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

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, DeltaH(sol), reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0 less than or equal to X-An less than or equal to 0.2, (2) high albite-like (C (1) over bar) ordering in intermediate plagioclases, and (3) anorthite-like (I (1) over bar) ordering in the An-rich region 0.7 < X-An less than or equal to 1. In regions 1 and 2, DeltaH(sol) decreases as a function of X-An, but increases in region 3 as a consequence of the C1 --> I (1) over bar ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, DeltaH(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, DeltaH(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. DeltaH(sol) values of I (1) over bar plagioclases (X-An > 0.7) can be "corrected" for the CT --> I (1) over bar ordering effect (Carpenter 1992a). When combining the corrected values with the DeltaH(sol) data which were actually measured on the C (1) over bar plagioclases (X-An < 0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C (1) over bar ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of IT 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 DeltaH(ex) in the K-plagioclase series. DeltaH(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 DeltaH(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 DeltaH(volmism)(ex) on composition, with W-volmism(H) = 48 +/- 13 kJ/mol.