High temperature single crystal investigation in a clinopyroxene of composition (Na0.5Ca0.5)(Cr0.5Mg0.5)Si2O6
Tribaudino, Mario; Nestola, Fabrizio; Ohashi, Haruo
European Journal of Mineralogy Volume 17 Number 2 (2005), p. 297 - 304
published: Apr 29, 2005
ArtNo. ESP147051702013, Price: 29.00 €
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 Beq vs 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 NaM 3+ 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.