ABSTRACT

Near-liquidus phase relations of an andesite and the andesitic mixtures with Na₂O, CaO, MgO, and iron oxide added to the andesite were determined under one-atmosphere, fO₂-controlled conditions. The stability relation between clino- and orthopyroxene was determined, and pyroxenes, plagioclase and coexisting liquids (glasses) were analyzed. The liquidus temperature of plagioclase decreased and that of pyroxenes increased by the addition of Na₂O, CaO and MgO, but they decreased by the addition of iron oxides. The effect of Na₂O on the crystallization of minerals from andesitic melts is explained in relation to the chemical potential of wollastonite component in the liquid. The normative composition is useful to deduce the first appearance temperature of liquidus phase, and will be a satisfactory measure for the subliquidus phase if the composition of the liquidus phase is known.