Thermal expansion behaviour of orthopyroxenes: the role of the Fe-Mn substitution

Abstract

AbstractTwo Pbca orthopyroxene samples, donpeacorite (DP N.1) and enstatite (B22 N.60) with chemical formulae Mn0.54Ca0.03Mg1.43Si2O6(XMn= 0.27) and Fe0.54Ca0.03Mg1.43Si2O6(XFe= 0.27), respectively, were investigated by single-crystal X-ray diffraction at high-temperature conditions.The nearly identical XFeand XMnmake the two samples the perfect candidates to investigate the effect of the compositional change at theM2 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 roomTand 1073 K. No evidence for phase transitions was found over that range. The two samples have been previously disordered with anex situannealing 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: V0= 853.35(4) Å3, αV,303K= 2.31(24) × 10–5K–1and V0= 845.40(6) Å3, αV,303K= 2.51(25) × 10–5K–1for 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 thebdirection is nearly identical for both samples, slight differences have been found alongaandclattice directions. The thermal expansion along theadirection is counterbalanced by that alongcbeing responsible for the changes in lattice expansion scheme from αb> αc> αaat room T, to αc> αb> αaat high T. Therefore, as a result of the different behaviour alongaandc, 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. Nestolaet al., 2011; Howellet al., 2010; Angelet al., 2014a, b, 2015). In fact, though the compressibility effect is still not known, the nearly identical thermal expansion coefficients will not affect the entrapment pressure (Pe).