Albite-rich domains in potash feldspar

Detection of sodium-rich areas in orthoclase by electron microprobe analyses supports a theory for perthite formation that involves a preexsolution stage with sodium atoms clustering to form albite domains coherent with the overall lattice. Orthoclase with a heterogeneous matrix, ranging in composition from one up to 14 weight percent albite, and ranging in size from 2 to 10 microns are present in a hypabyssal nepheline syenite from Beemerville, New Jersey, U.S.A. These become unstable at relatively low concentrations of albite (less than 15 weight percent albite) and exsolve forming incoherent albite grains.