High-temperature crystal chemistry of C2/c clinopyroxenes along the join CaMgSi2O6-CaAl2SiO6

Single-crystal in-situ data have been collected at high temperature on synthetic clinopyroxenes of composition Di(70)CaTs(30) and Di(50)CaTs(50). Sample Di(70)CaTs(30) was refined at 25, 300, 500 and 700 degrees C and sample Di(50)CaTs(50) at 700 degrees C. A strong deformation with temperature in the M2 polyhedron is induced by a shift in the tetrahedral chains along z, decreasing the M2 polyhedral coordination, similar to the high temperature behaviour of pure diopside and other C2/c clinopyroxenes. This deformation is opposite to that produced at room temperature by the entrance of CaTs into the diopside structure. The amount of the deformation in the M2 polyhedron is comparable to that observed in pure diopside and lower than in Ca-poorer clinopyroxene. The behaviour of B-eq vs T suggests the presence of positional disorder in the studied samples, mostly affecting the oxygen atoms.