ALBERT

Description: 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 M 2 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 , α V,303K = 2.31(24) x 10 –5 K –1 and V 0 = 845.40(6) Å 3 , α V,303K = 2.51(25) x 10 –5 K –1 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 α b 〉 α c 〉 α a at room T , to α c 〉 α b 〉 α 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. , 2014 a , 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 ).