Feldspar-liquid equilibria in peralkaline liquids; the orthoclase effect

Abstract

Recent published discussions of pantellerites and related experimental liquids have described compositional differentiation trends in terms of normative Q, or, and ab. This norm projection has at least three intrinsic deficiencies when used to depict paralkaline liquid compositions: (1) It exaggerates potential free silica in the liquid if ns forms part of the total composition. (2) K 2 O is arbitrarily allotted to or; peralkalinity is thus expressed only in terms of Na 2 O, which is thereby lost in the projection. (3) By ignoring part of the Na 2 O in peralkaline liquids the projection conceals an important relationship in the alkali balance between the liquids and their feldspar phenocrysts--the orthoclase effect.