Compositions of Granophyre, Myrmekite, and Graphic Granite

Abstract

Intergrowths of quartz and feldspar form three distinct types that differ in bulk composition as well as texture, occurrence, and genesis. Granophyric intergrowths involve quartz and alkali feldspar, intergrown on scales from submicroscopic to 1 or 2 mm. Approximately equal amounts of quartz, NaAlSi 3 O 8 and KAlSi 3 O 8 participate in most of these intergrowths, which have a truly granitic composition. Granophyric intergrowths occur as mesostasis, groundmass, and megacrysts, and result from relatively rapid simultaneous growth of quartz and alkali feldspar from a melt, vapor, or devitrifying glass. Myrmekite is an intergrowth of vermicular quartz in plagioclase, which forms a few volume percent of many igneous and metamorphic rocks. The intergrowths rarely exceed 1 mm in diameter. Myrmekite may form by sodium and calcium metasomatism of K-feldspar, or possibly by exsolution. The occurrence of rare calcic myrmekite in mafic rocks remains unexplained. Graphic granite is an intergrowth, on a scale of 1 mm to 1 m, of subhedral skeletal quartz prisms in an alkali feldspar or sodic plagioclase host, which occurs in granitic pegmatites and rarely as megacrysts in granites. Compositions of two types (averaging Qz 27 Or 52 Ab 19 An 2 and Qz 37 Or 5 Ab 51 An 7 ) as well as the texture suggest simultaneous crystallization of quartz and feldspar from a fluid. In each kind of intergrowth, one of the two phases grows commonly in crystallographic continuity with an earlier nucleus or substrate.