Thermoelastic behaviour of C2/c omphacite.

Omphacite shows an intermediate composition along the binary jadeite-diopside join of the Na-pyroxene solid solution. Most of the interest in this mineral has been focused on the order-disorder phenomena which it undergoes, causing a P2/n - C2/c phase transition [1]. Although omphacite plays a key role in high-pressure and high-temperature geological environments, in situ studies on its thermo-elastic behaviour under non-ambient conditions are definitively scarce. Some experimental results have been reported on the omphacite high-pressure in situ behaviour [2,3] and, more recently, the thermal expansion and the thermal equation of state were studied by [4] and by [5] respectively. However all these data were collected using different experimental conditions, by both powder materials and single crystals and often off the omphacitic composition. Therefore they do not provide a systematic on the high-pressure and high-temperature behaviour. In order to constrain the correct value of the compressibility and thermal expansion of omphacite we have studied a natural sample by high-pressure and high-temperature in situ single crystal X-ray diffraction (in two separate experiments). Two twin and inclusion-free single crystals of P2/n omphacite from the Münchberg Mass (Bavaria) eclogitic rock were selected for this study. Its composition, Jd48Di39Hd6En4CaTs3, is very close to pure Jd50Di50 omphacite. In order to obtain fully disordered C2/c omphacite crystals, isothermal annealing at 1000 °C for 300 hrs was carried out using a vertical temperature control furnace. Intensity data collections were performed to verify the absence of the h+k = 2n+1 reflections, and data were refined in the C2/c space group. Unit-cell parameters were determined by SCXRD at 16 different pressures up to about 7 GPa. A continuous decrease of the unit-cell parameters and volume was observed as a function of pressure up to the maximum P reached (volume decreases about 5.2% up to 7 GPa). A third order Burch Murnaghan equation of state was used to fit the experimental pressure-volume data. We obtained the following coefficients: V0 =421.04(7) Å3, KT0=118.65(1.84) GPa, K'=5.7(6). The compressibility scheme βb > βc > βa is similar to that of other clinopyroxenes. High-temperature experiments were carried out measuring lattice parameters at intervals of 50°C in the 25-800°C T range both on increasing and decreasing temperature. Continuous increasing of the unit-cell parameters and volume was observed as a function of temperature with a volume increase of about 2.3% up to 800°C. The thermal expansion obtained using the approach by [7] yielded α 3.03(3) x 10-6 K-1. We obtained two different behaviours as a function of pressure and temperature: 1) even if further experimental data are necessary for intermediate compositions, our data show that the bulk modulus value for disordered omphacite is in very good agreement with a linear combination of the bulk moduli of the Jd and Di end-members; 2) otherwise the thermal expansion value is slightly higher than that expected for a linear combination of the volumetric thermal expansion of the two end members. [1] Carpenter et al. (1990) Eur J Mineral 2, 7-18; [2] McCormick et al. (1989) Am Mineral 74, 1287-1292; [3] Pavese et al. (2001) Phys Chem Minerals 28, 9-16; [4] Pavese et al. (2000) Min Mag 64, 983-993; [5] Nishihara et al. (2003) . Am Mineral 88, 80-86; [6] Boffa Ballaran et al. (1998) Am Mineral 83, 419-433; [7] Gottshalk (1997) Eur J Mineral 9, 175-223