A comparison of the clinopyroxene compounds CaZnSi₂O₆ and CaZnGe₂O₆

Single crystals of CaZnSi₂O₆ (calcium zinc silicate) and CaZnGe₂O₆ (calcium zinc germanate) were synthesized at 1623 K and 2.5 GPa by slow cooling of the melts from 1473 K. Structure solution using Patterson methods revealed the two compounds to be isomorphous and thus isostructural. They adopt the clinopyroxene structure type with space group C2/c. The substitution of Ge⁴⁺ for Si⁴⁺ increases the distortion of the tetrahedra and octahedra. The increased size of the tetrahedral GeO₄ chain is mainly compensated by (i) increasing the kinking of the tetrahedral chain and (ii) lengthening the Zn-O bonds.

S1. Comment

The title compounds belong to the common clinopyroxene structure type with a general structural formula of M2M1T₂O₆ (M and T are octahedral and tetrahedral cations, respectively). Several of the most important Fe- and Mg-bearing rock-forming minerals occur within this mineral group, among them CaFeSi₂O₆ (hedenbergite) and CaMgSi₂O₆ (diopside). CaZnSi₂O₆ (petedunnite) is one of the less common clinopyroxene minerals and was first described by Essene & Peacor (1987), who only reported the lattice parameters. The three-dimensional crystal structure was determined later from a synthetic sample (Ohashi et al., 1996). We present here a new refinement of the crystal structure of synthetic CaZnSi₂O₆ and compare the results with the analogous germanate, CaZnGe₂O₆, whose structure has been determined for the first time.

CaZnSi₂O₆ and CaZnGe₂O₆ crystallize in the C2/c space group at room temperature and adopt the general structural topology of the clinopyroxenes. This consists of infinite chains of corner-sharing TO₄ tetrahedra (T = Si⁴⁺ or Ge⁴⁺) running parallel to the c axis, zigzag chains of edge-sharing M1O₆ octahedra (M1 = Zn²⁺) and eight-coordinate M2 sites hosting Ca²⁺ ions (Fig. 1). A more detailed discussion and a polyhedral representation of the clinopyroxene structure type is given by Clark et al., 1969), Cameron & Papike (1986) or, more recently, Redhammer & Roth (2004).

The average M1—O bond length in CaZnSi₂O₆ is smaller than that in CaFeSi₂O₆ but larger than that in CaMSi₂O₆ (M = Ni, Mg and Co; Table 3), reflecting the differences in M1 ionic radii. The distortion parameters, however, do not exhibit a common trend. Among the Ca silicate clinopyroxenes, the Zn²⁺ compound shows the largest bond-length distortion (BLD) and a large octahedral angle variance (Table 3). The M1 octahedra in CaZnSi₂O₆ and CaCoSi₂O₆ can be readily compared, as both compounds have similar M1 ionic radii and almost identical average M1—O bond lengths (Table 3). The large distortion of the M1 octahedron in CaZnSi₂O₆ corresponds to a (4 + 2) coordination by oxygen and is consistent with the fact that Zn²⁺ is more commonly found in tetrahedral coordination. Replacing Si⁴⁺ by Ge⁴⁺ causes a distinct increase of the Zn—O bond lengths, a doubling of the octahedral angle variance, and an increase in bond- and edge-length distortion (Table 3). The largest increase in bond length upon Si⁴⁺/Ge⁴⁺ substitution occurs for the Zn—O1(−x,1 − y, −z) bond, which is aligned parallel to the c axis. The stretching of this bond reflects the increased size of the tetrahedral chain, also running along the c axis.

The Ca²⁺ ion at the M2 site is in an eightfold coordination in the Ca clinopyroxene series, and there is no large structural change either upon changing the M1 cation or upon replacing Si⁴⁺ by Ge⁴⁺. The tetrahedron in CaZnSi₂O₆ is the same within experimental error as in other silicate Ca clinopyroxenes. The distortion parameters are quite large and show that the SiO₄ tetrahedra exhibit large deviations from ideal geometry, e.g. a large tetrahedral angle variance (TAV), mostly as a result of an elongation along the a axis (angle τ in Table 3). In all Ca clinopyroxenes, the tetrahedral chains are kinked to accommodate the size difference between the M1 octahedra and the tetrahedra. In CaZnSi₂O₆, the tetrahedral bridging angle [165.3 (1)°] compares well with that in CaCoSi₂O₆ but is smaller than in CaMgSi₂O₆. A decrease of this bridging angle with increasing size of the M1 cation can be observed (Table 3).

The replacement of Si⁴⁺ by Ge⁴⁺ in the Zn²⁺ clinopyroxene results in an increase of the average T—O bond length by 0.122 Å, which is close to the difference in ionic radius between Si⁴⁺ and Ge⁴⁺ (0.14 Å; Shannon & Prewitt, 1969). The GeO₄ tetrahedron appears to be more elongated along the a axis, resulting in a tetrahedral angle variance which is almost twice as large (Table 3). The tetrahedral bridging angle decreases from 165.3 (1)° in the silicate to 158.5 (1)° in the germanate in order to match the larger GeO₄ tetrahedra to the chain of ZnO₆ octahedra. Obviously this mechanism is not sufficient, since a distinct lengthening of the Zn—O bonds is also observed. Finally, the increase of the average T—O bond length is not as large as might be expected from the ionic radii alone. Besides increasing the kinking of the tetrahedral chains and the Zn—O bond length, this can be seen as a third mechanism to maintain size compatibility between the tetrahedral and octahedral chains.

Our lattice parameters of CaZnSi₂O₆ are similar to those given in the literature (Essene & Peacor, 1987; Ohashi et al., 1996; Huber et al., 2004). The latter authors have determined the lattice parameters of petedunnite, but no additional structure information. The substitution Ge⁴⁺ for Si⁴⁺ causes an increase of the unit-cell volume by 8.3%, which is mainly due to a lengthening of the a and c parameters by 3.78% (0.37 Å) and 3.54% (0.19 Å), respectively. The b lattice parameter increases only slightly (0.35%). The small expansion along b can be explained by the larger kinking of the tetrahedral chain in the germanate structure. As the tetrahedral chains run parallel to the c axis and the tetrahedral apices point towards the a axis, the large expansions of the unit-cell along these directions directly reflect the replacement of Si⁴⁺ by Ge⁴⁺.

S2. Experimental

Starting materials were prepared by mixing CaCO₃, ZnO, and SiO₂ or GeO₂ in the exact stoichiometry of the compounds. The oxide mixture of CaZnSi₂O₆ was placed in a small Pt tube (5 mm long, inner diameter 3 mm), welded tight at both ends and transferred to the piston-cylinder apparatus of the Institute of Crystallography, RWTH Aachen. The synthesis was performed at 1623 K and 2.5 GPa and yielded small transparent crystals up to 100 µm in size with a short prismatic habit. CaZnGe₂O₆ was produced by slow cooling from the melt. The starting material was placed in an open Pt crucible and heated slowly to 1473 K. This temperature was maintained for 24 h before cooling slowly to 1073 K at a rate of 0.5 K min⁻¹. Large crystals of up to 1 mm in size were recovered.

S3. Refinement

A second crystal of CaZnGe₂O₆ was investigated by single-crystal X-ray diffration and gave atomic coordinates and structural parameters that are the same within experimental error. Lattice parameters for CaZnGe₂O₆ were also refined from a powder (obtained by crushing several single crystals) using whole pattern fitting and are the same as those from single-crystal data with experimental error.

Computing details

For both compounds, data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Diamond (Brandenburg & Berndt, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

Fig. 1. Part of the CaZnGe2O6 structure at 298 K, with displacement ellipsoids drawn at the 90% probability level. [symmetry codes: (i) x,-y,1/2 + z; (ii) 1/2 − x,1/2 − y,-z; (iii) 1 − x, y, 1/2 − z; (iv) 1/2 + x,1/2 + y,z; (v) 1/2 + x,1/2 − y,1/2 + z; (vi) 1/2 − x, 1/2 − y, 1/2 − z; (vii) −1/2 + x,1/2 − y,-1/2 + z; (viii) −x,y,1/2 − z; (ix) −1/2 + x,1/2 + y,z; (x) −1/2 + x,1/2 − y,1/2 + z; (xi) 1/2 − x,1/2 + y,1/2 − z; (xii) 1/2 − x,1/2 − y,1 − z.]

(I)

Crystal data

CaZnSi2O6	F(000) = 504
Mr = 257.63	Dx = 3.856 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1256 reflections
a = 9.7955 (8) Å	θ = 2.1–28.2°
b = 8.9781 (8) Å	µ = 7.18 mm^−1
c = 5.251 (6) Å	T = 298 K
β = 106.033 (7)°	Cuboid, colourless
V = 443.8 (5) Å^3	0.08 × 0.07 × 0.06 mm
Z = 4

Data collection

STOE IPDS-I diffractometer	445 reflections with I > 2σ(I)
Radiation source: sealed X-ray tube	Rint = 0.029
ϕ or ω scans?	θmax = 28.1°, θmin = 3.1°
Absorption correction: numerical via equivalents (X-SHAPE and X-RED; Stoe & Cie 1996)	h = −12→12
Tmin = 0.56, Tmax = 0.66	k = −11→11
2095 measured reflections	l = −6→6
527 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0364P)^2 + 0.1449P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.021	(Δ/σ)max = 0.001
wR(F^2) = 0.058	Δρmax = 1.52 e Å^−3
S = 1.05	Δρmin = −0.58 e Å^−3
527 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
48 parameters	Extinction coefficient: 0.0095 (9)

Special details

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

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

	x	y	z	Uiso*/Ueq
Ca	0	0.29913 (8)	0.25	0.00825 (18)
Zn	0.5	0.40553 (4)	0.25	0.00613 (14)
Si	0.28648 (7)	0.09259 (7)	0.22949 (14)	0.00389 (17)
O1	0.1166 (2)	0.08882 (19)	0.1442 (4)	0.0057 (4)
O2	0.3603 (2)	0.2480 (2)	0.3220 (3)	0.0080 (4)
O3	0.35015 (19)	0.0188 (2)	−0.0063 (3)	0.0062 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.0084 (4)	0.0102 (4)	0.0047 (3)	0	−0.0006 (3)	0
Zn	0.0059 (2)	0.0076 (2)	0.0046 (2)	0	0.00094 (15)	0
Si	0.0034 (3)	0.0054 (3)	0.0032 (3)	−0.0002 (3)	0.0014 (2)	−0.0006 (2)
O1	0.0038 (9)	0.0087 (8)	0.0047 (8)	0.0002 (7)	0.0014 (6)	0.0004 (6)
O2	0.0091 (10)	0.0076 (9)	0.0078 (9)	−0.0027 (7)	0.0031 (7)	0.0001 (6)
O3	0.0044 (9)	0.0092 (9)	0.0050 (8)	−0.0009 (7)	0.0015 (7)	−0.0015 (6)

Geometric parameters (Å, º)

Zn—O1^i	2.070 (3)	Ca—O1	2.3512 (19)
Zn—O1^ii	2.070 (3)	Ca—O3^v	2.601 (2)
Zn—O2	2.0739 (18)	Ca—O3^ix	2.601 (2)
Zn—O2^iii	2.0739 (18)	Ca—O3^i	2.739 (2)
Zn—O1^iv	2.1613 (19)	Ca—O3^x	2.739 (2)
Zn—O1^v	2.1613 (19)	Si—O2	1.5850 (19)
Ca—O2^vi	2.325 (3)	Si—O1	1.600 (2)
Ca—O2^vii	2.325 (3)	Si—O3^xi	1.684 (2)
Ca—O1^viii	2.3512 (19)	Si—O3	1.670 (2)
O1^i—Zn—O1^ii	177.19 (10)	O1—Ca—O3^ix	136.73 (7)
O1^i—Zn—O2	89.39 (8)	O3^v—Ca—O3^ix	81.37 (8)
O1^ii—Zn—O2	92.52 (8)	O2^vi—Ca—O3^i	84.57 (8)
O2—Zn—O2^iii	94.03 (10)	O2^vii—Ca—O3^i	108.18 (7)
O1^i—Zn—O1^iv	84.78 (8)	O1^viii—Ca—O3^i	161.37 (6)
O1^ii—Zn—O1^iv	93.08 (8)	O1—Ca—O3^i	90.63 (6)
O2—Zn—O1^iv	171.15 (7)	O3^v—Ca—O3^i	59.37 (6)
O2^iii—Zn—O1^iv	92.88 (7)	O3^ix—Ca—O3^i	66.69 (7)
O2^iii—Zn—O1^v	171.15 (7)	O2^vi—Ca—O3^x	108.18 (7)
O1^iv—Zn—O1^v	80.82 (10)	O2^vii—Ca—O3^x	84.57 (8)
O2^vi—Ca—O2^vii	159.01 (10)	O1^viii—Ca—O3^x	90.63 (6)
O2^vi—Ca—O1^viii	83.54 (8)	O1—Ca—O3^x	161.37 (6)
O2^vii—Ca—O1^viii	79.62 (7)	O3^v—Ca—O3^x	66.69 (7)
O2^vi—Ca—O1	79.62 (7)	O3^ix—Ca—O3^x	59.37 (6)
O2^vii—Ca—O1	83.54 (8)	O3^i—Ca—O3^x	106.72 (8)
O1^viii—Ca—O1	73.15 (10)	O2—Si—O1	117.10 (10)
O2^vi—Ca—O3^v	137.38 (6)	O2—Si—O3^xi	103.70 (10)
O2^vii—Ca—O3^v	62.64 (6)	O1—Si—O3^xi	109.76 (10)
O1^viii—Ca—O3^v	136.73 (7)	O2—Si—O3	110.14 (10)
O1—Ca—O3^v	119.33 (6)	O1—Si—O3	110.90 (10)
O2^vi—Ca—O3^ix	62.64 (6)	O3^xi—Si—O3	104.25 (9)
O2^vii—Ca—O3^ix	137.38 (6)	Si^xii—O3—Si	135.84 (13)
O1^viii—Ca—O3^ix	119.33 (6)

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

(II)

Crystal data

CaZnGe2O6	F(000) = 648
Mr = 346.63	Dx = 4.791 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1520 reflections
a = 10.1659 (8) Å	θ = 2.1–28.2°
b = 9.0096 (7) Å	µ = 18.40 mm^−1
c = 5.4369 (4) Å	T = 298 K
β = 105.181 (4)°	Cuboid, colourless
V = 480.59 (6) Å^3	0.15 × 0.12 × 0.11 mm
Z = 4

Data collection

STOE IPDS-I diffractometer	532 reflections with I > 2σ(I)
Radiation source: sealed X-ray tube	Rint = 0.040
ϕ and ω scans?	θmax = 28.2°, θmin = 3.1°
Absorption correction: numerical via equivalents (X-SHAPE and X-RED; Stoe & Cie 1996)	h = −13→13
Tmin = 0.079, Tmax = 0.135	k = −11→10
1919 measured reflections	l = −7→7
572 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0389P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.025	(Δ/σ)max < 0.001
wR(F^2) = 0.057	Δρmax = 1.11 e Å^−3
S = 1.07	Δρmin = −0.88 e Å^−3
572 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
48 parameters	Extinction coefficient: 0.0352 (13)

Special details

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

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

	x	y	z	Uiso*/Ueq
Ca	0	0.30391 (9)	0.25	0.00883 (19)
Zn	0.5	0.40829 (5)	0.25	0.00854 (16)
Ge	0.28474 (3)	0.09791 (3)	0.22624 (5)	0.00586 (15)
O1	0.1094 (2)	0.0924 (2)	0.1355 (4)	0.0077 (4)
O2	0.36254 (19)	0.2616 (2)	0.3414 (4)	0.0110 (4)
O3	0.35821 (18)	0.0286 (2)	−0.0161 (4)	0.0091 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.0095 (4)	0.0084 (4)	0.0078 (3)	0	0.0009 (3)	0
Zn	0.0086 (3)	0.0093 (3)	0.0071 (3)	0	0.00108 (19)	0
Ge	0.0051 (2)	0.0067 (2)	0.00540 (19)	−0.00012 (8)	0.00073 (13)	−0.00014 (9)
O1	0.0058 (9)	0.0086 (10)	0.0087 (9)	−0.0005 (6)	0.0016 (7)	0.0001 (7)
O2	0.0135 (9)	0.0083 (9)	0.0107 (9)	−0.0050 (8)	0.0026 (8)	−0.0019 (8)
O3	0.0075 (9)	0.0124 (10)	0.0079 (9)	−0.0014 (7)	0.0030 (7)	−0.0033 (7)

Geometric parameters (Å, º)

Ca—O2^i	2.3687 (19)	Zn—O2^viii	2.076 (2)
Ca—O2^ii	2.3687 (19)	Zn—O1^iv	2.100 (2)
Ca—O1	2.370 (2)	Zn—O1^ix	2.100 (2)
Ca—O1^iii	2.370 (2)	Zn—O1^x	2.177 (2)
Ca—O3^iv	2.633 (2)	Zn—O1^vi	2.177 (2)
Ca—O3^v	2.633 (2)	Ge—O2	1.713 (2)
Ca—O3^vi	2.679 (2)	Ge—O1	1.721 (2)
Ca—O3^vii	2.679 (2)	Ge—O3	1.788 (2)
Zn—O2	2.076 (2)	Ge—O3^xi	1.809 (2)
O2^i—Ca—O2^ii	151.12 (10)	O2^ii—Ca—O3^vii	142.82 (7)
O2^i—Ca—O1	75.84 (7)	O1—Ca—O3^vii	133.54 (7)
O2^ii—Ca—O1	81.01 (7)	O1^iii—Ca—O3^vii	121.75 (6)
O2^i—Ca—O1^iii	81.01 (7)	O3^iv—Ca—O3^vii	65.87 (7)
O2^ii—Ca—O1^iii	75.84 (7)	O3^v—Ca—O3^vii	62.78 (3)
O1—Ca—O1^iii	73.00 (10)	O3^vi—Ca—O3^vii	81.82 (9)
O2^i—Ca—O3^iv	87.31 (7)	O2—Zn—O2^viii	100.94 (12)
O2^ii—Ca—O3^iv	109.44 (6)	O2—Zn—O1^iv	91.32 (8)
O1—Ca—O3^iv	88.85 (7)	O2^viii—Zn—O1^iv	88.45 (7)
O1^iii—Ca—O3^iv	160.31 (7)	O1^iv—Zn—O1^ix	179.65 (10)
O2^i—Ca—O3^v	109.44 (6)	O2—Zn—O1^x	168.48 (8)
O2^ii—Ca—O3^v	87.31 (7)	O2^viii—Zn—O1^x	89.43 (8)
O1—Ca—O3^v	160.31 (7)	O1^iv—Zn—O1^x	83.90 (8)
O1^iii—Ca—O3^v	88.85 (7)	O1^ix—Zn—O1^x	96.37 (7)
O3^iv—Ca—O3^v	110.07 (10)	O1^x—Zn—O1^vi	80.72 (11)
O2^i—Ca—O3^vi	142.82 (7)	O2—Ge—O1	118.29 (10)
O2^ii—Ca—O3^vi	65.18 (6)	O2—Ge—O3	109.14 (10)
O1—Ca—O3^vi	121.75 (6)	O1—Ge—O3	112.31 (9)
O1^iii—Ca—O3^vi	133.54 (7)	O2—Ge—O3^xi	101.66 (9)
O3^iv—Ca—O3^vi	62.78 (3)	O1—Ge—O3^xi	112.98 (9)
O3^v—Ca—O3^vi	65.87 (7)	O3—Ge—O3^xi	100.55 (6)
O2^i—Ca—O3^vii	65.18 (6)	Ge—O3—Ge^xii	128.54 (11)

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