Leucite-Albite Incompatibility

Abstract

It is well known that the assemblage of orthoclase and nepheline, but not that of leucite and albite, is possible in the calculation of the norm classification, based on petrological observations of leucite-bearing rocks (Cross et al. 1902). For example, Shand (1943) indicated that in leucite-bearing lavas the coexisting feldspar is either a calcium-rich plagioclase, or a potassium-rich alkali feldspar, or both. On thermochemical grounds, however, Miyashiro (1960) has postulated that the apparent incompatibility between leucite and albite may not exist at high temperatures. MacKenzie and Rahman (1968) described veins of leucite and sodium-rich feldspar in a basanite from the Massif Central, France, but they did not confirm this as a stable assemblage. Fyfe (quoted in MacKenzie and Rahman 1968) calculated that the leucite-albite assemblage may become stable below around 1000 °C. Experimental studies at ${{P}_{{{{H}_{2}}O}}}$ up to 1 kb in the potassium-rich portion of the NaA1SiO₄-KA1SiO₄-SiO₂ system (Fudali 1963) indicate that the bulk compositions from which leucite-albite pair should crystallize have low melting temperatures and at low water pressures the end products are nepheline_ss, potassium-rich feldspar, and sodium-rich leucite. The only other study in this system is of the liquidus relations of the system nepheline-kalsilite-SiO₂ at atmospheric pressure (Schairer 1950; Chap. 7.1).