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 (αV), 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 (αV) 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 aV(303K,1bar) = 2.68(3) × 10-5 + [1.1(1) × 10-8 × XJd]-[7.1(1.7) × 10-10 × X2 Jd] obtained from the αV at room-T (i.e., αV303K,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 αV value for P2/n omphacite αV(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) [αV(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.