Cellular intergrowth between quartz and sodium-rich plagioclase (myrmekite) — an analogue of discontinuous precipitation in metal alloys

Abstract

Gneissic rocks of the contact aureole of the Traversella Intrusion (N. Italy) reveal a cellular intergrowth of quartz and oligoclase (myrmekite), originating from alkali feldspar grain boundaries. The myrmekite occurs in a temperature range of 500 to 670°C. The size of the cells and the lamellar spacing of the quartz rods increase with rising temperature. A comparison of the characteristic features of myrmekite cells with discontinuous precipitation cells of alloys indicates many similarities between the reactions. A is concluded that the formation of myrmekite is a solid-state reaction; a supersaturated alkali feldspar decomposes into a cellular structure growing behind a moving incoherent grain boundary. Micro probe analyses of the original alkali feldspar, the reaction products, and the volume relationship of the reactants (quartz, sodium-rich plagioclase) show that an ion exchange occurs during the reaction (K⁺ removed; Na⁺, Ca²⁺ and Si⁴⁺ added). A model describing the early stages of the formation of myrmekite is presented.