New thermoelastic parameters of natural C2/c omphacite

Abstract

The compressibility at room temperature and the thermal expansion at room pressure of two disordered crystals (space group C2/c) obtained by annealing a natural omphacite sample (space group P2/n) of composition close to Jd 56 Di 44 and Jd 55 Di 45 , respectively, have been studied by single-crystal X-ray diffraction. Using a BirchMurnaghan equation of state truncated at the third order [BM3-EoS], we have obtained the following coefficients: V 0 = 421.04(7) Å 3 , K T0 = 119(2) GPa, K 0 = 5.7(6). A parameterized form of the BM3 EoS was used to determine the axial moduli of a, b and c. The anisotropy scheme is b c B b a B b b , with an anisotropy ratio 1.05:1.00:1.07. A fitting of the lattice variation as a function of temperature, allowing for linear dependency of the thermal expansion coefficient on the temperature, yielded a V(1bar,303K) = 2.64(2) 9 10 -5 K -1 and an axial thermal expansion anisotropy of a b ) a a [ a c . Comparison of our results with available data on compressibility and thermal expansion shows that while a reasonable ideal behaviour can be proposed for the compressibility of clinopyroxenes in the jadeite-diopside binary join [K T0 as a function of Jd molar %: K T0 = 106(1) GPa ? 0.28(2) 9 Jd (mol%) ], the available data have not sufficient quality to extract the behaviour of thermal expansion for the same binary join in terms of composition.

Volume thermal expansion along the jadeite–diopside join

“…In Fig. 3b the volume-temperature data obtained for the intermediate composition C2/c and P2/n samples of this work (Jd 40 , Jd 60 , Jd 80 , Jd 55 N5 P2/n) are reported together with those available from the literature, in particular Jd 55 N2 C2/c from Pandolfo et al (2012) and Omp P2/n from Pavese et al (2000).…”

“…The temperature calibration of the microfurnace was undertaken as described in Cámara et al (2012), Ferrari et al (2014), and Gatta et al (2014). The high-temperature device and the calibration equation used for these experiments are described in detail by Pandolfo et al (2012).…”

“…For the P2/n sample N5, once the structure refinement reached convergence, a structure refinement with chemical constraints was carried out using the data from the electron microprobe analysis with 1σ error in order to obtain reliable site partitioning that allowed to calculate the order degree (see Pandolfo et al 2012 for details about the refinement procedure). For the same sample, the mean atomic numbers calculated for the octahedral sites [m.a.n.…”

“…The authors suggested that the discrepancies between heating and cooling (10 %) are mainly due to order-disorder reaction in the sample. Very recently, Pandolfo et al (2012) investigated a natural disordered omphacite (space group C2/c) using the same experimental technique but fitting the data following the equation proposed by Berman (1988) (expressed in the form where α V,T r = a o + 2a 1 (T − T r )) obtaining an α V,303K value of 2.64(2) × 10 −5 K −1 .…”

“…In this study, we will (1) better constrain the α V evolution along the jadeite-diopside binary join investigating the thermal expansion behavior of the end-members as well as that of synthetic samples along the join, by using the very same device in order to avoid any effect due to changes in the experimental setup or temperature calibration of the device, and (2) evaluate the effect of the convergent ordering on the α V value, by investigating the thermal expansion behavior of a P2/n omphacite and comparing it with that obtained on a disordered C2/c sample (Pandolfo et al 2012).…”