Experimental Determinations of the Microcline--Low Albite Solvus

Abstract

The metastable, high-temperature portion of the microcline-low albite solvus has been experimentally determined at atmospheric pressure by the fused-salt alkali ion exchange technique (Bachinski), and by the homogenization—unmixing technique (Müller). Analytical smoothing of each set of pairs of coexisting compositions yields critical temperatures and compositions of 885 °C and 27 mol per cent KAISi3O8, and 887 °C and 24 mol per cent KAISi3O8, respectively. The differences between the solvi are real and the lack of coincidence of the two is attributable, to varying extents, to differences in the structural state, composition, twinning, and heat treatment of the starting materials. Previous workers' estimates of the position of the one-feldspar-two-feldspar field boundary are in general accord with the experimental solvi.