Acta Crystallographica Section B

Structural Science

Volume 64, Part 3 (June 2008)

research papers

Acta Cryst. (2008). B64, 261-271 [ doi:10.1107/S0108768108010434 ]

On the crystal chemistry of olivine-type germanate compounds, Ca_1 + xM_1 - xGeO₄ (M²⁺ = Ca, Mg, Co, Mn)

G. J. Redhammer, G. Roth, G. Amthauer and W. Lottermoser

Abstract: Germanate compounds, CaMGeO₄ with M²⁺ = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298 K and adopt the olivine structure. The M2 site is exclusively occupied by Ca²⁺, while on M1 both Ca²⁺ and M²⁺ cations are found. The amount of Ca²⁺ on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1 atoms per formula unit) and the largest in the Mn compound (0.20 atoms per formula unit), while in Ca₂GeO₄, also with olivine structure, both sites are completely filled with Ca²⁺. When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b has essentially the same values, regardless of the tetrahedral cation, meaning that b is independent of the tetrahedral cation. Structural variations on the octahedrally coordinated M1 site are largely determined by the size of the M1 cation, the average M1-O bond lengths being identical in Ca silicate and Ca germanate olivine. Increasing the size of the M1 cation induces an increasing polyhedral distortion, expressed by the parameters bond-length distortion, octahedral angle variance and octahedral quadratic elongation. However, the Ca germanate olivine compounds generally have more regular octahedra than the analogous silicates. The octahedrally coordinated M2 site does not exhibit large variations in structural parameters as a consequence of the constant chemical composition; the same is valid for the tetrahedral site.

Keywords: olivine; germanates.


	Structure factor file (CIF format) (25.0 kbytes) [ doi:10.1107/S0108768108010434/ws5059Ca2Co_2sup2.hkl ] Contains datablock ca2co_2

	Structure factor file (CIF format) (27.1 kbytes) [ doi:10.1107/S0108768108010434/ws5059cfg_rotsup3.hkl ] Contains datablock cfg_rot

	Structure factor file (CIF format) (29.3 kbytes) [ doi:10.1107/S0108768108010434/ws5059cazn_bsup4.hkl ] Contains datablock cazn_b

	Structure factor file (CIF format) (27.5 kbytes) [ doi:10.1107/S0108768108010434/ws5059camn_a5sup5.hkl ] Contains datablock camn_a5

	Structure factor file (CIF format) (27.3 kbytes) [ doi:10.1107/S0108768108010434/ws5059camn_a2sup6.hkl ] Contains datablock camn_a2

	Structure factor file (CIF format) (26.7 kbytes) [ doi:10.1107/S0108768108010434/ws5059camg_a1sup7.hkl ] Contains datablock camg_a1

	Structure factor file (CIF format) (26.1 kbytes) [ doi:10.1107/S0108768108010434/ws5059caco_a2sup8.hkl ] Contains datablock caco_a2

	Structure factor file (CIF format) (26.2 kbytes) [ doi:10.1107/S0108768108010434/ws5059caco_a1sup9.hkl ] Contains datablock caco_a1

	Structure factor file (CIF format) (25.5 kbytes) [ doi:10.1107/S0108768108010434/ws5059ca2mn_1sup10.hkl ] Contains datablock ca2mn_1

	Structure factor file (CIF format) (24.1 kbytes) [ doi:10.1107/S0108768108010434/ws5059ca2mg_1sup11.hkl ] Contains datablock ca2mg_1

	Structure factor file (CIF format) (26.4 kbytes) [ doi:10.1107/S0108768108010434/ws5059ca2ge_1sup12.hkl ] Contains datablock ca2ge_1

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