Enthalpies in (Na,Ca)- and (K,Ca)-feldspar binaries: a high-temperature solution calorimetric study

Abstract

A series of high structural state plagioclases (Ab–An) was crystallized from glasses. By exchanging Na for K in KCl melts, metastable K-plagioclases (Or–An) were prepared which possess the same structural state as the starting plagioclases. Both series were investigated at 980 K by lead borate solution calorimetry. Continuing the ideas of Carpenter and McConnell (1984) and Carpenter (1992a), the results can be interpreted as follows. In the high plagioclase series, the enthalpies of solution, ΔH_sol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0≤X_An≤0.2, (2) high albite-like (C1̄) ordering in intermediate plagioclases, and (3) anorthite-like (I1̄) ordering in the An-rich region 0.7<X_An≤1. In regions 1 and 2, ΔH_sol decreases as a function of X_An, but increases in region 3 as a consequence of the C1̄→I1̄ ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, ΔH^ex, to be positive in the plagioclase binary as a whole. Neglecting the existence of phase transitions at X_An=0.2 and X_An=0.7, ΔH^ex can be approximated by a two-parameter Margules model yielding W^H _AnAb=14±3 kJ/mol and W^H _AbAn=40±3 kJ/mol. ΔH_sol values of I1̄ plagioclases (X_An>0.7) can be "corrected" for the C1̄→I1̄ ordering effect (Carpenter 1992a). When combining the corrected values with the ΔH_sol data which were actually measured on the C1̄ plagioclases (X_An<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1̄ ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1̄ order. The solution experiments on the K-plagioclases resulted in similar characteristics as those found for the plagioclases. However, in addition to the ordering effects observed in the plagioclase binary, volume mismatch effects contribute to ΔH^ex in the K-plagioclase series. ΔH^ex can be represented by a Margules model with W^H _AnOr=60±10 kJ/mol and W^H _OrAn=91±14 kJ/mol when the phase transitions at X_An=0.2 and X_An=0.7 are again neglected. The contribution of the volume mismatch effect to ΔH^ex is considerable, as appears from the large difference between the K-plagioclase and the plagioclase Margules parameters. Their difference corresponds to a practically symmetrical dependence of ΔH^ex _volmism on composition, with W^H _volmism=48±13 kJ/mol.