A HT-XRD study of synthetic magnesian-ferrispodumene: transition temperature vs. cation composition and ordering. Nineth International Symposium on Experimental Mineralogy Petrology and Geochemistry

We synthesised magnesian-ferrispodumene in the MgO-LiO2-FeO-SiO2-H2O system. Single-crystal structure refinement at room T and EMP analyses show that this pyroxene is monoclinic P21/c with composition M2(Li0.85Mg0.09Fe2+0.06)M1(Fe3+0.85Mg0.15)Si2O6 (a = 9.638(3) Å, b = 8.709(2) Å, c = 5.258(2) Å, ? = 109.83(3)?, V = 415.2 Å3). Li is ordered at M2, and Fe3+ at M1, Mg and Fe2+ distribute over both octahedral sites. Structure refinements done at different temperatures show that at 105°C magnesian-ferrispodumene undergoes a reversible displacive phase transition P21/c ? C2/c. It is evidenced by the disappearance of the h+k = 2n+1 reflections and by abrupt changes in the unit cell parameters. It is known from previous HT-XRD work that in Li-clinopyroxenes the transition temperature is inversely related to the size of the M1 cation [70°C in LiCrSi2O6 (Behruzi et al., 1984); 10°C in LiGaSi2O6 (Sato et al., 1995); -44° in LiFe3+Si2O6, (Redhammer et al., 2001)]. The studied crystal has an aggregate ionic radius at M1 larger than LiFe3+Si2O6; therefore its transition temperature should be < -44°C. It is also known (Prewitt et al., 1971) that the transition temperature in ferromagnesian clinopyroxenes increases with decreasing aggregate cation radius at M1 and M2, and thus depends on the Fe2+ ordering between M1 and M2 (Cámara et al., 2002). The transition temperature experimentally measured for magnesian-ferrispodumene can be thus explained by the presence of significant Mg at M2 which shortens the aggregate radius at that site. This study suggests the possibility of a complex thermodynamic behaviour accompanying variable cation substitutions at both the M1 and M2 sites in clinopyroxenes. References (1) Behruzi M., Hahn T., Prewitt C.T., Baldwin K., Acta Crystallogr. A40, C.247, 1984. (2) Sato A., Osawa T., Ohashi H., Acta Crystallogr. C51, 1959-1960, 1995. (3) Redhammer G.J., Roth G., Paulus W., André G., Lottermoser W., Amthauer G., Treutmann W., Koppelhuber-Bitschnau B., Phys. Chem. Minerals 28, 337-346, 2001. (4) Prewitt C.T., Brown G.E., Papike J.J.,Geochim. Cosmochim. Acta, 1, 59-68, 1971. (5) Cámara F., Carpenter M.A., Domeneghetti M.C., Tazzoli V., Phys. Chem. Minerals In press, 2002.