Calorimetric determination of the enthalpy of Mg-Fe ordering in orthopyroxene

Abstract

The enthalpy of Mg-Fe ordering in En₅₀Fs₅₀ orthopyroxene was measured using the transposed temperature drop calorimetric method. Heat effects associated with two consecutive drops were recorded. In the first drop, synthetic orthopyroxene samples equilibrated at 823 K, 0.1 MPa and a f O₂ of the WI buffer were dropped from 823 K into the calorimeter, which was held at 1173 K. The measured heat effect corresponds to the enthalpy change due to the heat capacity of the sample from 823 to 1173 K and to the enthalpy associated with the (dis)ordering of Mg and Fe²⁺. In the second drop, the samples, with an Fe-Mg order corresponding to 1173 K, were dropped again from 823 to 1173 K. From the difference of the heat effects measured in the two experiments, the enthalpy of disordering associated with the temperature change from 823 to 1173 K was calculated to be −1.73±0.04 J mol⁻¹. The observed enthalpy corresponds to a change in the mole fraction of iron on the M2 site, ΔX _Fe,M2=−0.096 ± 0.001, which leads to of ΔH ⁰ _exch of 18.0 ± 0.4 kJ mol⁻¹ for the exchange reaction:

The degree of Fe-Mg order was characterized by ⁵⁷Fe Mössbauer resonance spectroscopy. In order to minimize the error due to the thickness of the absorber, the iron concentration of the absorber was reduced step by step from 5 to 1 mg Fe cm⁻². The iron distribution extrapolated to zero thickness was used for the calculations of the enthalpy of exchange reaction.