Low-temperature crystal structure evolution of (Na,Ca)(Cr,Mg)Si_2O_6 pyroxene

摘要

The crystal structure of a clinopyroxene with composition (Na_(0.75)Ca_(0.25))(Cr_(0.75)Mg_(0.25))Si_2O_6 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~+M~(3+)Si_2O_6 (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 NaTiSi_2O_6 and NaGaSi_2O_6 compounds. The refinements were done in the C2/c space group (wR~2 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 alpha_b >= alpha_a > alpha_c. The M2, M1 and T polyhedra expand with alpha_(M2) > alpha_(M1) > alpha_T, as generally observed in pyroxenes. A discontinuity in the Ml polyhedral volume is observed between 200 and 250 K, similar to the one observed in NaGaSi_2O_6 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 CaMgSi_2O-NaCrSi_2O_6 join suggests significant positional disorder for the 01 oxygen, due to repulsion of the 2p orbitals of O1 and the non-bonding 3d electrons of Cr.