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

The enthalpy of Mg-Fe ordering in En(50)Fs(50) orthopyroxene was measured u sing the transposed temperature drop calorimetric method. Heat effects asso ciated with two consecutive drops were recorded. In the first drop, synthet ic orthopyroxene samples equilibrated at 823 K, 0.1 MPa and a fO(2) of the WI buffer were dropped from 823 K into the calorimeter, which was held at 1 173 K. The measured heat effect corresponds to the enthalpy change due to t he heat capacity of the sample from 823 to 1173 It and to the enthalpy asso ciated with the (dis)ordering of Mg and Fe2+. In the second drop, the sampl es, with an Fe-Mg order corresponding to 1173 K, were dropped again from 82 3 to 1173 K, From the difference of the heat effects measured in the two ex periments, the enthalpy of disordering associated with the temperature chan ge from 823 to 1173 It was calculated to be -1.73 +/- 0.04 J mol(-1). The o bserved enthalpy corresponds to a change in the mole fraction of iron on th e M2 site, Delta X-Fe,X-M2 = -0.096 +/- 0.001, which leads to of Delta H-ex ch(0) of 18.0 +/- 0.4 kJ mol(-1) for the exchange reaction: FeM2MgMISi2O6 --> MgM2FeMISi2O6. The degree of Fe-Mg order was characterized by Fe-57 Mossbauer resonance sp ectroscopy. In order to minimize the error due to the thickness of the abso rber, the iron concentration of the absorber was reduced step by step from 5 to mg Fe cm(-2). The iron distribution extrapolated to zero thickness was used for the calculations of the enthalpy of exchange reaction.