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, ΔHsol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0≤XAn≤0.2, (2) high albite-like (C1¯) ordering in intermediate plagioclases, and (3) anorthite-like (I1¯) ordering in the An-rich region 0.7<XAn≤1. In regions 1 and 2, ΔHsol decreases as a function of XAn, but increases in region 3 as a consequence of the C1¯→I1¯ ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, ΔHex, to be positive in the plagioclase binary as a whole. Neglecting the existence of phase transitions at XAn=0.2 and XAn=0.7, ΔHex can be approximated by a two-parameter Margules model yielding WHAnAb=14±3 kJ/mol and WHAbAn=40±3 kJ/mol. ΔHsol values of I1¯ plagioclases (XAn>0.7) can be "corrected" for the C1¯→I1¯ ordering effect (Carpenter 1992a). When combining the corrected values with the ΔHsol data which were actually measured on the C1¯ plagioclases (XAn<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1¯ ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1¯ 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 ΔHex in the K-plagioclase series. ΔHex can be represented by a Margules model with WHAnOr=60±10 kJ/mol and WHOrAn=91±14 kJ/mol when the phase transitions at XAn=0.2 and XAn=0.7 are again neglected. The contribution of the volume mismatch effect to ΔHex 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 ΔHexvolmism on composition, with WHvolmism=48±13 kJ/mol.