Low-temperature crystal structure evolution of (Na,Ca)(Cr,Mg)Si 2 O 6 pyroxene F. NESTOLA 1, *, A. MADSEN 2 , M. TRIBAUDINO 3 , T. BALIC ´ -Z ˇ UNIC ´ 4 , H. OHASHI 5 , L. SECCO 1 AND A. DAL NEGRO 1 1 Dipartimento di Geoscienze, Universita ` 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, Universita ` 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 [Received 26 June 2008; Accepted 9 September 2008] ABSTRACT The crystal structure of a clinopyroxene with composition (Na 0.75 Ca 0.25 )(Cr 0.75 Mg 0.25 )Si 2 O 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 2 O 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 2 O 6 and NaGaSi 2 O 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 a b 5 a a > a c . The M2, M1 and T polyhedra expand with a M2 > a M1 > a 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 2 O 6 between 190 and 235 K. The atomic displacement parameters are scaled according to the following pattern: U M2 > U O2 > U O3 5 U O1 > U T 5 U M1 . Comparison with previous data along the CaMgSi 2 O-NaCrSi 2 O 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. KEYWORDS: single-crystal, X-ray diffraction, silicate, low-temperature, clinopyroxene, structure refinements. Introduction SODIUM and lithium silicates, with the general formula A + M 3+ Si 2 O 6 (with A = Li or Na and M = transition elements, space group C2/c at ambient conditions), have recently been investigated at low temperature as they represent low-dimensional magnets. They are characterized by quantum spin phenomena and show interesting and little- investigated magnetic properties as also found for some germanate clinopyroxenes (Isobe et al., 2002; Millet et al., 1999). Moreover, some naturally occurring members of this structural family (e.g. kosmochlor NaCrSi 2 O 6 ) are important rock-forming minerals. In fact, structurally, the above Na and Li silicates belong to the family of clinopyroxenes, one of the most abundant mineral family in mineralogy. Therefore, the clino- pyroxene crystal structure is well-known (Cameron et al., 1973) and is made up by three distinct structural crystallographic oxygen-coordi- nated polyhedra (Fig. 1): M2, M1 and T. M2 is an irregular polyhedron 6, 7 or 8-fold coordinated (based on which cation occupies it); M1 is an octahedron, smaller and more regular than M2. The M2 and M1 polyhedra are linked to a tetrahedral polyhedron (T). M1 octahedra and T tetrahedra make infinite chains along the c axis. In Na and Li silicates, Na and Li are hosted in the M2 * E-mail: Fabrizio.nestola@unipd.it DOI: 10.1180/minmag.2008.072.3.809 Mineralogical Magazine, June 2008, Vol. 72(3), pp. 809–816 # 2008 The Mineralogical Society