Low-temperature crystal structure evolution of (Na,Ca)(Cr,Mg)Si (sub 2) O (sub 6) pyroxene

The crystal structure of a clinopyroxene with composition (Na (sub 0.75) Ca (sub 0.25) )(Cr (sub 0.75) Mg (sub 0.25) )Si (sub 2) O (sub 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 (super +) M (super 3+) Si (sub 2) O (sub 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 (sub 2) O (sub 6) and NaGaSi (sub 2) O (sub 6) compounds. The refinements were done in the C2/c space group (wR (super 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 (sub b) > or =alpha (sub a) >alpha (sub c) . The M2, M1 and T polyhedra expand with alpha (sub M2) >alpha (sub M1) >alpha (sub 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 NaGaSi (sub 2) O (sub 6) between 190 and 235 K. The atomic displacement parameters are scaled according to the following pattern: U (sub M2) > U (sub O2) > U (sub O3) > or = U (sub O1) > U (sub T) > or = U (sub M1) . Comparison with previous data along the CaMgSi (sub 2) O-NaCrSi (sub 2) O (sub 6) 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.