electrons availableftom specific combinations of cations; hollow circles:[Fe]; triangles: [Fe + Zn]; hollow squares: Be + Zn + W+(Mn + Ti + Cr + Co)l; horizontal line: Be + Zn + IW+ +Lil; full circles: [Fe + Zn + IW+ + Li + Mg]. Where spe-cific symbols are missing, they overlap symbols alreadypresent [i.e., the additional cation contributes few elec-trons to the possible Z(2) scatteringl. The diagonal lineindicates 1:1 correspondence between the number ofelec-tons from the structue refinement and the electrons fromthe fonnula unit, Hence one can read from this diagramwhich cations nwt occw atT(z).

(b)

71-62R tr

a

- - - ' - 8 " '

' " - . . j --_ _------o

tion of B. For the uncoupled model, the site-populationof I(2) should stay unchanged; for the coupled model,it should increase with increasing B as the degree ofM(4A)-M(48) order becomes more pronounced. Forstaurolite 1,77L89, the observed (2) scattering values(in epfu) favor the uncoupled model; the values forstaurolite 7L-62R are perhaps intermediate betweenthe two models, but are still closer to the uncoupledmodel.

Thus the arguments of the last two paragraphssupport the locally uncoupled model. As will beshown later. the reason for the Fez* disorder betweenT(2) and M(4) is connected with the fact thatoccupancy of the M(4A) nd M(48) sites is locallyuncoupled.

(2) Srrs

The results of the site-scattering refinements aregiven in Table 5. There is no unique way of assigningspecific chemical site-populations to (2) simFly fromthe diffraction results, and consequently we must use acombination of the diffraction results" electron- andion-microprobe results, and stereochemical evidence.

S o me qualit ativ e c ons i de rations

As a first step, we need to compare the numbers ofelectrons known to be present atthe T(2) site from therefinement results with the numbers of electrons avail-able from each different cation fusually considered asoccupying the T(2) sitel, as indicated from the elec-tron- and ion-microprobe results. This is done inFigure 15; not all crystals are shown or the detailwould be obscured, but a range of chemical types isrepresented. The obvious conclusions from Figure 15

(a)

1 17189

" e " " ' t r

' O - - - - - - - - u

. t ' a a

" . . - t r . - - - " ' - - " - - - - - - - - - -o

Is.oI

90.1I

90.2I

90.1

. o - f i c - +

Frc. 14. Variation n fQ) site-scat0ering (expressed in electrons pfu) as a function of p angle for staurolite samples (a) 117189,and O) 71-62R; the observed data (full circles) and the ideal variation for both coupled (hollow squares) and uncoupledQrollow circles) occupancy of M(4A) ellrdM(48) are shown.

l l90.3 90.4

I

90"2

80

5

6

c

o

oooooo|F

?

:io ootuJ

dN?l:?

5

tz$ ::o

tIo

a

:ooA

o.oN

6

576 TIIE CANADIAN MINERALOGIST

TABLE 9. SITE.POPULATIONS IN STAUROLITE CRYSTALS

s(r ) s(2) s(3) s(4) s(5) s(6) s(7) s(8) s(e) s(10) s(12) s(13) s(rs) s(16) s (17) s (18)

l r ( r ) Al 7.81Hg 0.07Fe* 0.12

r,f (2) Ar 7.79I'lg 0.02Fe* 0. 19

M(3 ) A l 7 .77Hg 0.03Fe* 0.0I

l l 2 . re

l l (4) Fe* 0.43

l l 3 . 57

T ( l ) s i 7 . 65Al 0.35

T(2) Fe* 2,37Zn 0 .03l4g 0. 16Lt 0.40A l 0 .19

[ ] 0.85sum T(2) 4.00

e* 68.1

eot 71.3

7.85 7.84 7.430 .07 0 .06 0 .390 .08 0 .10 0 .18

7 .84 7 .89 7 .680 .02 0 .02 0 . l 10 . 14 0 .09 0 .21

1 .65 1 ,67 1 .800 .03 0 .03 0 .200 .07 0 .09 0 .032 . 2 4 2 . 2 t 7 . 9 7

0 .40 0 .41 0 . 123 .50 3 .59 3 .88

7 .68 7 .69 7 ,730 .32 0 .3 r 0 .27

2 .35 2 .39 2 .550 .04 0 .04 0 .060 . 1 5 0 . 1 5 0 , 9 20 .40 0 .40 0 .024 .26 0 .20 0 .200.80 9,84 0.244.00 4.00 1.00

68.7 69.0 82.1

70.5 71.9 85.4

7 .47 7 .49 7 .700 .45 0 .40 0 . i 90 .06 0 .u 0 . i l

7 . 75 7 .74 7 .810 . l s 0 . 1 1 0 . 0 50 . 1 0 0 , 1 5 0 . 1 4

1 . 8 8 t . 7 9 1 . 9 70 .20 0 ,20 0 .090 .01 0 .04 0 .00t . 9 l I . 97 1 .94

0 . l l 0 . u 0 , 0 93 .89 3 .49 3 .91

7 .67 7 .71 7 .670 .33 0 .29 0 .33

2 . 7 8 2 . 6 9 3 . t 50 .06 0 .05 0 .040 .88 0 .94 0 .450 .01 0 .02 0 .090.01 0.08 0.090 .22 0 .22 0 .184 .00 4 ,00 4 .00

85.3 83,8 90.0

84.9 86.0 92.3

7 .7E 7 .73 7 .730 .17 0 .27 0 .260 . 1 0 0 . 0 0 0 , 0 1

7.85 7.77 7 ,8 ' l0 . 05 0 .07 0 .070 . r l 0 . 1 6 0 . 0 6

1 .97 1 .87 1 .900 .09 0 . 13 0 .130 .04 0 .01 0 .001 . 9 0 1 . 9 9 t . 9 7

0 . 1 0 0 . 1 3 0 . 1 43 .90 3 .87 3 .85

7 .64 7 .48 7 .440 .36 0 .52 0 .56

3 .?0 ? ,28 2 ,420 .04 0 .64 0 .650 .40 0 .64 0 .610 . 1 6 0 . 0 1 0 . 0 10 .00 0 . l 7 0 .040 .20 0 .26 0 .274 .00 4 ,00 4 .00

89.7 88.4 90.3

91 ,3 89 .8 90 .7

7 ,69 7 ,57 7 ,790 .25 0 .26 0 .090 .05 0 .07 0 . 12

7 .7& 7 .75 7 .800 , 1 2 0 . 1 6 0 . 0 20 . 1 0 0 . 0 9 0 . 1 8

1.89 l .88 1.870 .07 0 .08 0 .040 .00 0 .02 0 .032 .04 2 .02 2 ,06

0 .10 0 .09 0 .003 .90 3 .91 4 .00

7 .68 7 .70 7 .780 .32 0 ,30 0 .22

2 . t 2 2 .07 2 .42r . 33 1 .40 0 . l l0 . 31 0 .29 0 .2 t0 .06 0 .06 1 .020 .00 0 .00 0 .240 .21 0 .18 0 .004.00 4.00 4.00

98 .5 99 .4 75 .0

102.6 r03.9 78.1

7.70 7 .540.09 0.340 . 2 1 0 . 1 2

7.76 7 .680.02 0 .090.22 0 .23

1.98 1 .800.04 0 . t70.00 0.031.98 2 ,00

0.00 0. 194.00 3 .81

7.78 7 .660.22 0 .34

2.32 2 .300.09 0.030.22 0 .82t . 15 0 .034,22 0.440,00 0.384.00 4 .00

72.0 76.4

72,6 77 _O

7 .710 . t 70 , 1 2

7 .850 .050 . t 0

I O ?

0 .08

I . 99

0 . 1 43.86

7 .420 .58

3 .180 .030 .400 .090 .020 .284 .00

89;0

90. I

Fe*' Fe?' + l4n + Co + Cr + Tl + Fe3';eA - nunber of electrons at T(2) from assigned site-populatlons;e"' - number of electrons at t(z) from sltelscatterinb i.eflnemeni

are as one would expect. In most cases (i.e., Fe2+- andZn-ich staurolite crystals), the majority of Fe and Znmust be assigned to the T(2) site to account for theobserved scattering. However, it is also apparent thatif all the Fe and Zn is so assigned, there is still notenough scattering power assigned to T(2). As therefinement results also indicate small amounts oftran$ition metals at octahedrally coordinated sites,it is necessary that other cations (e.g., Mg Al, Ti,Mn, Li, but not necessarily all of them) oca;r at T(2).The Mg-rich crystal S(41) also is of interest. It isapparent that the amount of Mg available greatlyexceeds the amount of scattering at T(2), and thusmuch Mg must occur at otier sites in the structure;this is in line with our previous assignment of largeamounts of Mg to the M(4A) and M(48) sites inthis crystal. It also is apparent for S(41) that theuncoupled model for M(4A) and M(48) occupancy isnot compatible with either the observed stoichiometryor the refined scatiering atthe T(2) site. Obviously thevery high pressure of formation for this particularsample has strongly affected this aspect of its short-fimge order.

The position of Li in staurolite

The results of the site-scattering refinements andtle mean bond-lengths sholv that significant amountsof Li do not occur at the M(1,2,3) sites. Thus for acation sum of 30 apfu, there are two "end-membey''possibilities for the distribution of Li: (i) Li occurs atT(2), (ii) Li occurs at M(4A) and M(48) in a coupledarTangemenl

Site-scattering refinement of orystals S(15) and5(16) show essentially no electron density at theM(4A) and M(48) sites. One atom of Li distributedover M(4A) and M(48) would result in an aggregateM(4) scatteing of three electrons, similar to thatobserved in the Li-poor staurolite crystals; no suchelecffon density is observed in S(15) and 5(16). ThusLi must occur at the T(2) site, and the scattering fromthis site in Li-rich staurolite crystals is in agreementwith this.

Vacancies at T(2)

There are four (2) sites pfu. However, it is com-

TIM CRYSTAL CHEIVtrSTRY OF STAUROLITE 577

TABLE 9. CONTINUM.

s(20) s(22) s(23) s(24) s(25) s(26) s(27) s(30) s(33) s(35) s(36) s(3e) s(40) s(41) s(42)

M(l ) Ar 7 .70ttg 0.23Fe* 0.07

l{(2) Al 7.e4|,lg 0.06Fe* 0.00

r,r(3) Al 1.8iMg 0. 12Fe*[ ] 2 , o l

14(4) Fe* 0.17i l 3.83

T ( 1 ) S i 7 . 5 rA1 0.49

T(2) Fe* 3.00Zn 0.06|'1g 0.55L i 0 .08At 0.02n 0 .34

Surn T(2) 4.03

e*' 86.9

eo ' 83 .2

7 ,76 7.700.24 0 .250.00 0 .05

7.82 7 .730.08 0 .050. t0 0 .22

1.89 1 .900. l l 0 . l0

2 .00 2 .00

0. 14 0 . 13J . 6 0 J . O '

7.57 7 .490.43 0 .52

3.03 2 .930. t l 0 .080.57 0 ,420 . 1 0 0 . 1 00.00 0 ,220.28 0 .264.09 4 ,01

89.5 86 .8

90.2 88.2

7 .73 7 .730.20 0 .200.07 0 .07

7 . 8 4 7 . 7 70.06 0 .050 . l 0 0 . 1 8

1 . 9 0 1 . 9 40 . 1 0 0 . 1 0

2.00 1 .96

0. 14 0 .203.86 3 .80

7.48 7 .670.52 0 .33

3.03 2 .820.08 0 .200.49 0 .470.09 0 .120.04 0 .020.28 0 .404.01 4 .03

87.9 85 .6

87.7 85.5

7 . 66 7 . 74 7 , 76

0 . l 8 0 .23 0 .23

0 .16 0 .03 0 .01

7 .65 7 .85 7 .83

0 . 0 5 0 . I I 0 . 1 1

0 .30 0 .04 0 .06

I . 84 1 .88 1 .900 .09 0 . 12 0 . l 0

2 .07 2 .00 2 .00

0 . 2 4 0 . 1 2 0 . 1 4

3 .76 3 .8S 3 .86

7 .62 7 .53 7 .520 .38 0 .47 0 .48

2 .47 3 .39 3 .36

0 .22 0 .03 0 .03

0 .43 0 ,24 0 .260 . 1 2 0 . 1 1 0 . u0 .42 0 .01 0 .000 .48 0 ,24 0 .28

4 . 16 4 .01 4 .04

82.1 92.5 91.7

84 .3 91 .4 90 .7

7 .70 7 .750. l8 0 . t80 . 1 2 0 . 0 7

7 . 7 t 7 . 7 50.05 0 .050,24 0 .20

1 0 l I 0 1

0 .09 0 .09

2 .00 2 .00

0 . 1 9 0 . 2 53 .81 3 .7s

7 .58 7 .580 .42 0 ,42

2 .85 2 .590 .09 0 .10

0 .41 0 .42

0 . 1 1 0 . 1 30 . 17 0 .280 .38 0 .504 .01 4 .02

84 .3 79 .5

85.9 79.8

7 .78 ' t . 79

0 . 16 0 . 170 ,06 0 .04

7 .77 7 .850 .04 0 .050 . 19 0 . r 0

1 . 6 4 l . 9 l0 .08 0 .090 . t 22 , 1 6 2 . 0 0

0 . 1 5 0 . 1 93 .85 3 .81

7 .58 1 .610 .42 0 .39

2 .75 2 ,480 .04 0 .540 .38 0 .420 .00 0 .160 .52 0 .030 .30 0 .384 .00 4 .01

84.3 86.5

88.1 87.1

7.64 7 .78 7 .950 . 1 7 0 . 1 40. 19 0 .08 0 .05

7.69 7 .84 7 .890.05 0 .050 . 2 6 0 . 1 1 0 . l l

1 .84 I .98 1 .850.08 0 .020.042.04 2 .00 2 . t5

0 . l4 t .04 ' 0 . 193.86 2 .96 3 .81

7.74 7 .96 7 .970.26 0 .04 0 .03

2,730.04 0 .02 2 .450 . 4 0 1 . 7 6 0 . 1 90.21 0 .94 0 .510.38 - 0 . t70 .28 1 .28 0 .384.04 4 .00 3 .70

82.5 24.7 79,5

87,7 25.? 81.1

Fe* - Fe?'+ l{n + Co + Cr + 1l + Fe3';e*l - nunber of Electrons at T(2) from asslgned slte-populatlons;e"t . nunber of electrons at T(2) fron s'lte-scattering refinenent' M q

monly observed in unit formulae that the number ofcations conventionally assigned to this site is signifi-cantly less than 4 apfu. In some cases, this will be dueto the presence of undetermined Li. However, in gen-eral it indicates the presence of vacancies, and \ileknow from previous arguments (Smith 1968) thatoccupancy of. the M(4A) and M(48) sites must beaccompanied by vacancies at the locally associated(2) sites.

Above we have given fwo ooend-member" modelsfor the occupancy-vacancy association of M(4A),M(48) ndT(2),the uncoupled model and the coupledmodel. Calculated vacancies are given Table 9, assum-ing the uncoupled model. Of course, these vacanciesdepend on the correct assigpmsal of cations to M(4A)and M(48) to account for the observed scattering;however, if there are major errors in any of the assign-ments, we should end up with an inconsistency in ourscattering, chemical and stereochemical models.At this stage in the process of site-populationassignments, it is not clear which occupancy-vacancymodel is appropriate, and so we will retain both in ourarguments.

Vac ancie s from stoichiomztry

The results of the site-scattering refinements andthe electron-microprobe analyses indicate the follow-ing sites to be tully occupied: T(1), M(lA), M(l'B) ndM(2), givfig a total of 24 cations at these sites. TheM(3A) and M(38) sites have a combined rank of 4,and the 7(2) site has a rank of 4. The resultant sum ofsites corresponding to the formula unit with 48 [O,OH, Fl is 32. This situation is complicated by the factthat the M(3A), M(38) and I(2) sites are not fullyoccupied by cations, together with the occrrrence ofthe M(4A) and M(48) sites that also show only partialoccupancy. As the M(3) site is on aggregate only(approximately) half occupied, and the vacancies atthe T(2) site exceed the cations at M(4), the number ofcations in the formula unit (CATSUM) generallyranges between 29.3 and 30.0 apfu; values for thestaurolite crystals examined here are given in Table 6.These analytical values give us a check on our crystal-chemical assignment of vacancies; obviously we placemore reliance on the compositions for which there arehydrogen determinations available.

578 T}IE CANADIAN MINERALOGIST

TAELE 10. TMNSITION-METAL SITE.PREFERENCEIN STAUROLlTE

Fe"'Fe"'Mn'Zn

Ti*

T(2) >> r '1(4)M( l ) , r '1 (3 ) > M(?) > T(3)r (2 )T(2)T(2)M(2) >> t ' t(1), i t(3) ?

Assignment of cations

At this stage, we will assign all the remaining unas-signed cations to the Z(2) site and see how well theyagree with the expected stoichiometry and refinedscattering results; this is done in Table 9. The amountof Al at the (1), M(LA), M(IB), M(2), M(3A) andM(34) sites was calculated from the refined site-scar-tering (Table 5), and Mg was assigned to the M sitesas discussed above; the Al assigned to these sites wassubtacted from the total Al calculated from the micro-probe analyses, and the remainder was assigned to theI(2) site. A similar procedure was followed with theother cations. To simplify tlis process, all transitionmetals (Fe2+, Fe3*, Mn, Coo Cr, Ti) are expressed asFe*; once we have assigned Fe' site-populations, wecan examine the behavior of the different minor transi-tion-metal species. Above, we have shown that Li alsooccurs at the (2) site in Li-rich $taurolir€ crystals; thiswas so assigned for all crystals. This procedure pro-duced slight negative occupancies for Al in three ofthe crystals. These uegative amounts were within theuncertainty of the assignment procedure, and the Mgcontents of the M(1,2,3) sites were adjusted slightly toremove tlese negative values; t}te amounts involveddid not significantly affect the relationships shown inFigures 7,1,1 and, L2.

Summing the assigned contents of the (2) site forall of the refined crystals gave cation (+ vacancy)sums between 3.88 and 4.00 apfu, compared to tleideal value of 4.00 apfu. We may ascribe these dis-crepancies to the presence of Fe" atoms at the M(3)site, which we have not yet been able to assign, lack-ing any basis on which to do so. At this stage of theprocedure, this situation has now changed. The devia-tions ofthe assigned I(2) cations (+ vacancy) from anideal value of 4.0 are due to an excess of Fe* and adeficiency of Al at Z(2); this may be corrected byassigning to the M(3) site an amount of Fe* equal tothe difference between the T(2) su'n and 4.0, and re-assigning twice that amount of Al to T(2).In this wayothe scatiering at each site is kept unchanged, whereasthe scattering species at the T(2) site (including theassigned vacancy) now sum to tle ideal value of 4.0apfu; thus we have finally been able to assign the Feu

content of the M(3) site. The site-populations of Table10 were assigned in this way. We note that for crystalsS(17) to S(42), for which we assumed 3 H ptu, devia-tion from the assumption will adversely affect the (2)site assignment, especially in the estimation of the A1content. This may be the origin of the high r(2)Al con-tents in a small number of crystals. Despite the verycircuitous process of assignment for the Z(2) site-pop-ulations, the number of electrons arthe T(2) site calcu-lated from the assigned populations corresponds rea-sonably well to the refined site-scattering values[R.M.S. deviation of 2.4 epfu with a mean value of-84 epfu over the whole data setl.

Srrs AssrcNMmr.lTs ronTRANSnToN-ME"IAL Carrons

We have so far discussed the site populations of thetransition metals in terms of the ioint scatterinespecies Fe* = pg2+ + Fe3+ + Mn + Co + Cr + Ti. Winow need to assign specific chemical species to thevarious sites in the structure; we emphasize that thisprocedure is dependent on a wide variety of evidencein addition to the site-scattering refinement results dis-cussed above.

F&+

Fe is dominantly in the divalent state in staurolite,and is also strongly ordered atthe TQ) site; hence Fe2+occurs at T(2).Fe also occurs atthe M(4A) and M(48)sites; both these sites are octahedrally coordinated, andas extensively discussed above, the observed meanbond-lengths indicate Fe2+ to occur at tlese sites. Thecombined site-scattering and observed mean bond-lengths at the M(lA), M(LB), M(3A) and M(38) sitesare incompatible with any significant Fe2+ occupancyof these sites. Thus Fe2* occurs at r}re T(2), M(4A) andM(48) sites Clable l0).

Fd+

There have been a variety of proposals in the litera-fure as to the location (and even existence) of Fe3+ inthe staurolite structure: as the amounts of Fe3+ are verysmall, (i) it is very dfficult to be sure (as distinct fromguess) about the location of Fe3+, and (ii) we are notconvinced by previous analytical and spectroscopicevidence that staurolite generally contains significantFe3+. Caucia et al. (1994) have shown that combinedoxidation - dehydroxylation occurs in staurolite (uponheating) vla the process Fe2+ r OH -+ Fe3+ + 02- +HT. the resulting Fe3* primarily occupies a new tefra-hedrally coordinated site, the (3) site (Table 1). Thusif small amounts of Fe3+ do occur in staurolite, theymay either represent the original composition of thesample or the resul* of small amounts of postcrystal-lization oxidation-dehydroxylation.

T}IE CRYSTAL CHEI\{ISTRY OF STAUROLITE 579

As the Z(2) site accommodates small amounts ofAl, it seems a reasonable argument that the Z(2) sltecould also accommodate small amount of Fer+.Howevero there is no direct evidence for this possibi-lity, as Fe3+ can also occur at the tetrahedrally coor-dinated (3) site, and spectroscopic techniques (asdistinct from scattering techniques) lack the spatialinformation to resolve this problem.

The total scatiering due to transition metals at theM(l.A), M(LB), M(2), M(34) and M(38) sites isalways in excess of the amount of (Ti+Cr), whichalso occur at these sites. Hence there must besmall amounts (-0.1-{.2 apfu) of Fe3+ at these sites. IfTi4+ occurs primarily at M(2), as suggested byHenderson et aI. (L993), this would mean that t6lFe3*is strongly ordered at the M(LA), M(lB), M(3A) andM(38) sites.

Mnz*

Staurolite is not a characteristic mineral of highlyoxidizing environments, and it is reasonable to assumethat Mn is in the divalent state. Mn2+ is unlikely tosubstitute for A1 at the M(LA), M(LB), M(2), M(3A)and M(38) sites, leaving the possible locationsfor Mn2+ as the (2), M(4A) and M(48) sites. Onthe basis of size and charges arguments, one expectsMn2+ to behave in the same way as Fd+. As Fe2+ isstrongly ordered at T(2), it is reasonable to proposethat Mn2+ shows the same behavior: as the Mn2+contents of staurotte are low (<0.15 apfu), only the(2) site will show significant Mn2+ occupancy. Notethat tetrahedral coordination of Mn2+ is uncommon butby no means unknown: ordered [a]Mn2+ occurs inwillemite and in akatoreite (Burns & Hawthorne1993). These arguments are bome out by the results ofHenderson et al. (1993), who showed via XAS thatMn in staurolite is tefrahedrally coordinated and (fromthe observed mean bond-lengfh of 2.01 A) in the diva-lent state.

Zn

The crystal structure results for natural Zn-richstaurolite (Hanisch 1966, this study) show Zn to beordered atfte T(2) site. This behavior was also foundfor trace Zn in Fe-rich staurolite by Henderson er a/.(1993), although they (in line with other previoussuggestions in the literature) indicate that I6lZn alsocould be present. We regard the latter suggestion asunlikely. Zn has a much stronger preference fortetrahedral coordination than Fe2+" and there isalways sufficient Fe2+ to account for the scatteringat the M(4A) ard M(48) sites in the structure ofany of the Zn-rich staurolite sample examined.A direct positive signal for I6lZn needs to be recog-nized before this occupancy can be considered asconfirmed.

cd+

From the occurence and structure of lusakite@rinehurst & Griffen 1986), Co2+ shows solid-solu-tion with Fe2+ and occurs at the Z(2) site.

Ti4+

Trivalent Ti is extremely rare in terrestrial environ-ments flMaychunas 1987); Ti is assumed to be in thetetrahedral state in staurolite. As discussed above, theargument proposed for Tia occupancy of (2) in stau-rolite is not convincing. As tetrahedrally coordinatedTi# is rare i1 minerals, it is a reasonable assumptionthat Tia is octahedrally coordinated in staurolite. TheM(4A) and M(48) oc.tahedra in staurolite are intrinsi-cally quite large, and it seems unlikely that Ti# willoccur at either of these sites. This leaves the M(IA),M(lB), M(2), M(3A) nd M(38) sites as possible loca-tions for titanium: however" as tfie amounts of Tiainvolved are small (-0.10 apfu), our site-scatteringrefinements provided no direct evidence on this point,except to indicate that Ti# could occur at any or all ofthese sites. The XAS results of Henderson at a/.(1993) support this argument, and further suggest thatTi# is strongly ordered atthe M(2) site.

CoNcr-ustoNs

1. Staurolite is monoclinic, C2lm, with p (observed)varying in the range 90.G-90.49'; the symmetry differ-ences are reflected in the relative degree of order ofthe constituent cation species between the (pseudo-)equivalentA and B sites in the structure.

2. The B angle correlates linearly with the differentialoccupancy and size (mean bond-lengths) of the M(3A)and M(38) octahedra; it is thus a measure of thedegree of order in the structure. Other order-disorderpairs [e.g., M(4A) afi M(48), possibly f(2) site-splittingl also correlate with the p angle.

3. The mean bond-length of the I(1) tetrahedronshows only very limited variation, reflecting the smallvariation in the A1 content ofthe Z(l) site.

4. Site-scattering refinement shows small but signifi-cant amounts of transition metals at the octahedrallycoordinated M(LA), M(1"8), M(2), M(3A) and M(38)sites, which are dominantly occupied by Al.

5. Mean bond-length - composition relationships indi-cate significant occupancy of the M(1), M(2) and M(3)sites by Mg; the slopes of these relationships allowquantitative assignment of Mg site-populations. In allexcept very Mg-rich staurolite crystals, approximatelyhalf of the Mg in the formula unit is in octahedralcmrdination.

580 TIIE CANADIAN MINERALOGIST

6. There is a considerable range n M(4) siie-popula- We are indebted to the reviewers and to the editor whotions with varying bulk composition; expressed in insisted that we clarify our fuzzy prose, and who veryterms of site scattering, the variation is 0-11.5 epfu. carefully checked the very involved arguments in thisSeveral axguments indicate thatthe M(4) sites are gen- wor( all erors are their fault. The CNR Centro dierally occupied by Fe2+ and vacancieso except for one Studio per la Cristallochimica e la Cristallografia,very Mg-rich staurolite crystal (Mg : 3.0 apfu), in Pavia, provided an idyllic environment for this work.wfuch M(4) is occupied by Mg. In terms of atoms, the Financial support was provided by CNR-NATO, andrange of M(4) site-populations is 0-{.43 Fe2+ apfu. the Natural Sciences and Engineering Research

Council ofCanada.7. The variation in M(4) occupancy shows a strongpositive correlation with the H contetrt.

8. There is variable total occupancy of the M(3) sites,spanning the range L.70-2.09 cations pfu. Low totaloccupancy correlates with increased H content.

9. The positional disorder of the (2) cations ca::not beadequately resolved by conventional refinement of X-ray datz. Even with data collected to maximum resolu-tion (20* = 135o), the model obtained is a functionof the starting pammeters of the refinement; this indi-cates convergence at local minima rather than at theglobal minimum. Consequently, although the dataindicate cation disorder atTQ), the quantitative resultsderived from normal procedures of refinement are notreliable.

10. Local order involvine the T(2) nd M(4) sites is avery important feature of the staurolite structure.Where M(4A) or M(48) is occupied, the neighboringtarc T(2) sites must be vacant (and, vice versa). T-hestoichiometry of the staurolite crystals examined hereindicates that a (locally) occupied M(4) site is associ-ated with vacant adjacent M(4) sites. Thus the associa-tion of two Z(2) vacancies with an occupied M(4) s\teis a long-range (as well as a short-range) feature of thestructure, and allows quantitative assignment ofvacancies to the I(2) site from the assigned site-popu-lations of the M(4) site.

1 1. Li occupies the I(2) site in Li-bearing staurolite.

l2.T\e I(2) site is occupied byFe,Zn, Li, Co, tr, Ivtgand Al, and may be considered as chemically promis-cuous.

AcKNOWLEDGEMEl,lrs

We thank Luisa Ottolini oPiero Bottazzi and MarcoPalenzona for their help with the ion-microprobe GREw, E.S. & Sarononn, S. (19g4): A staurolire-talc assem-analysis, and all of the people who provided samples blage in tourmaline - phlogopite - chlorite schist fromfor this study: Christian Chopin, Barbara Dutrow, Ed northern Victoria Land, Antarctic4 and its petrogeneticGrew, Mike Holdaway, Bruno Lombardo, Annibale significance. Contrib, Mineral Petrol.87,337-350.Mottana and Cbris Ward. We thank Mike Hendersonand coworkers for providing us with a copy of their GnrnFen, D.T. (19-8.1):^SynthetcFelZn stauroliles and the

paper otr XAS of staurolite prior to publication. BaJb ionic radius of NZnt2+' Arn' Mineral' 66,932-937 '

Dutow and Mke Holdaway suggested that FCH work - GosNsy, T.c. & pru:-ps, w.R. (1982): The chemicalor staurolite many years ago; they will pay for that! fo;;iu;f;u1u;-it^royt .e^ui*ra1.ff1.292_297.

RnpBnrNcss

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