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Low-temperature crystal structure evolution of (Na,Ca)(Cr,Mg)Si₂O₆ pyroxene

F. Nestola^1,*, A. Madsen², M. Tribaudino³, T. Bali-uni⁴, H. Ohashi⁵, L. Secco¹ and A. Dal Negro¹

¹ Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, 35010, Padova, Italy
² Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100, Copenhagen, Denmark
³ Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, 43100, Parma, Italy
⁴ Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
⁵ Hashi Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan

^* E-mail: Fabrizio.nestola{at}unipd.it

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 _{b a} > _c.

The M2, M1 and T polyhedra expand with _M2 > _M1 > _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.

KEYWORDS: single-crystal, X-ray diffraction, silicate, low-temperature, clinopyroxene, structure refinements