337

Canadian MineralogistVoL 31, pp. 337-346 (1993)

THORTVEITITE AND ASSOCIATED Sc-BEARING MINERALSFROM RAVALLI COUNTY. MONTANA

EUGENEE. FOORD

U.S. Geological Survey, Denver Federal Center, MS 905, Denver, Colorado 80225, U.S.A.

SCOTT D. BIRMINGHAM

Boulder Scientific Cornpany, P.O. Box 548, Mead, Colorado 80542-0548, U.S.A.

FRANCESCO DEMARTIN

Isthuto di Chimica Strutturisrtca Inorganica, Universitd degli Smdi, Via Venezia 21, I-20133 Milano, Italy

TTILLIO PILATI

Centro CNR per la Sndio delle Relazioni fra Strum,ta e Reaxivitd Chtunica, Vin Golgi 19, I-2013 j Milano' haly

CARLO M. GRAMACCIOLI

Dipanimento di Scienze delh Tena, [Jniversitd degli Studi, Via Bouicelli 23, I-20133 Mil'ano, Italy

FREDERICKE. LICTITE

tJ.S. Geological Survey, Denver Federal Center, MS 975, Denver, Colorada 80225' U.S.A.

ABSTRACT

The rare Sc mineral thofieitite, (Sc,Y12Si2O7, occurs as pm- to mm-sized crystals in fluorite-bearing granitic pegmatites and

the host melagabbro within the Crystal lrtoirn6in fluorite deposit ltavalti County, Montana. Thortveitite is found as colorless and

clear to smok! and translucent, suLhedral to euhedral prisms up to 3 mm in length in the massive fluorite, as mm-sized anhedra to

subhedra in diopside and edenite, and as pm-sized droplet-like crystals in actinolite. Micrometric textures suggest that some

thorweitite exsoived from actinolite, which contains between 1.2 and 2.9 tut.Vo Sc2O3. Associated fenoan diopside contains as

much as 4.8, edenite as much as 2.1, actinolite as much as 2.0, allanite as 4gch as 0.5 and titanite as much as 0.4wt.Vo Sc2O3'respectivd. The source of the Sc is believed to be magmatic, rather than from assimilation and extraction of Sc from adjacent

anr,phibolite xenoliths. Indices of refraction, determined using spindle-stage techniques , arct a7.752, p 1.780, 1 1.804' all fl.m3'

T-cl = 0.052. The mineral is biaxial negative, 2Vr*. = JQo, 2V"*. = 84". The empirical formula derived on the basis of 7 atoms

of o*ygen from electron-microprobJand tasei-iep analysej is (Sc1.83Y0.05Fe3*o.orRq.mLz.oo(Sir.g4 .d>r.go lvhere R

represeits tvtn, Ca Mg, Ti, Na Ii, Nb, Sn, Ce, Pr, Nd, Snr, Eu, Gd, Tb, Dy, {o, Er' rm' Yb, Lu, Th, u,Ht'7'r, P' Li' Be' B' v'

Cr, Co, Ni, Cu, Ba Ga Ge, As, Rb, Sr, and In. Du = 3.50, Dc = 3.50 g cm-3. The mineral contains *EE = 4.63 wt.7o and is

enriched in the heaw rare-eartli elements. Cell dimensions are: a 6.5304(4), b 8.5208(4)' c 4.6806(5) A' p 102.630(7)" 'V 254.1(l)

N,Z=Z,spa"egroop C2lm.'tlrcwit-cellvolumeissmallerthanthatofthorweititefrommostotherlocalitiesbecauseofitsScenrichment. Simitartj, Oe metal--oxygen distances (2.088 to 2.199 A) are smaller than those for most natural specimens of

thorweitite.

Keryords: thortveitite, scandium silicate, rare-earth mineral, fluorite, actinolite, diopside, granitic pegmatite, X-ray dat&

crystal-structure refinement, Crystal Mountain mine, Ravalli County' Montana-

Souuanr

Nous avons trouv6 la thorweitite, (Sc,Y)2Si2O7, min6ral rare de Sc, en cristaux de taille microm6trique l millim6tnque dans

des pegmatites granitiques i fluorite et dansleur encaissant m6lagabbroique du gisement de fluorite de Crystal Mountain, comtd

de Fiava1i, au Montani. Etle forme des prismes sub-idiomorphas i idiomorphes, incolores et transparents i fumdset translucides'jusqu'i 3 mm en longueur dans la fluorite massive, en cristaux millim6triques x6nomorphes ou sub-idiomolphes dans la diopside

et t;gOenite, et en cri-staux micromdtriques en forme de goutelette dans I'actinolite. Les textures h l6chelle rnicrom6rique font

penserqu'ils'agitd'uneorigineparexsolutionipartirdel'actinolite,quicontientent"e l.2et297odeSc2O3enpoids.Ladiopside

338 THE CANADIAN MINERALOGIST

ferreuse associ6e en contientjusqu'A4.8Vo,l'6denite,jusqu'12.1 7o, I'actinolite,jusqu'd2,0%o,l'allanite,jusqu'h 0.57o, et la titanite,jusqu'i 0.47o. L'enrichissement en Sc serait primaire (magmatique) plut6t que due d l'assimilation et i l'extraction du Sc desx6nolithes d'amphibolite associds. Les indices de r6fraction, d6termin6s avec platine munie d'une aiguille, sont: a 1.752, P 1.780,1 I .804, avec dans chaque cas, et y- o = 0.052. C'est un min6ral biaxe n6gatif, 2Vn*.7C.,2V"4". 84". Les donndes obtenues parmicrosonde 6lecfonique er par plasma i couplage inductif avec laser, mbnent i la formule empirique suivante, pour sept atomesd'oxygdne, (Sc1.63Ys.65Fer+s.osRo.oz):z.oo(Sir.s7Ab.0rr2.o0Ou. Ici, f repr6sente Mn, Ca Mg, Tr, Na K, Nb, Sn, Ce, Pr, Nd, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U, Hf, Zr, P, Li, Be, B, V, Cr, Co, Ni, Cu, Ba, Ga, Ge, As, Rb, Sr et In. ta densitd mesur6eest de 3.50, et la densite calcul6e, 3.50. La thortveitite contient 4.637o en poids de terres raxes, et montre un enrichissement enterres rares lourdes. Les dimensions de la maille 6l6mentaire sont: a 6.5304(4), r 8.5208(4), c 4.6806(5) A, p tOZ.OlOlZ;", V254.1Q)43,2=2,groupespatial C2lm,I*volumedelamaille6l6mentaire,piuspetitquecequia6t6documeniddanslaplupartdes autres exemples connus, serait d0 d l'enrichissement en Sc dans ce cas. De mOme, les distances entre mdtal et oxygbne (2.088d 2.199 A) sont plus counes que dans la plupart des autres exemples de ce min6ral.

(Iraduit par la R6daction)

Mots-clds: thortveitite, silicate de scandium, min6ral de terres rares, fluorite, actinolite, diopside, pegmatite granitique, donn6es dediffraction X, affinement de la structure cristalline. mine de Crvstal Mountain" comt6 de Ravalli. Montana.

IxmooucnoN

The occurrence of the rare Sc mineral thortveitite.(Sc,Y)rSirO7, in the Crystal Mountain fluorite deposit ofRavalli County, Montana (Fig. 1), was fust reported byParker & Havens (1963). Fluorite was extracted fromthe deposit from 1952 to 1973 (Taber 1952), but noadditional data were obtained on the thortveitite until thedeposit was evaluated in 1984 as a potential Sc resource(Birminghanl unpubl. dat4 1984). This occurrence, inthe southern Sapphire Mountains, is the only onereported for North AmericA and one of fewer than adozen areas in the world where thortveitite has beenpositively identified (Voloshi! et aL l99l,Bianchi et al.1988, Yamada et al. 1980, Amli 1977, Kainov 1973,Mathiesen 1 970, Oftedal l969,lkol et al. 1969,Phan etal. 1967, Sakurai et al. 1,962, Neumann 1961). Thispaper presents new data on the thortveitite and associ-ated Sc-bearing minerals at Crystal Mountain. Samplesused in this study were provided by Boulder ScientificCompany, the present owner ofthe deposits.

OccunnsNcn

Thortveitite at Crystal Mountain is dispersed over a Ikm' area in several tabular bodies of fluorite cut bv manvminor faults of small displacemenl and in smaliMie;of intrusive melagabbro. The fluorite bodies, nowmostly removed by open-pit mining, extended down-ward to less than 200 m below the surface; only a smallexposure of the melagabbro is preserved at Crystal Point(Fie. l).

Ftc. 1. (A) Index map and simplified geological map of theCrystal Mountain thortveitite deposits. (B) Simplified geo-logical map showing fluorite deposits as of 1952 (nowmostly removed), and an intrusive body of granite in the pitfloor approximately I 00 m below the 1 952 surface. Adaptedin pan from Taber (1952).

l lBldltc-granodlclte

lT--l fl rort"etute'ueartnr

THORTVEITITE FROM RAVALLI COUNTY. MONTANA 339

The fluorite and thortveitite at this locality arebelieved to be related 1) to hypidiomorphic-granulargranitic pegmatites consisting primarily of quartz, bio-tite, plagioclase, and microcline, and 2) to associatedthortveitite-bearing melagabbro. The granitic rocks arepoor in Sc; the thortveitite occurs primarily within areasof the fluorite bodies that are enriched in biotite,magnetite, fergusonite, allanite-(Ce), epidote, xenotime,clinopyroxene (fenoan diopside), and amphibole (acti-nolite and calcic edenite). Other minerals identified inthe fluorite bodies are: apatite, chalcopyrite, spes-sartine-almandine, pyrite, titanite, rutile, zircon, thorite,uranothorite, molybdenite, and possible thorianite. Themelagabbro associated with the fluorite.bearing granitepegmatites contains edenite - edenitic hornblende,quartz, titanite, magnetite, biotite, and thortveitite. Thediopside, edenite - edenitic hornblende, actinolite, alla-nite-(Ce) and titanite contain variable amounts of Sc.

The area surrounding the Crystal Mountain fluoritepegmatites is composed primarily of a biotite granodio-rite pluton of Tertiary to Cretaceous age that is presumedto be related to the Bitterroot lobe ofthe Idaho batholith(Y,lallace et aI. 1982). This pluton contains large foliatedxenoliths of biotite - quafiz - plagioclase gaeiss andhornblende-plagioclase gneiss that resemble thethortveitite-bearing melagabbro, but are poor in Sc.Dikes of similar age consisting primarily of microclineand quaro, and hornblende gabbro dikes are known tothe eastin the Sapphire Wilderness Study Area, and maybe cogenetic with the Crystal Mountain pegmatites.However, they are not known to contain fluorite and alsoare poor in Sc: the hornblende gabbro dikes typicallycontain about 30 ppm Sc, and the microcline-quartzpegmatites, about 15 ppm (Campbell et al. 1983).Crystal Mountain is adjacent to the Late CretaceousSapphire batholith, an epizonal complex of six comag-matic peraluminous intermediate plutons. Wallace et al.(1982) reported that monzogranite and ganodiorile in

Frc. 2. Photomicrograph ofthortveitite. Scale bar represens I 00pm. Substrate is double-sticky tape.

the westem part of Sapphire Wilderness Sfidy Areacontain high concentrations ofSc and the rare earths, butthortveitite has not been identified in this area; whole-rock analytical resulc presented by Campbell et al.(1983) show $0 ppm Sc and, in most cases, <10 ppm.Several other fluorite-bearing deposits occur along theeastern margin of the Idaho batholith in Mineral,Missoula. and Ravalli counties, Montana (Ekambaramet al. 1986'1, but thorweitite has not been identified atany of these localities, and they show no anomalousconcentrations of Sc (Birmingham, unpubl. data, 1984).Rocks of the Idaho batholith may generally be slightlyenriched in Sc; some batholith-derived heavy-mineralplacers in central Idaho reportedly contain 200-1,500ppm Sc, mainly in tantalite, columbite, euxenite, andilmenite (Sondermayer l976,Parker & Adams 1973).

PHYSICAL AND OYnceL PnoPsnrrcs

Thortveitite in the massive fluorite bodies is found assubhedral to euhedral prisms up to 3 mm long. Theyrange from colorless and clear to smoky and translucent.Some crystals have well-developed {110} prisms, butwell-developed pyramids are uncommon; the {110}cleavage is indistinct (Fig. 2). Some crystals exhibitpolysynthetic twinning, with { I l0} as the compositionplane. The optical properties of thorweitite from CrystalMountain were determined in blue-filtered white light.The indices of refraction, determined using spindle-stage techniques, are as follows: a 1.752, B 1.780, 11.804, all t 0.003, nro = 0.052. The mineral is biaxialnegative, 2Vr* .=70o t2",t?V"a". - 84o. The mineralis colorless and shows no pleochroism. Dispersion r > vis distinct. The optical orientation scheme isl X A c =-6o, Y = D. Drrn = 3.50, Dc = 3.50 g cmr. The thortveititeis commonly enclosed by fluorite, and may envelopsmall (up to 100 pm) crystals, mainly of thorite anduranothorite, but also purple fluorite, amphibole, clino-pyroxene, and titanite. The inclusions are commonlyvery numerous, evenly distributed, and randomly orien-ted. Thorweitite in the melagabbro occurs as anhedral tosubhedral crystals up to 2 mm across within edenite, andcontains the same inclusions found in thortveitite fromthe fluorite deposits. In some of the melagabbro,thorweitite is found as small, droplet-like anhedralcrystals (<10 pm across) within actinolite that may be aresult of exsolution (Fig. 3).

ExprnDrsNTAL PnocsDURE

Chemical analyses

Electron probe microanalyses were carried out onpolished grain-mounts using ARL-SEMQ instrumentsat the Italian National Research Council (C.N.R.) at theCentro di Studio per la Stratigrafia e laPefiografia delleAlpi Centrali, Niilan, at the Department of Geological

340 THE CANADIAN MINERALOGIST

Ftc. 3. Photomicrograph of thortveitite-amphibole intergrowth.White: calcite, light grey: thortveitite, medium grey: actino-lite, and dark grey: quartz. Scale bar in lower right comer isl0 pm.

Sciences at the University of Texas at Austin, and at theU.S. Geological Survey in Denver, Colorado. Standardoperating conditions were as follows: accelerating volt-age from 15 to 20 kV; sample current on brass of 0.01pA; beam spot diameter of 5 or 15 pm; I 0- or 20-secondcount time at peak position, and l0- or 20-second counttimes on background positions, respectively. PET, LiF,TAP, and RAP spectrometer crystals were used. Acombination of natural minerals and synthetic com-pounds, as described in Jarosewich et al. (1980),Ingamells ( I 978), and Maruu wi et al. ( I 986), were usedas standards. Data were reduced using MAGIC IV andMAGIC V programs (Colby 1968).

This study also utilized a VG PlasmaQuad PQ+inductively coupled plasma mass sFctrometer equippedwith a Spectrum l,aser Systems pulsed Nd/YAG laseroperaled at the primary frequency of 1.068 pm (Anow-smith 1987). A single mm-sized crystal of the mineralwas ablated using the Q-swirched mode of the laseroperated at 158 mJoules/pulse and pulsed at 10 Hz for a33-second integration. All masses except r/z = 40 wereintegrated. Calibration of the instrument was accom-ptshed using an internal reference-standard glass GSE(Myers et aI. 1976). Composile results of analyses ofthortveitite, diopside, edenite, actinolite, allanite, andtitanite using several different crystals and both analyti-cal techniques (EPMA and ICP-MS) are presented inTable 1.

Cry stal- stntcture refinement

A crystal 0.10 x0.12 x 0.20 mm, from aheavy-mineral concenfate from the fluorite-processingmill at Crystal Mountain, \pas mounted on an Enraf-Nonius CAD-4 diffractometer, and twenty-five intensereflections havinga20 value in the range 12.03-37.48"were centered using' graphite-monochromated MoKcrradiation (7,, = 0.7fi73 A). The diffracted intensities[1618 with b3o(f)] were collected atroomtemperaturewith variable scan speed (maximum scan-time for eachreflection: 90 s) by exploring the hemigphere of thereciprocal lattice with 0 < h < ll, -15 3 /< ( 15, and-8 < I < 8, out to a maximum 20 angle of 80o. Thediffracted intensities were corrected for Lorentz, polari-zation and background effects. The data and otherpertinent information are presented in Tables 2-5.

An empirical absorption correction was applied byperforming a psi-scan correction (North er aL 1968),followed by a DIFABS correction (Walker & Stuart1983), according to the procedure already described inDemartin et al. (1991). After averaging the symmetry-related data whose agreement was 0.87o on an Fo basis,825 independent reflections were obtained; ofthese, 797with b3o(t) were considered for the structure refine-ment. Scattering factors for neufral aloms and anoma-lous dispersion correction factors were taken fromCromer & Waber (1974) nd Cromer (1974), respecti-vely. The structure was refined, starting fromthe atomicpositions reported inBiancbi et al. (1988)" by full-matrixleast-squares, minimizing the function >w(Fd-klF"D2.Weights assigned to individual observations were1/o2@o), where o@o)=[o2(D + GI)2]r2l2FJ.p, o21g isthe standard deviation for each reflection as derivedfrom counting statistics, k (=0.04) is a coefficient forimproving the goodness of fit" and Lp is the Lorentz-po-larization factor. Anisotropic displacement parameterswere assigned to all the atoms. The refined extinctioncoefficient was 1.59 x l0-5. The occupancy of Sc wasrefined to the value of 1.12, thus confirming partialreplacement of this elementby Y andheavy lanthanides.All the calculations were performed on a PDPI ll73computer using the SDP-Plus Sfructure DeterminationPackage @reru et al. l98O). We found the maximumresidual in the final difference Fourier synthesis to bea.4 et L3 . The final R (unweighted) and R* indices were0.015 and 0.029, respectively.

D$cussIoN

The behavior of Sc3+ in igneous systems cannot bereadily explained in terms of simple diadochic substitu-tion. Although some correlations may be possible withFe2*, Fe3*o Mg2* and Al3*, a stronger relationship maybe found with total Fe because the relative proportionsof Fe3* and Fdn determined in whole-rock analvses may

341TIIORTVEITITE FROM RAVALLI COUNTY. MONTANA

ad

o

6

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x

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o8

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q

ea

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554 FN N A V

c i c t . F qq q h :- 6 - F f , b

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o l 11116 ; ; ; dd + =

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d 6 6 6 6

9 Z Z Z 9

d$gggEe1 .9

En xqg !l6 9 da3siBcEic=ei*$r HdsZ Q € r s

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e. g3E Bb6 9l^ Eo o-

E*qiEEi E E?E€gg:33ggtei*iggP tEEE5gg?g3 gi ig&; c$fg;. €=. :'l

ffa3*Pei ;i 5g;-s113Fd Et 1€EtgPdFi ,qh tu-gFaccgF3 +q €

xE EElSeirF :€ A

gEEFEqige€$$E E 6 e € g g € o - . o - "

e! gE€33€F.l - i"$$$gE:tFgF.4"8"j ' ' i dEE:E:E HEJ=.F=

o N o N o o o o o ! t P g P A 5E u t $ a N d d d S i d I c , i ' b - $ f t

O Fq q q =E O E f r

F O @ F O @ Fo N o o o o o o @ o; d c i c i c t d d c t c t d

O @ O F O F @ o @O O N O O O F O F; c j c t c t . j . i t . ; c i c i I t I

. N 9 S' 8 i d t $ f r

t t

f , - t s <qo l o l =I o E I E f r

o o N N N O A O O {; d q i c i N c i c t c t F : dO g N N

t o ( o o o o N o o o oc \ i s q c t q d d c t c i d

o @ N 6 o * * o 9 o - I I g* d u i - ; c i ; ; c i , j d t t F t N *S F F F V O O

@ 6 t t s o o N { o N o @o 6 0 0 0 0 0 6 0 N o 6ci ; - . i ' j c i c i c i c i c i d - t

O @ @ N F F F oo O O 6 O o F Oc , i c t c t c r c i c i t c i I c i | | |

q { r - c { q . r e € q q *S - : 9 F o o F o o t I l E l E t r

O O N O o N N o Fo o o o o o o o oJ d c i c i < i c i t d c i c i t

9 F @ O O O o O F oc i - - d s ; c i e i c i c i d I

N

E* E?q i *I N | 8 F E f ; b

t : . : 8 c Sr o r R a R f t

o F o - t s * o a F o h s s $S ; o i o t o t c t c t . { c i { | | d g ' d E

@ N O * n F a o o F F@ S t O O O F O F O Oo l q q q q q q c 9 q qF o o o o a o o 9 F a I t

Eo . ^"B i ;bbbbbb .Bbb F E g=

$ d r . i c t c i c t c i d $ c j | | g t 8 i f r Q

E!8$$9rgg3g$$.EEgE FE

N O O N O N O O O 6 0 N@ O F @ N O O F O N O OF: ct - ci ; ct ct ct ci ci ci ci r

O O F F t s N O N F O F t sci - oi ts'j ct ct ci ci oi ci ci tA F F V V

O O F F F O @ O N A oo @ o t s o o N @ o N oN ; 6 i o i ; d d c i c i d c i t t

a q q q a a q q - q . : o lS @ 9 N F o F F A F A As F F V

EEqqqE{hq8eq@ F N N F A g O O g O O

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5

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342 TI{E CANADIAN MINERALOGIST

TABLE 6. ANISOIROPIC DISPTACEMENT PAMME'TERS

0

0

0

0.0019(2)

Sc

sl

o(r)

o(2)

o(3)

Cry€aal cy$emSps@ Grqpc 6 )! (A)c6)p ( tV (A1zD.DrMk! taNrfss&n hcoorS@n trEdearugamwldth 0lh€ta-nngs Orullfml spa€ oqrqedr@sr€d r€d€cdoculqa obsdr€d 1d6{dm vrnh l>3o0)Ffd R rd Rr hdb€ INo. d ErbU6GOF A

rqtodhlcQlm5.5304 (4)85208 (4)4.6800 (5)r@.630 c4254.1 (1)23.503.60 g m{0.E7u - n0.8 + 0.35tan 01 " 4 0lL rh 1lI 706nn0.015' 0.0293it1.238

0.0052(5)

o.qxs(8)

0.0070(3)

o.@76(2)

0.0r11(2)

0.00091 (4)

o.mo57(6)

o.oo6s(3)

{.m10(2)

0.0026(1)

0

0

0.0022(1)

0.00di4(5) 0.00618(5)

0.m570(8) 0.0042414

0.03re(6) 0.0204(5)

0.m71e) 0.650(2)

0.0069(2) 0.002(2)

Tho tgnrraijre iac,tor b In lh9 tdm:qpt,2'ih1?u(1,1) + l?b2ul2,2l F rc?u(3,3) + 2hka'b'u(t2) + 2hla'c'u(1,3) +

2tdb'c'U(2.3))l whsr€ a', b', ard c' 49 rodpr@l lard@ 6dant&

. F-EC.*lF.l)/FJ R.-lx(F"*lF.lFDflF"1'F! coF- Itx(F""k I F. lFlN*,N-Jt'"

w= l/(d(FJ)?, o(FJ = to'(l) + (0.04)1jFl2F"1.

TABLE 3. TNTERATOMIC DISTANCES (A) AND ANGLES O

may be concentrated in the late stages of pegmatiteformation in thorweitite, bazzite (the Sc-analog of beryl,Be3Sc2Si6O,s), and other Sc-bearing minerals thatrarelyoccur in granitic pegmatites (Schetelig 1,922, lacroix1923, Neumann 1961, Fryklund & Fleischer 1963, Krolet al. 1969, Juve & Bergstpl 1 990, Voloshin et al. l99 l).Similarly, in carbonatites, Sc is fractionated by maficminerals but may be enrichqd in the final differentiates(Mitchell & Brunfelt 1974, Amli 1.977).For pegmatiteswith notable concentrations of Sc. assimilation of am-phibolites or other rocks enriched in Sc by the intrudingmagma, or Sc extraction from wallrocks by pegmatiticfluids, commonly is cited as amechanismforenrichment(Phan et al. '1967, Schaller et al. 1962, Goldschmidt1934). For the thorfveitite-bearing pegmatites in theIveland-Evje district, Norway, Goldschmidt (1934)proposed that Sc had been extracted by the graniticrnagma from adjacent amphibolite. Phan er al. (1967)found that some Malagasy Republic (Madagascar) peg-matites enriched in Sc are very closely associated withamphibole-rich rocks, which acted to collect Sc. Scenrichment occurs in beryl replacing schist (200 ppm)compared to beryl replacing granite (50 ppm) at LakeGeorge, Colorado (W.R. Griffttts, unpubl. data" 1962).Similar enrichment in Sc also was found for beryl infeldspathic veins compared to that in pegmatites atseveral localities in Colorado and New Mexico (Staatzet al. 1965). More recently, the occurrence of bazzite,scandian ixiolite, and scandian pyrochlore in'ocleave-landite" - "amazonite" pegmatites at Heftetjern, TOrdal,Telemmk, Norway, was attributed to Sc enrichmentduring the passage of pegmatites through a maficvolcanosedimentary sequence (Bergstpl & Juve 1988,Juve & Bergstdl 1990). At the surface at CrystalMountain, the bodies of thortveitite-bearing pegmatiteare isolated from large masses of mafic rocks, except foroccasional xenoliths (<100 m across) ofbiotite - quartz- plagioclase gneiss and hornblende-plagioclase gueiss,by a large expanse of felsic biotite granodiorite (<20ppm Sc). The xenoliths have <40 ppm Sc and aregenetically distinct from thortveitite-bearing melagab-bro. The field relationships at Crystal Mountain areunlike those of Iveland-Evje and Heftetjern, Norway,with respect to possible source-rocks for the Sc, and thus

1.6060)r.6260)1.63r (1)

o(1)-o(2) 2.543(r)o(1)-o(30€ 2.615(1)o(2)*o(3))a z7m(lo(3)-o(3r 2-678(11

Sc"o(2)1c 2.ils(i)ScO(3)5e 2.088(1)Sc-o(3)14 2.199(1)

o(2)-'opr 2.682(1)o(2)"-o(3r 3.071(1)

o(r)€r€(2) 103.83(3)o(1)€ro(3))e 107.81(2)o(4€ro(3))@ 113.28(2)o(3)€ro(3r 110.34(4)

sro(1)sr€(2)sr€(3))a

o(2)Lo(3f 3.034(1)o(2)"o(3f 2.7J5(1)o(3)-o(3r 3.28s(1)

o(2F€co(2fo(2F&€(3)12O(2f€co(3)14oef"gco(3)5eo(3fsco(3fO(3fSco(3)!eo€r&€6)1e

78.80(3)sl.s4(2)&.42(2t%.6s(a103.76(3)r15.60(2)74.s(21

o(3)'o(3)' 3.628(r)o(3)rc(3)' 2.5e7(1)Symmdy tEnsiormdom s lolloF: al x.-y,zi bl -112+\1/2ry,z; c) -)qy,z;dl " t /2 tx"'l /2-y,1 +.i el 1 l2-u1 /2-'t,1-zt lt 1 /2-x.1 /2t,.zi S) ,"y,1 + z

TABLE 4. ATOMIC POSMONS AND EOUVATENT U'S

Sc

o(t)

o(2)

o(3)

0.0@ 0.30841(2) 0.500 0.488(2)

o.^ag\ 0.m {.@7a(6) 0.s(3)

0.000 0.@ 0.m0 15r(2)

0.@r(r) 0.0@ 02211(21 0547(el

023549(8) 0.15714(6) {2s54(t} 0.6n(rl

Fw lho alom hsF herdorcd, alsoroplc dbpawt parurEds U0'|s hfls b@n6ed: tlw m reported h Tat s 5, In tha last cdma m botoplc dll.d€lotrdbdamsrn pa€mdsd€rln€d s: Uq(A - 1/3 4UfFF,rqr b,epoded.

not accurately represent the proportion of either valencestate at the time of crystallization @ryklund & Fleischer1963, Norman & Haskin 1968). This being so, Scnormally behaves as a compatible element and is readilyaccommodated in mineral gtoups such as amphibole,pyroxene, and biotite; however, in systems rich in Sc, it

IHORTVEITITE FROM RAVALLI COIINTY, MONTANA 343

it seems doubtful that the pegmatites share the assimila-tion--extraction mechanism for Sc enrichment proposedby Goldschmidt (1934), Bergstol & Juve (1988) andJuve & Bergst@l (1990). Electron-probe microanalysesof edenite and actinolite within thortveitite-bearingrocks from Crystal Mountain showed between 1.6 and2.0wt.Vo SqO, (Table l). The Sc probably was derivedfrom the magma responsible for the melagabbro, and theunusual texture of the thortveitite in these rocks may bethe result of exsolution from amphibole upon cooling.Although most thortveitite in this association has irregu-lar outlines, some blebs seem to follow crystallographicdirections within the amphibole. There is an excess ofcations, just about equal to the amount of Sc, forthe threesamples of edenite and one of actinolite (Table l).Because thortveitite has just about the same amount ofSiO2(44.6wtVo) as the edenite, it is not simple to detectthe presence ofthortveitite in edenite. The polished thinsections containing edenite were examined carefully ina scanning electron microscope; no evidence for asecond Sc-bearing mineral within the edenite was found.The distribution of Sc in the amphibole seems to beuniform. TEM studies will be necessary to possiblyresolve the answer as to why there is a stoichiometricexcess of cations that happens to equal the amount of Scpresent. We contend that the source of Sc is magmaticand that thortveitite formed because the limit of Sc solidsolution in amphibole and other minerals was exceeded.

In synthetic scandium-enriched pargasite, fluor-pargasite, fluor-eckermannite, and fluor-nydbite, Sc hasbeen shown to be strongly ordered in the M(2) site insubstitution for octahedral Al @audsepp et al. 1987a,b,

1989). A composition of 0.8 a.f.p.u. has been found forscandian fluor-pargasite and 0.6 a.f.p.u. for scandianpargasite (Raudsepp et al. 1989). No natural analogs ofthese compounds have been reported, but Sc contents ofabout 10G400 ppm (0.015{.062 wt.Vo Sc2O3) havebeen noted for amphibole from several localities (Bori-senko & Rodionov 1961" Borisenko & Delitsin 1965,Senet al.1959, Nekrasov & Lebedeva l970,Krylovaetal. 1970D. Thus it seems that the edenite - edenitichornblende and actinolite associated with thortveitite atCrystal Mountain are anomalously rich in Sc. Krylovaet al. (1970) reported a weak correlation betweenconcentrations of Sc andlvAl in amphibole, but sugges-ted that Sc also may occupy the uAl sites. However, thepossibility oftetrahedrally coordinated Sc in minerals isnot supported by crystallographic evidence. On the basisof ionic radius, vrsc (0.83 A) would be more likely tosubstiture for vrAl (0.61 A) than IvAl (0.47 A), but thecomplexity of amphibole compositions makes assign-ment to a particular site arbitrary (radii from Whittaker& Muntus 1970).

A chondrite-normalized ratio (CNR) plot of the i?EE(Fig. 4) shows the strong fractionation of the f/REE bythortveitite, and the strong negative Eu spike. Thepattern is attributable to the similarity in ionic radii, ioniccharges, and other properties of Scr+ and the I/REEcapable ofbeing admitted into the thortveitite structure.Although thortveitite is enriched in HREE and Y, astrong geochemical correlation of these elements withSc has not been estabtshed for igneous or sedimentaryrocks in general (Norman & Haskin 1968). A CNRpattern for associated titanite also is shown in Figure 4.

o to:Ju

a 3.5o

d-

M h h t u U l l h l b I l n

- "-

lhilhoitite -"- Tilanile

Fto. 4. Plot of chondrite-normalized concentrations of the rare-earth elements in coexistingthortveitite and titanite.

344 T}IE CANADIAN MINERALOGIST

CNR patterns for associated purple fluorite (ZREE +Y=473 ppm) and fenoan diopside areessentially flat" wittlstrong Eu-depletion spikes.

Schneider et al. (1975,1977), M0ller et al. (1976),and Ekambaram et al. (1986)haye used log Tb/C aversuslog Tb/I-ato define three genetic types of fluorite. A highratio of Tb/Ca is believed to reflect a differentiatedsource-region such as that for granite pegmatites,whereas low values suggest a depleted, sedimentary-type source. Intermediate values are interpreted to be ofhydrothermal origin. The precise ranges are given inMbller et al. (1976). Using this method, Ekambaram elal. (1986) have recently shown ,that the fluorite -carbonate - quartz deposits of Mineral, Missoula, andRavalli counties in western Montana are hydrothermalin origin. Fluorite from Crystal Mountain is of apparenthydrothermal origin on thebasis of log Tb/CaversuslogTb/La using our analytical data. However, the Tb/Laratio for Crystal Mountain fluorite is lower than for anyof the other examples of fluorite in southwesternMontana.

EDS and ICP-MS analyses of ferroan diopside asso-ciated with thorweitite in a heavy-mineral concentratefrom the fluorite-processing mill show a range in Sccontent ftom 0 to 4.8 wt.Vo SqO.. BSE images show noother minerals to be present within the diopside, andX-ray-diffraction studies ofpurified separates also showno other mineral phases. An average result of threeanalyses for one grain containing close to the maximumamount of Sc2O, also is given in Table l. As the Sc2O3and Na2O contents increase, the FeO and CaO contentsdecrease, respectively, reflecting the coupled substitu-tion: Sc3* + Na+ = Fe2* + Ca2* to maintain chargebalance. The ionic radius of Sc3* (O.g:A) is close to tfrtof Fe2* 10.76 A). NazO contents range from 0.92 to 2.53wt.%o,CaO conten8 range from 19.7 to22.6wt.VooaadFeO contents range from 9.3 to 11.6 wt.Vo. The mineraljervisite (Mellini et al. 1982) is a scandium-rich py-roxene from the Baveno Granite, Italy. Synthetic Sc-analogs of aegirine and spodumene were characterizedby Ito & Frondel (1968). In such a Sc-rich environment,it is not surprising that the ferroan diopside containsappreciable amounts of Sc, and likewise associatedamphiboles, allanite and titanite. Scandium-bearing al-lanite (0.80 wt.Vo SqO) from Finland was described byMeyer (l9ll). Likewise, Sc-bearing (0.22 wt.Vo Sc)titanite was reported from a pegmatite in Kazakhstan(Semenov et aL 1966).

Thethortveitite from Crystal Mountain is notparticu-larly enriched in Sn, as is the thortveitite reported fromthe Norwegian localities (Oftedal 1969). Likewise, thereappears to be no correlation between Sc and Sn, Li, orBe in rocks from Crystal Mountain" in contrast to thatreported for theTprdal area ofNorway (Bergst@l & Juve1988). Sc at Crystal Mountain best correlates, in de-creasing order, with F, Fe}, Th, Y, and Yb. Yb and Yare known only in the thortveitite; Th correlates with Scbecause of the common inclusion of thorium minerals

within the thortveitite, and in associated allanite-(Ce)and fergusonite.

The unit-cell dimensions of thorweitite from Mon-tana are smaller than those of most thorweitite, but areclose to those of thortveitite from the Malagasy Republic(Biancbret al. 1988). The composition is similar, with amarked prevalence of Sc and low content of Y and therare-earth elements.

A contraction of the cell volume of thorfveitite withincreasing scandium content is evident in the study byBianchi et al. (1988), and can be explained on thegrounds of the smaller ionic radius of Sc$ with respectto Y3*.-Similarly, the metal-oxygen distances (2.088 to2.199 A) are smaller than for most natural specimens ofthortveitite, but do not significantly differ from theconesponding values for thortveitite from MalagasyRepublic.

The Sc site contents (a.p.f.u.) of thortveitite fromCrystal Mountain (CM) and Malagasy Republic (MR)are nearly identical (see Table I and Bianchi et al. 1988):Sc 1.83 I (CM), 1.836 (MR); Y 0.054 (CM), 0.054 (MR);Fe? 0.049 (CM), 0.M6 (MR); and R (all others bydifferenEe) 0.066 (CM),0.064 (MR).

AcKNowLEDGBMENTS

The authors thank Larry L. Jackson (USGS) for theH2O* and HrO- determinations on the scandium-bearingferroan diopside associated with thortveitite. Theauthors also thank R.C. Erd and J.J. Fitzpatrick (both ofthe USGS) for reviews of the manuscript. Additionalreviews by Mati Raudsepp and Sven Bergst6l furtherimproved the manuscript.

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Received January 16, 1992, revised manwcript acceptedJu|y9,1992.