No abstract

7.1(1.7) × 10 −10 × X 2 Jd ] 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) 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.

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.

A single-crystal X-ray diffraction (XRD) study, using a diamond-anvil cell at high pressure and room temperature, was performed on a crystal from a natural space group P2/n omphacite sample with composition very close to Jd 55 Di 45 and with a high degree of order in cation distribution. Unit-cell parameters were determined at 13 different pressures up to about 7.5 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) fitted to the P-V data yielded V 0 = 421.43(4) Å 3 , K T0 = 122(1) GPa, and K′ = 5.1(3). The K T0 value for this sample lies between the data obtained for the two end-members jadeite and diopside, and describes a slight positive curvature trend.During the same experiment, intensity data were collected and crystal structures were refined at 5 pressures up to 7.3 GPa. Both M1 and M2 polyhedra volumes showed a slight but significant change in slope at about 4 GPa. This behavior can likely be explained in terms of tilt angle variation of TA and TB tetrahedral, which also showed a change in slope with pressure, rather than in terms of bond length compression anomaly.

Two Pbca orthopyroxene samples, donpeacorite (DP N.1) and enstatite (B22 N.60) with chemical formulae Mn 0.54 Ca 0.03 Mg 1.43 Si 2 O 6 (X Mn = 0.27) and Fe 0.54 Ca 0.03 Mg 1.43 Si 2 O 6 (X Fe = 0.27), respectively, were investigated by single-crystal X-ray diffraction at high-temperature conditions.The nearly identical X Fe and X Mn make the two samples the perfect candidates to investigate the effect of the compositional change at the M2 site (i.e. Fe-Mn substitution) on the thermal expansion behaviour of orthopyroxenes.Therefore, the unit-cell parameter thermal expansion behaviour of both samples has been investigated in the temperature range between room T and 1073 K. No evidence for phase transitions was found over that range. The two samples have been previously disordered with an ex situ annealing at~1273 K.The unit-cell parameters and volume thermal expansion data, collected on the disordered samples, have been fitted to a Fei Equation of State (EoS) and the following coefficients obtained: V 0 = 853.35(4) Å 3 , a V,303K = 2.31(24)610 À5 K À1 and V 0 = 845.40(6) Å 3 , a V,303K = 2.51(25)610 À5 K À1 for DP N.1 and B22 N.60, respectively.While there is no difference in the volume thermal expansion coefficient as a function of composition and the expansion along the b direction is nearly identical for both samples, slight differences have been found along a and c lattice directions. The thermal expansion along the a direction is counterbalanced by that along c being responsible for the changes in lattice expansion scheme from a b > a c > a a at room T, to a c > a b > a a at high T. Therefore, as a result of the different behaviour along a and c, the unit-cell volume thermal expansion for both samples is identical within estimated standard deviations. The negligible effect of the Fe-Mn substitution on the bulk thermal expansion can be applied when dealing with geothermobarometry based on the elastic host-inclusion approach (e.g. Nestola et al., 2011;Howell et al., 2010; Angel et al., 2014a Angel et al., ,b, 2015. In fact, though the compressibility effect is still not known, the nearly identical thermal expansion coefficients will not affect the entrapment pressure (P e ).