Schaurteite, Ca₃Ge(SO₄)₂(OH)₆·3H₂O

This report presents the first crystal structure determination of the mineral schaurteite, ideally Ca₃Ge(SO₄)₂(OH)₆·3H₂O, tricalcium germanium bis­(sulfate) hexa­hydroxide trihydrate. This single-crystal X-ray diffraction study investigated a natural sample from the type locality at Tsumeb, Namibia. Schaurteite is a member of the fleischerite group of minerals, which also includes fleischerite, despujolsite, and mallestigite. The structure of schaurteite consists of slabs of Ca(O,OH,H₂O)₈ polyhedra (site symmetry mm2) inter­leaved with a mixed layer of Ge(OH)₆ octa­hedra (-3m.) and SO₄ tetra­hedra (3m.). There are two H atoms in the asymmetric unit, both located by full-matrix refinement, and both forming O—H⋯O hydrogen bonds.

Related literature

For the original description of schaurteite, see: Strunz & Tennyson (1967). For descriptions of related minerals: fleischerite (Frondel & Strunz, 1960); despujolsite (Gaudefroy et al., 1968); mallestigite (Sima et al., 1996). For structural refinements of related minerals: despujolsite (Barkley et al., 2011); fleischerite (Otto, 1975). For analysis of anisotropic displacement parameters, see: Downs (2000).

Experimental

Crystal data

• Ca₃Ge(SO₄)₂(OH)₆(H₂O)₃

• M_r = 541.05

• Hexagonal, P 6₃ /m m c

• a = 8.5253 (4) Å

• c = 10.8039 (6) Å

• V = 680.03 (6) ų

• Z = 2

• Mo Kα radiation

• μ = 3.79 mm⁻¹

• T = 296 K

• 0.05 × 0.03 × 0.03 mm

Data collection

• Bruker APEXII CCD diffractometer

• Absorption correction: multi-scan (SADABS; Sheldrick, 2005) T_min = 0.656, T_max = 0.747

• 17243 measured reflections

• 536 independent reflections

• 456 reflections with I > 2σ(I)

• R_int = 0.061

Refinement

• R[F² > 2σ(F²)] = 0.022

• wR(F²) = 0.045

• S = 1.15

• 536 reflections

• 35 parameters

• All H-atom parameters refined

• Δρ_max = 0.40 e Å⁻³

• Δρ_min = −0.42 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O3—H1⋯O2^i	0.73 (3)	2.12 (3)	2.823 (2)	164 (3)
O4—H2⋯O1^ii	0.79 (3)	2.04 (3)	2.789 (2)	158 (3)
Symmetry codes: (i) y, x, -z; (ii) .

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

Schaurteite, Ca₃Ge(SO₄)₂(OH)₆.3H₂O, is a rare germanium mineral found in oxidized germanite ores at the Tsumeb Corporation mine, Tsumeb, Namibia (Strunz & Tennyson, 1967). Schaurteite belongs to the fleischerite group of isotypic minerals, along with mallestigite, Pb₃Sb(SO₄)(AsO₄)(OH)₆.3H₂O (Sima et al. 1996), despujolsite, Ca₃Mn⁴⁺(SO₄)₂(OH)₆.3H₂O (Gaudefroy et al. 1968), and fleischerite, Pb₃Ge(SO₄)₂(OH)₆.3H₂O (Frondel & Strunz, 1960). Of these four minerals, only despujolsite (Barkley et al. 2011) and synthetic fleischerite (Otto, 1975) have reported structures. This study represents the first structural report for schaurteite.

The crystal structure of schaurteite consists of slabs of Ca(OH)₄O₂(H₂O)₂ polyhedra (mm. symmetry), interconnected by mixed layers of Ge(OH)₆ octahedra (3m. symmetry) and SO₄ tetrahedra (3m. symmetry) (Figures 1,2). The mean Ca—O, Ge—O, and S—O bond lengths are 2.487 Å, 1.895 Å, and 1.468 Å, respectively. There are two separate hydrogen atoms, H1 bonded to the O3 atom coordinating Ge, and H2 bonded to the O4 atom (mm. symmetry) which generates an H₂O molecule. Both H atoms form hydrogen bonds, O3—H1···O2 and O4—H2···O1 (Figure 3), mimicking those seen in despujolsite (Barkley et al. 2011).

In the original description of schaurteite, Strunz and Tennyson (1967) noted systematic absences in X-ray photographs consistent with three different space groups: P6₃/mmc, P6₃mc, and P62c. Supposing an isostructural relationship with despujolsite (Barkley et al. 2011), this study initially refined schaurteite in space group P62c with a favorable R_obs of 0.0251 for 45 parameters. Despite the relatively low R factor, the positional parameters for H2 did not converge, and a twin model showed a racemic component of 47%, indicating centrosymmetry in the structure. In space group P6₃/mmc, the structural refinement converged rapidly for all atomic coordinates including those for H, and R_obs dropped to .0219 for 35 parameters. Consequently, this study proposes P6₃/mmc symmetry for schaurteite, in contrast to P62c symmetry reported for both fleischerite (Otto, 1975) and despujolsite (Barkley et al. 2011). Curiously, schaurteite, Ca₃Ge(SO₄)₂(OH)₆.3H₂O, and fleischerite, Pb₃Ge(SO₄)₂(OH)₆.3H₂O, are both single locality mineral species occurring at the same deposit, notably lacking significant solid solution between Pb and Ca in their reported analyses (Strunz and Tennyson, 1967; this study; Frondel and Strunz, 1960). If unbonded lone pair electrons belonging to Pb in fleischerite disrupt the centrosymmetry seen in schaurteite, this would explain an ostensibly limited solid solution between fleischerite and schaurteite.

Previous studies of fleischerite group minerals noted relatively large displacement parameters for O atoms in the SO₄ tetrahedron. For this reason, Otto (1975) proposed a split site for the O1 atom in the structure of synthetic fleischerite. When Barkley et al. (2011) reported the structure of the isotypic Mn⁴⁺ analog despujolsite, they also noted large displacement parameters for O1. However, Barkley et al. (2011) proposed a single O1 site for despujolsite after an analysis of the displacement parameters demonstrated that the SO₄ group behaves as a rigid body with significant translation (0.72 Å) and libration (7.95°). Using translation-libration-screw motion (TLS) modelling software (Downs, 2000), the SO₄ group in schaurteite similarly shows rigid body behavior with 0.73 Å of translation and 7.97° of libration. Consequently, this study proposes a single O1 atom model for schaurteite (Figure 4).

Related literature

For the original description of schaurteite, see: Strunz & Tennyson (1967). For descriptions of related minerals: fleischerite (Frondel & Strunz, 1960); despujolsite (Gaudefroy et al., 1968); mallestigite (Sima et al. 1996). For structural refinements of related minerals: despujolsite (Barkley et al., 2011); fleischerite (Otto, 1975). For analysis of anisotropic displacement parameters, see: Downs (2000).

S2. Experimental

The schaurteite specimen used in this study came from the Tsumeb Corporation mine, Tsumeb, Otavi Mountains, Namibia and remains on deposition (sample R120104) with the RRUFF project (http://www.rruff.info). Schaurteite forms colorless fibrous crystals in a matrix of quartz, calcite, and germanite.

Chemical analyses were performed on a Cameca SX-100 electron microprobe at the Lunar and Planetary Laboratory, University of Arizona. The electron microprobe sample and the single-crystal fragment came from the same parent sample. Like despujolsite (Barkley et al. 2011), schaurteite was fugitive under the electron beam, leading to the choice of the following operating conditions: 20 kV exciting voltage, 6 nA operating current, and a spot size of 5 microns. The average of 13 analyses gave GeO₂ 17.97%, CaO 30.41%, and SO₃ 29.22%. Normalizing to 17 O atoms (which includes 6 moles H₂O per formula unit), the combined empirical and structural chemical formula becomes Ca_3.01Ge_0.95(S_2.03O₈)(OH)₆.3H₂O. Further details of the electron microprobe analysis and formula calculations are available on the RRUFF web site (http://www.rruff.info/R120104).

S3. Refinement

To model possible variable occupancy of cations, site occupancies were allowed to vary without restraints. The results justified a model with full occupancies of Ca, Ge, and S, corroborated by electron microprobe data showing only these elements (with Z > 8). Two hydrogen atom positions were refined without restraints, both with reasonable isotropic displacement parameters.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. Polyhedral view of the schaurteite structure down [100], with slabs of light blue Ca(OH)4O2(H2O)2 polyhedra, interleaved with a mixed layer of green Ge(OH)6 octahedra and yellow SO4 tetrahedra. Unit cell outline shown in black.
	Fig. 2. Polyhedral view of the schaurteite structure down [001], showing a single layer of light blue Ca(OH)4O2(H2O)2 polyhedra, a single mixed layer of green Ge(OH)6 octahedra and yellow SO4 tetrahedra, and dark blue H atoms in both layers.
	Fig. 3. Hydrogen bonding in schaurteite, showing O4—H2···O1 at 2.04 Å and O3—H1···O2 at 2.12 Å.
	Fig. 4. Ellipsoidal view of the schaurteite structure down [001], showing red O atoms, yellow S atoms, green Ge atoms, and light blue Ca atoms. Unit cell outline shown in black. The large circular ellipsoid at [2/3,1/3,z] represents the O1 atom (3 m. symmetry), which forms the apex of a rigid SO4 tetrahedron with significant translation (0.73 Å) and libration (7.97°).

Tricalcium germanium bis(sulfate) hexahydroxide trihydrate

Crystal data

Ca3Ge(SO4)2(OH)6(H2O)3	Dx = 2.642 Mg m^−3
Mr = 541.05	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mmc	Cell parameters from 2603 reflections
Hall symbol: -P 6c 2c	θ = 2.8–28.0°
a = 8.5253 (4) Å	µ = 3.79 mm^−1
c = 10.8039 (6) Å	T = 296 K
V = 680.03 (6) Å^3	Hexagonal prism section, colourless
Z = 2	0.05 × 0.03 × 0.03 mm
F(000) = 544

Data collection

Bruker APEXII CCD diffractometer	536 independent reflections
Radiation source: fine-focus sealed tube	456 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.061
ϕ and ω scans	θmax = 33.1°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 2005)	h = −10→13
Tmin = 0.656, Tmax = 0.747	k = −13→13
17243 measured reflections	l = −16→16

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.022	Hydrogen site location: difference Fourier map
wR(F^2) = 0.045	All H-atom parameters refined
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0131P)^2 + 0.5886P] where P = (Fo^2 + 2Fc^2)/3
536 reflections	(Δ/σ)max < 0.001
35 parameters	Δρmax = 0.40 e Å^−3
0 restraints	Δρmin = −0.42 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ge	0.0000	0.0000	0.0000	0.00762 (11)
Ca	0.30387 (8)	0.15193 (4)	0.2500	0.01107 (12)
S	0.3333	0.6667	0.52599 (7)	0.00963 (16)
O1	0.3333	0.6667	0.1106 (2)	0.0167 (5)
O2	0.4786 (2)	0.23930 (11)	0.06925 (14)	0.0190 (3)
O3	0.10006 (10)	0.2001 (2)	0.10982 (13)	0.0106 (3)
O4	0.50772 (16)	0.49228 (16)	0.7500	0.0190 (5)
H1	0.142 (2)	0.284 (4)	0.075 (2)	0.028 (8)*
H2	0.543 (2)	0.457 (2)	0.695 (3)	0.051 (11)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ge	0.00878 (15)	0.00878 (15)	0.0053 (2)	0.00439 (7)	0.000	0.000
Ca	0.0098 (3)	0.0131 (2)	0.0091 (2)	0.00491 (13)	0.000	0.000
S	0.0108 (2)	0.0108 (2)	0.0073 (3)	0.00538 (11)	0.000	0.000
O1	0.0217 (9)	0.0217 (9)	0.0067 (11)	0.0109 (4)	0.000	0.000
O2	0.0114 (7)	0.0243 (7)	0.0169 (7)	0.0057 (4)	0.0039 (6)	0.0020 (3)
O3	0.0131 (5)	0.0086 (7)	0.0087 (6)	0.0043 (3)	−0.0004 (3)	−0.0008 (5)
O4	0.0239 (9)	0.0239 (9)	0.0143 (10)	0.0158 (10)	0.000	0.000

Geometric parameters (Å, º)

Ge—O3	1.8949 (14)	Ca—O3^iii	2.4890 (9)
Ge—O3^i	1.8949 (14)	Ca—O3^vii	2.4890 (9)
Ge—O3^ii	1.8949 (14)	Ca—O4^viii	2.6284 (12)
Ge—O3^iii	1.8949 (14)	Ca—O4^ix	2.6284 (12)
Ge—O3^iv	1.8949 (14)	S—O2^x	1.4651 (16)
Ge—O3^v	1.8949 (14)	S—O2^xi	1.4651 (16)
Ca—O2	2.3404 (15)	S—O2^xii	1.4651 (16)
Ca—O2^vi	2.3404 (15)	S—O1^vi	1.475 (3)
Ca—O3	2.4890 (9)	O3—H1	0.73 (3)
Ca—O3^vi	2.4890 (9)	O4—H2	0.79 (3)
O3—Ge—O3^i	180.0	O2—Ca—O3^vii	147.24 (3)
O3—Ge—O3^ii	95.05 (6)	O2^vi—Ca—O3^vii	79.95 (4)
O3^i—Ge—O3^ii	84.95 (6)	O3—Ca—O3^vii	105.61 (6)
O3—Ge—O3^iii	84.95 (6)	O3^vi—Ca—O3^vii	61.87 (7)
O3^i—Ge—O3^iii	95.05 (6)	O3^iii—Ca—O3^vii	74.96 (5)
O3^ii—Ge—O3^iii	180.00 (10)	O2—Ca—O4^viii	73.04 (3)
O3—Ge—O3^iv	95.05 (6)	O2^vi—Ca—O4^viii	73.04 (3)
O3^i—Ge—O3^iv	84.95 (6)	O3—Ca—O4^viii	83.33 (5)
O3^ii—Ge—O3^iv	84.95 (6)	O3^vi—Ca—O4^viii	83.33 (5)
O3^iii—Ge—O3^iv	95.05 (6)	O3^iii—Ca—O4^viii	139.12 (3)
O3—Ge—O3^v	84.95 (6)	O3^vii—Ca—O4^viii	139.12 (3)
O3^i—Ge—O3^v	95.05 (6)	O2—Ca—O4^ix	73.04 (3)
O3^ii—Ge—O3^v	95.05 (6)	O2^vi—Ca—O4^ix	73.04 (3)
O3^iii—Ge—O3^v	84.95 (6)	O3—Ca—O4^ix	139.12 (3)
O3^iv—Ge—O3^v	180.00 (14)	O3^vi—Ca—O4^ix	139.12 (3)
O2—Ca—O2^vi	113.10 (8)	O3^iii—Ca—O4^ix	83.33 (5)
O2—Ca—O3	79.95 (4)	O3^vii—Ca—O4^ix	83.33 (5)
O2^vi—Ca—O3	147.24 (3)	O4^viii—Ca—O4^ix	116.09 (10)
O2—Ca—O3^vi	147.24 (3)	O2—Ca—Ge	73.16 (4)
O2^vi—Ca—O3^vi	79.95 (4)	O2^x—S—O2^xi	110.32 (7)
O3—Ca—O3^vi	74.96 (5)	O2^x—S—O2^xii	110.32 (7)
O2—Ca—O3^iii	79.95 (4)	O2^xi—S—O2^xii	110.32 (7)
O2^vi—Ca—O3^iii	147.24 (3)	O2^x—S—O1^vi	108.61 (7)
O3—Ca—O3^iii	61.87 (7)	O2^xi—S—O1^vi	108.60 (7)
O3^vi—Ca—O3^iii	105.61 (6)	O2^xii—S—O1^vi	108.60 (7)

Symmetry codes: (i) −x, −y, −z; (ii) x−y, x, −z; (iii) −x+y, −x, z; (iv) y, −x+y, −z; (v) −y, x−y, z; (vi) x, y, −z+1/2; (vii) −x+y, −x, −z+1/2; (viii) −x+1, −y+1, −z+1; (ix) y, −x+y, −z+1; (x) y, −x+y+1, z+1/2; (xi) −x+1, −y+1, z+1/2; (xii) x−y, x, z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H1···O2^xiii	0.73 (3)	2.12 (3)	2.823 (2)	164 (3)
O4—H2···O1^xiv	0.79 (3)	2.04 (3)	2.789 (2)	158 (3)

Symmetry codes: (xiii) y, x, −z; (xiv) y, −x+y, z+1/2.