The crystal structure of CaFe3+SiAlO6 and the crystal chemistry of Fe3+—Al3+ substitution in calcium Tschermak's pyroxene

The crystal structure of a synthetic CaFe³⁺Al-SiO₆ pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe³⁺ and 0.18 Al³⁺. Within the tetrahedral T site, Si⁴⁺ (0.50), Al³⁺ (0.41) and Fe³⁺ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe³⁺ ions with respect to Al³⁺ ions in CaFe³⁺AlSiO₆ is about 10 kcal/mole, which is much higher than that found in Y₃Al _x Fe_5–2O₁₂ garnets. Topologically the structure of CaFe³⁺AlSiO₆ is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO₆. For CaM³⁺ AlSiO₆ clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.