• journal_title：Mineralogical Magazine
• Contributor：F. Nestola ; A. Madsen ; M. Tribaudino ; T. Balić-Žunić ; H. Ohashi ; L. Secco ; A. Dal Negro
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2008.072.3.809
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：72
• issue：3
• firstpage：809
• section：Articles

The crystal structure of a clinopyroxene with composition (Na_0.75Ca_0.25)(Cr_0.75Mg_0.25)Si₂O₆ was refined at 100, 150, 200, 250 and 298 K. The work was performed in the context of an investigation on the low-temperature behaviour of A⁺M³⁺Si₂O₆ (with A dominant in Na and M = transition elements) pyroxenes in order to provide new insights concerning the phase transition and anomalies recently found in the low-temperature behaviour of NaTiSi₂O₆ and NaGaSi₂O₆ compounds. The refinements were done in the C2/c space group (wR² between 0.048 and 0.068), and no change of symmetry was observed down to 100 K. Highly-anisotropic axial thermal expansion occurs with the scheme α_b ≥ α_a > α_c.

The M2, M1 and T polyhedra expand with α_M2 > α_M1 > α_T, as generally observed in pyroxenes. A discontinuity in the M1 polyhedral volume is observed between 200 and 250 K, similar to the one observed in NaGaSi₂O₆ between 190 and 235 K.

The atomic displacement parameters are scaled according to the following pattern: U_M2 > U_O2 > U_O3 ≥ U_O1 > U_T ≥ U_M1. Comparison with previous data along the CaMgSi₂O-NaCrSi₂O₆ join suggests significant positional disorder for the O1 oxygen, due to repulsion of the 2p orbitals of O1 and the non-bonding 3d electrons of Cr.