Abstract

The structure of a synthetic clinopyroxene of composition (Na0.5Ca0.5)(Cr0.5Mg0.5)Si2O6 was investigated at room and high temperature. At room temperature a high resolution data collection (2𝛉 ≤ 110°) was done; at high temperature the data were collected up to 2𝛉 ≤ 50° and at T = 25, 200, 400 and 620°C.

The observed symmetry was always C2/c, and no evidence of a reduction to the long range ordered P21/n symmetry was found. The analysis of the evolution of the Beqvs T suggests instead the presence of local clustering of NaCr and CaMg pairs.

A comparison with the high temperature behaviour of diopside, kosmochlor and jadeite shows a significant decrease in volume expansion with the coupled NaM3+ substitution for CaMg. This is mainly due to a decrease in the 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 higher stiffness of the O1A1 - O1B1 shared edge between the M1 polyhedra.

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