Mineralogy and Petrology (1997) 60:121-132 Mineralogy a n ( 1

Petrology © Springer-Verlag 1997 Printed in Austria

Ternesite, Ca5(SiO4)2SO4, a new mineral from the Ettringer Bellerberg/Eifel, Germany

E. Irran 1,*, E. Tillmanns 1, and G. Hentschel 2

1 Institut fiir Mineralogie und Kristallographie, Universitat Wien, Austria 2 Sinzig, Federal Republic of Germany

With 1 Figure

Received February 12, 1997; accepted April 14, 1997

Summary

The new mineral ternesite, Ca5(SiO4)2SO4, has been found at the Ettringer Bellerberg near Mayen, Eifel, Germany. The crystal structure, already known from the synthetic analogue, was refined from single crystal X-ray data: orthorhombic, space group Pnma with a= 6.863(1)A, b=15.387(2)A, c=10.181(1)A, Z=4, R=0.058, Rw=0.046 for 820 unique reflections with F0> 3o-(F0) and 96 variable parameters. The strongest peaks in the powder pattern are (d-value (A), I, hkl): 2.830, 100, (033)/2.853, 63, (230)/2.565, 55, (060)/3.198, 42, (132)/1.892, 39, (035) + (125). The mineral is optically biaxial negative with refractive indices n× = 1.630(1) (parallel [100]), ny = 1.637(2) (parallel [001]), and nz = 1.640(1) (parallel [010]). The optical angle 2Vx was measured as 63.5(5) °

Zusammenfassung

Ternesit, Ca5(Si04)2S04, ein neues Mineral vom Ettringer Bellerberg, Eifel, Deutsch- land

Das neue Mineral Ternesit, Ca5(SiO4)2SO4, wurde am Ettringer Bellerberg bei Mayen, Eifel, Deutschland gefunden. Die schon vom synthetischen Analogen her bekannte Kristallstruktur wurde aus Einkristalldaten yon nattirlichem Material verfeinert: Das Mineral ist orthorhombisch, Raumgruppe Pnma mit a= 6.863(1)A, b=15.387(2)A,

* Present address: Laboratorium ftir Anorganische Chemie I, Universit~it Bayreuth, Federal Republic of Germany

122 E. Irran et al.

c=10.181(1)A, Z=4, R=0.058, Rw=0.046 ftir 820 unabh~ngige Reflexe mit Fo> 3cr(Fo) und 96 variablen Parametern. Die st~rksten Maxima im Pulverbeugungs- diagramm sind (d-Wert (A), I, hkl): 2.830, 100, (033)/2.853, 63, (230)/2.565, 55, (060)/ 3.198, 42, (132)/1.892, 39, (035) + (125). Das Mineral ist optisch zweiachsig negativ mit Brechungsindizes n× : 1.630(1) (parallel [100]), ny : 1.637 (2) (parallel [001]), und nz : 1.640(1) (parallel [010]). Der optische Achsenwinkel 2Vx wurde zu 63.5(5) ° gemessen.

Introduction

Recently, two Ca-rich xenoliths containing an unknown light blue mineral were found at the Ettringer Bellerberg near Mayen (Eifel/Germany) by Mr. B. Terries (Mayen). The X-ray diffraction pattern of the blue mineral resembles that of synthetic Cas(NiO4)2SO 4 (PDF 26-1071) which is known as the main component of the green sulphate rings accumulating in cement rotary kilns. This phase is isotypic to Cas(PO4)2SiO4 ('silicocarnotite') and to some other Ca or Cd containing compounds.

The mineral was named after Mr. Ternes, who not only found the mineral and provided samples for this investigation but also contributed a lot to our knowledge of the minerals of the Eifel area. Mineral and mineral name have been approved by the IMA Commission on New Minerals and Mineral Names prior to publication (# 95-015). Type material is deposited at the Institut ftir Mineralogie und Kristal- lographie, Universit~it Wien and at the Naturhistorisches Museum, Wien, Austria.

Occurrence and paragenesis

The Ettringer Bellerberg volcano is part of the Laacher See area in the Eastern Eifel which was active in the Quaternary (Frechen, 1976). Its leucite tephrite lava contains many inclusions of different kinds of rock altered by high-grade thermo- metamorphism. Of special interest are the Ca-rich xenoliths, to which the rocks containing ternesite belong, because of their unusual paragenesis of rare minerals (Jasmund and Hentschel, 1964).

The analysis of the host rock by powder diffraction patterns and by optical examination of a thin section showed that it consists mainly of ternesite, ellestadite and a solid solution between ettringite and thaumasite. In addition other calcium silicates like mayenite, tobermorite, calcio-olivine, and larnite, also portlandite, magnetite, and hematite were detected.

The Ca-rich xenoliths found at the Bellerberg are possibly marly limestone incorporated into the leucite tephrite lava and decarbonized at high temperatures (Jasmund and Hentschel, 1964). This high temperature paragenesis is represented by mayenite, known as the phase C12A7 of tile cement clinker, by larnite, which is restricted to the hornfels facies (Turner, 1981), and by brownmillerite. As ternesite is the only water-free sulphur-containing component of the xenoliths, it can be supposed that it also belongs to the high temperature paragenesis. Ellestadite and ettringite/thaumasite were formed by subsequent hydratization at lower tempera- tures. Additional alteration products of the original paragenesis are tobermorite, portlandite, and calcio-olivine.

A new mineral from the Ettringer Bellerberg/Eifel 123

Synthetic Ca5(SiO4)2SO4

Ca5(SiO4)2SO 4 was described for the first time by Sundius and Peterson (1960) as the colouring component of the green sulphate rings in cement rotary Kilns. It is one of the complex sulphates formed in the Mueller-Kuehne process where CaSO4 is used as Ca supplier instead of CaCO3 (Smith and Gutt, 1971). These complex sulphates subsequently decompose to the essential clinker minerals Ca3SiOs, CazSiO4, and Ca3A1206. Pryce (1972) describes crystals of Ca5(SiO4)2SO4 from lime kiln wall coating where the kiln feed was calcite and quartz. Here the sulphur necessary for the formation of Ca5(SiO4)2SO4 was supplied by the fuel which was an oil with a high sulphur content. Experiments performed by Gutt and Smith (1966, 1967) showed that Ca5(SiO4)2SO4 is formed at temperatures of about 1000°C and decomposes at 1150°C in an open system and at about 1300°C in closed platinum crucibles. For the reaction CaSO4 + 2Ca2SiO4-=-Ca5(SiO4)2SO4 an activation energy of (157 :k 3) kJ/mol was derived (Hanic et al., 1986).

Natural ternesite therefore should be formed between 1000 and 1300 °C at low pressure, which is consistent with the conditions found for the paragenesis of the xenoliths. Sulphur theoretically can be supplied by sulphates in the original rock or by sulphur containing gases from the host rock. As stated by Jasmund and Hentschel (1964) the Ca-rich xenoliths from the Ettringer Bellerberg seem to receive the sulphur from the sulphur rich gases of the magmatic host rock.

Physical properties and chemical composition

Ternesite forms aggregates of radially arranged prismatic crystals elongated along [100]. Their size is up to 0.2 mm in length and about 0.05 mm in diameter. The crystals show no cleavage, the Moh's hardness is 4.5 to 5. The density determined by mixing of heavy liquids under the microscope is 2.94(2) Mg/m 3, the calculated density is 2.97 Mg/m 3. The colour is bright blue only in aggregates, single crystals are colourless. No fluorescence was observed.

The optical constants of ternesite were determined on a spindle stage using light with a wavelength of 586 nm. The mineral is optically biaxial negative with refractive indices nx = 1.630(1) (parallel [100]), ny -- 1.637(2) (parallel [001]), and nz = 1.640(1) (parallel [010]). The optical angle 2Vx was measured as 63.5(5) °, the theoretical angle calculated from the refractive indices gives 66(5) ° . The optical constant of Ca5(8iO4)2SO 4 from a lime kiln determined by Pryce (1972) are nx = 1.632(1), ny = 1.638(1), nz = 1.640(1) and 2Vx = 60(1) °, which agrees well with the constants of the natural sample.

The chemical composition was determined by EDX measurement on a JEOL JSM-6400 scanning electron microscope (operating voltage 20 kV, channel width 20 eV). The standards were wollastonite for CaO and SiO2 and anhydrite from Stassfurt/Germany for SO3. The measured data were corrected for attenuation effects by the program system LINK exL10 (Link Analytical Ltd., B u c k s , England). Table 1 shows the range and average chemical composition obtained from seven analyses and the theoretical composition of Ca5(SiO4)2SO4. On the basis of 12 oxygen atoms the empirical chemical formula was calculated as Ca5.04Si2.01S0.98012, idealised Ca5(SiO4)2SO 4.

124 E. Irran et al.

Table 1. Chemical composition of ternesite (in weigth percent)

Range Average Standard deviation Theoretical

CaO 57.32-60.35 58.90 0.83 58.34 SiQ 23.05-27.23 25.22 1.28 25.00 SO3 13.80-18.04 16.34 1.25 16.66

Z= 100.46 Z= 100.00

As none of the major element seems to be responsible for the bright blue colour of ternesite, a XRF analysis was performed to look for traces of transition elements which could not be detected by the EDX measurements. The analysis was carried out on a PHILIPS PW2400 X-ray spectrometer with Rh-anode, 3 kW generator and LiF(200) analysator (voltage 60 kW, current 50 mA). Traces of iron and titanium were found, but due to the small sample no quantitative analyses could be performed. As it is supposed to be the cause of the blue colour of sapphire and cyanite (Nassau, 1978), ternesite is likely coloured by a charge transfer be- tween Fe 3+ and Ti 4+. In contrast synthetic Ca5(SiO4)2SO 4 usually is green, the chemical analysis of Ca5(SiO4)2SO4 synthesised from high purity Ca2SiO4 and CaSO4 by Gutt and Smith (1966) showed a FeO-content of 0. l weight%. Samples of Ca5(SiO4)2SO4 from lime kiln wall coating analysed by Pryce (1972) showed a high Fe203- (0.5 weight%) and FeO- content (100 ppm), but only 50 ppm TiO2 and similar amounts of other metallic oxides. Therefore it can be assumed that the green colour of synthetic Ca5(SiO4)2SO4 is caused only by iron ions.

The agreement between measured density, refractive indices and chemical composition was checked by the Gladstone-Dale relationship using the constants of Mandarino (1976). The compatibility index (1-kp/kc) after Mandarino (1981) gives -0.054, which falls into the category "good".

X-ray investigations

A portion of ternesite was ground, fixed on a fiat cut silicon single crystal sample holder, and measured with a Philips PW3020 X-ray diffractometer. The peak po- sitions were corrected with silicon as internal standard, and the program NBS*- AIDS80 (Mighell et al., 1983) was used to index the powder diffraction pattern and refine the lattice constants of the orthorhombic cell (a = 6.863(1) A, b = 15.387(2) A, c = 10.181 (1) *). A detailed powder pattern has been deposited for the Powder Diffraction File with JCPDS/ICDD, it is also available from the authors upon request. The strongest peaks in the powder pattern are (d-value (A), I, hkl): 2.830, 100, (033)/2.853, 63, (230)/2.565, 55, (060)/3.198, 42, (132)/1.892, 39, (035) + (125). The powder diffraction pattern of ternesite shows good agreement with synthetic Ca5(8iO4)2SO 4 (PDF 26-1071), although the highest peaks differ some- what in intensity because of a preferred orientation of the natural sample due to the elongation of its crystallites parallel [100].

A crystal large enough for single crystal diffraction was chosen by oscillation photographs, and was used for data collection on a STOE AED2 automatic four

A new mineral from the Ettringer Bellerberg/Eifel 125

Table 2. Summary of crystal data, X-lay measurement and structure refinement of ternesite

Formula Ca5(SiO4)2SO4 Molecular weight 480.62 Space group Pnma (No. 62) Formula units per cell 4 a[,~] 6.863(1) b[]~] 15.387(2) c[A] 10.181(1) Volume [~3] 1075.1 Calculated density [Mg/m 3] 2.968 Crystal size [mm 3] 0.054 x 0.090 x 0.179 Range of data collection [°20] 3-55 Steps per peak 35 Step width [°20] 0.03 Background measurement 2 x 5 steps Measuring time per step [s] 1-3 Measured reflections 2732 Unique data set 1231 Data with F0 > 3(F0) 820 F(000) 960.0 Rin t 0.0600 R 0.0582 Rw 0.0459

circle diffractometer. The lattice constants were determined by exact measurements of the positions of nine reflections and their symmetrically equivalent ones, they were in good agreement with the more accurate constants derived from the powder data. The measured intensities were corrected for Lorentz and polarisation effects as well as for absorption by empirical ~-scans. The non-extinction rules are consistent with the possible space groups Pn21a and Pnma, the latter was confirmed by the crystal structure refinement. Table 2 gives some details of the crystal data and the refinement procedure.

For refinement the program SHELX76 (Sheldrick, 1976) was used, the atomic scattering factors were taken from the International Tables of Crystallography, Vol. C (Wilson, 1992), the anomalous scattering factors from the International Tables for X-ray Crystallography, Vol. IV (Ibers and Hamilton, 1974) and the absorption coefficients from Vol. II (Kasper and Lonsdale, 1956). The atomic coordinates of Cas(PO4)2SiO4 ('silicocarnotite') given by Dickens and Brown (1971) were used as starting model of the structure refinement. At the beginning the sulphur atoms were placed on the tetrahedra T(1)O4 on the mirror plane (Wyckoff position 4c) and silicon on the tetrahedra T(2)O4 in general position (Wyckoff position 8d) in contrast to Cas(PO4)2SiO4 which shows a more compli- cated distribution of the cations over the two tetrahedral position. During the refinement the cation-oxygen distances of the tetrahedra did not contradict this distribution, so it was kept till the end of the refinement. The refinement converged at a residual of 0.058 and a weighted residual of 0.046. A final difference Fourier

126 E. Irran et al.

Table 3. Atomic coordinates and equivalent isotropic displacement factors (~2). For the definition of Beq see Fischer and Tillmanns (1988)

x y z Beq

Cal 4c m 0.0502(3) 0.25 0.1911(2) 1.90

Ca2 8d 1 0 . 1 4 2 3 ( 2 ) -0.0966(1) 0.1588(1) 1.95

Ca3 8d 1 0 . 3 6 2 2 ( 2 ) 0 .0861(1) 0.0657(1) 1.85

Si 8d 1 0 . 3 4 5 7 ( 3 ) 0 .0747(1) 0.3659(2) 1.68

S 4c m 0.0171(5) 0.25 0.5885(2) 2.17

01 4c m 0.233(1) 0.25 0.6028(8) 3.21

02 4c m -0.043(1) 0.25 0.4519(6) 2.23

03 8d 1 - 0 . 0 6 2 ( 1 ) 0 .1717(3 ) 0.6504(5) 3.19

04 8d 1 0 . 3 8 6 1 ( 7 ) -0.0095(3) 0.2699(4) 1.93

05 8d 1 0 . 1 8 0 5 ( 7 ) 0 .0511(3) 0.4751(4) 1.77

06 8d 1 0 . 2 8 8 3 ( 7 ) 0 .1542(3) 0.2678(4) 1.92

07 8d 1 0 . 5 3 1 9 ( 7 ) 0 .1092(4) 0.4537(5) 2.13

0 3 map revealed no electron density larger than 1 e/A-, confirming the ordered distribution of S and Si. The refined atomic coordinates and equivalent isotropic displacement factors are given in Table 3, a list of anisotropic displacement factors is available from the authors on request.

Crystal s tructure

The crystal structure of ternesite is identical with the structure of synthetic Ca5(SiO4)2SO4 determined by Brotherton et al. (1974). It consists of isolated SiO4 and SO4 tetrahedra connected via Ca ions which are coordinated by six or seven oxygen atoms, respectively.

Some interatomic distance and angles of the tetrahedra are listed in Table 4. The tetrahedra T(1)O4 lying on the mirror plane have an average interatomic distance between the central atoms and the oxygen atoms of 1.467 A, which corresponds to the average S-O distance of 1.473 A given by Baur (1981). In the tetrahedra T(2)O4 in general position the average distance between the central atom and the oxygen atoms is 1.638 A, which fits well to an expected Si-O distance for isolated

o

SiO4 tetrahedra of 1.637 A, calculated after Baur (1978). There are three differently coordinated calcium ions, Ca1 is situated on a mirror

plane, while Ca2 and Ca3 occupy a general Wyckoff position, respectively. Some interatomic distances of the Ca polyhedra are listed in Table 4. The Ca1 ions are coordinated by seven oxygen atoms, which form an irregular one-capped trigonal prism. A further oxygen atom (02 I) is rather near (Cal-O2' = 3.149(7) A), but its distance is 27 percent larger as the average distance of the other ones, so it is not considered to be part of the coordination polyhedron. Ca2 is coordinated by six oxygen atoms, but there is also a further oxygen 03 I, that does not belong to the coordination of Ca2 (Ca2-O3 ~ : 3.106(7) A). The six ligands form a polyhedron

A new mineral from the Ettringer Bellerberg/Eifel

o

Table 4. Some interatomic distances [A], bond angles [ o], intrapolyhedral 0 - 0 distances (given in brackets) and bond valences s (after Brown and Altermatt, 1985)

127

[°] [A] s [o] [A] s

S-O2

S-O3

S-O1

<S-O>

2x

O1-S-O2 112.1(5)

O1-S-O3 2x 109.1(3)

O2-S-O3 2x 107.9(3)

O3-S-OY 110.8(4)

<O-S-O> <109.5>

1.451(7) 1.60 Si-O6 1.628(5) 0.99

1.464(5) 1.54 Si-O5 1.629(5) 0.99

1.489(8) 1.45 Si-O4 1.646(5) 0.94

<1.467> Z=6.13 Si-O7 1.647(5) 0.94

<Si-O> <1.638> 2=3.86

[2.439(10)] O4-Si-O5 110.,3(2) [2.688(6)]

[2.405(9)] O4-Si-O6 105.5(2) [2.607(7)]

[2.356(7)] O4-Si-O7 116.4(3) [2.800(7)]

[2.410(7)] O5-Si-O6 114.7(3) [2.742(6)]

<2.395> O5-Si-O7 104.0(3) [2.581(7)]

O6-Si-O7 106.2(3) [2.619(7)]

<O-Si-O> <109.5> <2.673>

Cal-O6 2x 2.335(5) 0.37

Cal-O6" 2x 2.362(5) 0.34

Cal-O7 2x 2.624(6) 0.17

Ca 1-02 2.731 (7) 0.13

<Cal-O> <2.482> Y,=1.89

Ca2-O3 2.327(5) 0.38

Ca2-O4 2.327(5) 0.38

Ca2-O5 2.338(5) 0.37

Ca2-O7 2.414(5) 0.30

Ca2-O4' 2.424(5) 0.29

Ca2-O1 2.575(3) 0.19

<Ca2-O> <2.401> Y~=l.91

Ca3-O5 2.288(5) 0.42

Ca3-O7 2.303(5) 0.40

Ca3-O5' 2.322(5) 0.38

Ca3-O6 2.364(4) 0.34

Ca3-O4 2.552(4) 0.21

Ca3-O2 2.611(2) 0.18

Ca3-O3 2.617(5) 0.17

<Ca3-O> <2.437> Y,=2.10

O1 Y~=1.83 02 2=2.09

03 2=2.09 04 Y~=1.82

05 Y~=2.16 06 Y~=2.04

07 Y~=l.81

which can be called a transition between an octahedron and a trigonal prism. The Ca3 ions are unambiguously surrounded by seven oxygen atoms, which form a distorted pentagonal bipyramid.

The oxygen atoms O1, 02 and 03 form the apices of the SO4 tetrahedra and they are coordinated by Ca and S in different ways: O1 and its three ligands (S and two Ca atoms) form a flat trigonal pyramid, 03 also has three ligands, but is trigonal planarly coordinated, the sum of the ligand-O3-1igand angles being

128 E. Irran et al.

Fig. 1. Crystal structure of ternesite projected along [100] (program ATOMS, Dowty, 1995)

360.0 °. 02 is surrounded by S and three Ca atoms which form a heavily compressed tetrahedron. In contrast, all oxygen atoms which are bound to silicon (04 to 07) are coordinated by only slightly distorted tetrahedra of Si and three Ca atoms, with average ligand-oxygen-ligand angles from 108.6 to 109.3 °. The bond valence sum of all seven oxygen atoms are given in Table 4.

Discussion

Gutt and Smith (1968) already recognised the similarity of the diffraction pattern of synthetic Ca5(8iO4)2SO4 to the pattern of Cas(PO4)2SiO 4 ('silicocarnotite'), which was confirmed by the crystal structure determination of both compounds (Brotherton et al., 1974; Dickens and Brown, 1971). In addition there is a number of synthetic compounds isotypic to ternesite which are listed in Table 5, some interatomic distances of these compounds are given in Table 6.

The tetrahedra

In ternesite the distribution of the small atoms over the tetrahedra is as expected by the formula, the sulfur atoms occupy the tetrahedra T(1)O4 in special position and the silicon atoms the tetrahedra T(2)O4 in general position. In contrast, in 'silicocarnotite', Cd5(PO4)2SiO4 and Cds(PO4)2GeO4 the T(1)O4 tetrahedra are filled by P only, but the other half of the P together with Si and Ge, respectively,

A new mineral from the Ettringer Bellerberg/Eifel

Table 5. Isotypic compounds, their lattice constants and volume

129

a [/k] b [A] c [A] Vol. [A 3] Lit.

Cas(SiO4)zSO 4 6.863(1) 15.387(2) 10.181(1) 1075.1 1) Cas(PO4)2SiO 4 6.737(1) 15.508(2) 10.132(1) 1058.57 2) Cas(CrO4) 3 6.733(2) 16.097(3) 10.334(2) 1120.01 3) Cas(CrO4)l.8(SiO4)l.2 6.725(4) 15.812(4) 10.249(10) 1089.83 3) CdzsCazs(PO4)2SiO4* 6.655 1 5 . 3 2 8 1 0 . 1 0 4 1030.69 4) Cds(PO4)2SiO 4 6.692(2) 14.99(1) 10.140(5) 1017.17 4)

Cds(PO4)2GeO 4 6.734(1) 15.107(2) 10.210(2) 1038.67 4) Cds(AsO4)2SiO4* 6.751 1 5 . 3 2 8 1 0 . 3 2 6 1068.53 4) Cds(AsO4)2GeO4* 6.800 1 5 . 4 2 7 1 0 . 4 1 6 1092.68 4) Nafd4(PO4) 3 6.670(2) 15.10(3) 10.04(2) 1011.20 5) NaCd4(AsO4)3* 6.759 1 5 . 6 1 9 1 0 . 3 8 2 1096.02 4)

NaCd9(PO4)sSiO4* 6.654 1 5 . 0 6 4 1 0 . 1 2 1 1014.49 4)

* Only powder data available, standard deviation below 0.002 ,~. Literature: 1) this work; 2) Dickens and Brown (1971); 3) Adendorffet al. (1992); 4) Engel and Fischer (1985); 5) Ben Amara et al. (1979)

share the T(2)O4 tetrahedra (Dickens and Brown, 1971; Engel and Fischer, 1985). As could be shown for Cd5(PO4)2SiO4 and Cds(PO4)2GeO4 this ordering is perfect, while the average distances of the tetrahedra of 'silicocarnotite' may show a slight disorder. Adendorff et al. (1992) suppose that in Cas(CrO4)3 the tetrahedra T(1)O4 are filled by formally tetravalent chromium and the tetrahedra T(2)O4 by formally hexavalent chromium as indicated by the average central atom - oxygen distance, but the distribution of the atoms shows a certain degree of disorder, too. The distribution of the tetrahedrally coordinated atoms in Ca5(CrO4)l.8(SiO4)1.2 is more complicated, the occupation factors of Cr and Si on the different tetrahedral sites could be determined, but not the distribution of the different oxidation states of chromium (Adendorffet al., 1992). In any way, the similar average distances show a high degree of disorder. At least in NaCd4(PO4)3 the tetrahedra are occupied by P only, although the T(1)O4 tetrahedra are slightly smaller than the T(2)O4 (Ben Amara et al., 1979).

This review shows that the distribution of the small atoms over the tetrahedra is mainly controlled by their size. The smaller P, Cr, and S atoms prefer the tetrahedra T(1)O4, while the larger ones (Si, Ge, Cr) rather fill the T(2)O4 tetrahedra. This results in an atomic distribution also found in normal, inverse, or partially inverse spinels, though in the 'silicocarnotite'-group it is just a distribution between dif- ferent kinds of tetrahedra and not between octahedral and tetrahedral voids as in spinel.

The large cations

In the different compounds of the 'silicocarnotite'-group not only the size of the coordination polyhedron of the large cations is varied, but also the number of

130 E. Irran et al.

Table 6. Comparison of interatomic distances of compounds isotypic to ternesite [A]; numbers in square brackets are not used for calculation of the average cation-anion bond lengths

Cas(SiO4)~ Cas(PO4)2 Ca~(CrO4)3 Ca~(CrO4b Cds(PO4) 2 Cds(PO4) 2 NaCd4 SO4 SiO4 (SIO4) 1.2 SiO4 GeO4 (PO4)3

T1-O2 1.451(7) 1.569(3) 1.673(5) 1.684(5) 1,545(6) 1.529(8) 1.527(3) T1-O3 2x 1.464(5) 1.546(2) 1.705(3) 1.668(3) 1.527(5) 1.521(7) 1.524(2) T1-O1 1.489(8) 1.5574(_43 1._.fi82(4) 1.676(6) 1.531(8) 1.551(12~ 1.553(4) <Yl-O> <1.467> <1.555> <1.691> <1.674> <1.533> <1.531> <1.532>

T2-O6 1.628(5) 1.600(3) 1.735(2) 1.660(3) 1.578(4) 1,651(6) 1.535(6) T2-O5 1.629(5) 1.596(2) 1.770(2) 1.659(3) 1.597(4) 1.628(6) 1.570(2) T2-O4 1.646(5) 1.585(2) 1.721(2) 1.651(3) 1.577(4) 1.633(6) 1.540(2) T2-O7 1.647(5) 1,608(2.23 1_~.736(3) 1.655(3) ~ 1.6587(7) 1.555(2.) <T2-O> <1.638> <1.597> <1.741> <1.656> <1.586> <1.643> <1.550>

M1-O6 2x 2.335(5) 2.416(3) 2.459(2) 2.422(3) 2.271(4) 2.261(6) 2.329(3) M1-O6" 2x 2.362(5) 1.417(3) 2.462(2) 2.434(4) 2.334(4) 2.334(6) 2.413(3) M1-O7 2x 2.624(6) 2.570(3) 3.552(3) 2.564(3) 2.708(5) 2.746(8) 2.809(3) M1-O2 2.731(7) 2.654(3) 2.741(5) 2.681(5) 2.380(6) 2.370(8) 2.381(3) M1-O2' [3.149(7)_] 2.907(3) 2.811(5) 2.81k(_6.) [3.095(6)] [3.1790) ] 3.030(3) <M1-O> <2.482> <2.546> <2.562> <2.534> <2.429> <2.436> <2.564>

M2-O3 2.327(5) 2.284(2) 2.298(2) 2.283(3) 2.193(4) 2.197(6) 2.185(2) M2-O4 2.327(5) 2.443(2) 2.425(2) 2.396(3) 2.291(4) 2.290(6) 2.321(2) M2-O5 2.338(5) 2.375(2) 2.384(2) 2.342(3) 2.395(4) 2.407(6) 2.410(2) M2-O7 2.414(5) 2.517(2) 2.557(2) 2.514(3) 2.460(5) 2.464(7) 2.425(2) M2-O4 2.424(5) 2.518(2) 2.529(2) 2.486(3) 2.496(4) 2.492(6) 2.463(2) M2-O1 2.575(3) 2.423(2) 2.479(2) 2.508(2) 2.307(3) 2.288(5) 2.307(2) M2-O3' [3.105(7).] 2.694(3) 2.600(3) 2.704_C4~ I2.967(6)_1 [2.952(9).] [2.878(3)_1 <M2-O> <2.401> <2.465> <2.467> <2.462> <2.357> <2.365> <2.352>

M3-O5 2.288(5) 2.317(2) 2.238(2) 2.306(3) 2.259(4) 2.261(6) 2.278(2) M3-O7 2.303(5) 2.299(2) 2.325(3) 2,306(3) 2.208(4) 2.216(7) 2.230(2) M3-O5' 2.322(5) 2.324(3) 2.367(2) 2.328(3) 2.284(4) 2.310(5) 2.315(3) M3-O6 2.364(4) 2.408(2) 2.442(2) 2.433(3) 2.366(4) 2.339(7) 2.290(2) M3-O4 2.552(4) 2.624(2) 2.651(2) 2.599(3) 2.768(4) 2.809(6) 2.791(2) M3-O2 2.611(2) 2.485(2) 2.562(1) 2.540(1) 2.491(2) 2.522(3) 2.421(2) M3-O3 2.617(5) 2,493(2) 2.494(2) 2.517(3_) 2.350(4) ~ 2,386(2) <M3-O> <2.437> <2.421> <2.440> <2.433> <2.389> <2.400> <2.387>

ligands can be changed. At the M1 position (conferring to the Ca l position in ternesite) in ' s i l icocamoti te ' , Ca5(CrO4)3, and Ca5(CrO4)l.8(SiO4)l.2 the ca lc ium ions are coordinated by eight oxygen atoms, in contrast to the seven-coordinated Ca l in temesite. In Cds(PO4)2SiO4, and Cds(PO4)zGeO4 the smaller Cd ions have seven ligands, the larger Na ion in NaCd4(PO4)3 again is coordinated by 8 oxygen atoms. In all cases o f 8-coordination, the eighth l igand is an 0 2 atom. A similar situation is found for the M2 position, in Cas(PO4)2SiO 4, Cas(CrO4)3, and Cas(CrO4)l.s(SiO4)l.2 the Ca2 ions have one more l igand than the Cd2 and the

A new mineral from the Ettringer Bellerberg/Eifel 131

Ca2 ions in Cds(PO4)2SiO4, Cds(PO4)2GeO4, and ternesite, respectively, resulting in a seven-coordination instead of a six-coordination in the latter compounds. The additional ligand in all cases is an 03 atom. In contrast, the M3 position is co- ordinated in the same way for all compounds of the group.

The oxygen atoms

As a consequence of the higher coordination of the Ca ions in Cas(PO4)2SiO4, Cas(CrO4)3, and Cas(CrO4)1.8(SiO4)l.2 the oxygen atoms 02 and 03 have one ligand more than in the other compounds, resulting in a 5- and 4-coordination, respectively. As both 02 and 03 are ligands of the T(1)O4 tetrahedra only, the average coordination number of the oxygen atoms of both the T(1)O4 and T(2)O4 tetrahedra is four. The bond lengths between the central atom and the ligands in a coordination polyhedron depends strongly on the coordination number of the lig- ands, so the size of the T(1)O4 and T(2)O4 tetrahedra should be more similar in the Ca-containing Cas(PO4)2SiO4, Cas(CrO4)3, and Ca5(CrO4)l.8(SiO4)l. 2. This may be the reason for the higher disorder of the tetrahedrally coordinated atoms. The reason, why ternesite shows a fully ordered occupation of the tetrahedra though it also contains Ca may be the large difference between the effective ionic radii of Si (0.27 A) and S (0.12 A) (ionic radii after Shannon, 1976).

Acknowledgements

Samples of ternesite were kindly provided by Mr. B. Ternes, Mayen, Germany. Thanks are due to Dr. G. Giester, Dr. Ch. Lengauer, Dr. E. Libowitzky, and Mr. P. Nagl for fruitful discussions and help with experimental procedures. Financial support by JCPDS/ICDD, Newtown Square, PA, USA under grant 90-03 to ET is gratefully acknowledged.

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Authors' addresses: E. Irran, Laboratorium ftir Anorganische Chemie I, Universit~it Bayreuth, Universit~itsstrasse 30, D-95440 Bayreuth, Federal Republic of Germany; E. TiIlmanns, Institut ftir Mineralogie und Kristallographie der Universit/it, Geozentrum, Althanstrasse 14, A-1090 Wien, Austria; G. Hentschel, Schillerstrasse 1, D-53489 Sinzig 2, Federal Republic of Germany.

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