Volume thermal expansion along the jadeite-diopside join

An in situ single-crystal high-temperature X-ray diffraction study was performed on clinopyroxene crystals along the jadeite, (NaAlSi₂O₆ Jd)–diopside (CaMgSi₂O₆ Di) join. In particular, natural samples of jadeite, diopside, P2/n omphacite and three C2/c synthetic samples with intermediate composition (i.e., Jd₈₀, Jd₆₀, Jd₄₀) were investigated. In order to determine the unit-cell volume thermal expansion coefficient (α V), the unit-cell parameters for all these compositions have been measured up to c.a. 1,073 K. The evolution of the unit-cell volume thermal expansion coefficient (α V) along the Jd–Di join at different temperatures has been calculated by using a modified version of the equation proposed by Holland and Powell (J Metamorph Geol 16(3):309–343, 1998). The equation αV(303K,1bar)=2.68(3)×10⁻⁵+[1.1(1)×10⁻⁸×XJd]−[7.1(1.7)×10⁻¹⁰×X²Jd] obtained from the α V at room-T (i.e., α V303K,1bar) allows us to predict the room-T volume thermal expansion for Fe-free C2/c clinopyroxenes with intermediate composition along the binary join Jd-Di. The observed α V value for P2/n omphacite α V(303K,1bar) = 2.58(5) × 10⁻⁵ K⁻¹ was compared with that recalculated for disordered C2/c omphacite published by Pandolfo et al. (Phys Chem Miner 1–10, 2012) [α V(303K,1bar) = 2.4(5) × 10⁻⁵ K⁻¹]. Despite the large e.s.d.’s for the latter, the difference of both values at room-T is small, indicating that convergent ordering has practically no influence on the room-T thermal expansion. However, at high-T, the smaller thermal expansion coefficient for the C2/c sample with respect to the P2/n one with identical composition could provide further evidence for its reduced stability relative to the ordered one.