Compositions and unit-cell parameters of synthetic non-stoichiometric tschermakitic clinopyroxenes

Clinopyroxenes with substantial numbers of M1 and М2 vacancies have been synthesized at 1300-1450°C and 25.0-32.3 kbar from compositions on the joins CaAl₂SiO₆-SiO₂ and [(CaAl₂SiO₆)_0.7(CaMgSi₂O₆)_0.3]-SiO₂. The non-stoichiometry, which is a stable phenomenon, is of much greater extent in clinopyroxenes on the latter join. In a composition which yielded clinopyroxene and quartz the pyroxene has the formula Ca^M2_0.95(Mg_0.28Al_0.68)_M1(Al_0.49Si_1.51)^TO₆ with a small predominance of М2 over M1 vacancies. If anorthite and quartz coexist with the pyroxene the vacancies are exclusively in М2.

Powder diffraction patterns of the product clinopyroxenes were indexed in the space group C2/c. A stoichiometric clinopyroxene with an atomic Al/(Al + Mg) ratio of 0.81 gave the cell parameters: a = 9.635 (1), b = 8.722 (2), с = 5.2815 (4) A, β = 106.18(1)°, V = 426.26 (6) A³. A clinopyroxene of the same Al/(Al+Mg) ratio but with substantial Ml and М2 vacancies (formula above) has a considerably smaller cell volume: a = 9.588 (2), b = 8.720 (2), с = 5.2638 (6) A, β = 106.45 (2)°, V = 422.10 (8) A.

The degree of non-stoichiometry increases with increasing pressure due to two factors. Firstly, the non-stoichiometric pyroxenes have relatively small cell volumes and, secondly, increasing pressure favors increasing Al₂O₃ solubility in the pyroxene and aluminous clinopyroxene can more readily accept Ml and М2 vacancies than alumina-free pyroxenes.