High temperature single crystal investigation in a clinopyroxene of composition (Na (sub 0.5) Ca (sub 0.5) )(Cr (sub 0.5) Mg (sub 0.5) )Si (sub 2) O (sub 6)

The spectrum of a synthetic clinopyroxene of composition (Na (sub 0.5) Ca (sub 0.5) )(Cr (sub 0.5) Mg (sub 0.5) )Si (sub 2) O (sub 6) was investigated at room and high temperature. At room temperature, a high resolution data collection was done; at high temperature, data was collected up to 2theta < or = 50 degrees and at T = 25, 200, 400 and 620 degrees C. The observed symmetry was always C2/c, and no evidence of a reduction to the long range ordered P2 (sub 1) /n symmetry was found. The presence of local clusters of NaCr and CaMg pairs is suggested. Comparison with the high-temperature evolution of diopside, kosmochlor and jadeite shows a significant decrease in volume expansion with the coupled NaM (super 3+) substitution for CaMg. This is mainly due to a decrease in expansion along the b axis, and is related to a smaller expansion in the M1 polyhedron for the substitution of a trivalent cation in the M1 site and to the consequent greater stiffness of the O1A1-O1B1 shared edge between the M1 polyhedra.