FAYALITE AND KIRSCHSTEINITE SOLID SOLUTIONS IN MELTS FROM BURNED SPOIL-HEAPS, SOUTH URALS, RUSSIA

Abstract

Individual grains of calcian fayalite and ferroan kirschsteinite, as well as fayalite-kirschsteinite intergrowths are observed in the groundinass of basic crystallised melts, or parabasalts, from burned spoil-heaps of the Chelyabinsk brown-coal basin. Exsolved fayalite and kirschsteinite rims surround the grains of fayalite and early Mg-Fe olivine. The chemical study of the olivines has shown that during their crystallisation they were becoming enriched in fayalite and larnite and depleted in forstetite. The intergrowths of ferroan kirschsteinite (> 20 wt.% of CaO) and calcian fayalite (< 8.5 wt.% of CaO) are the exsolution products of an initially homogeneous Ca-Fe olivine with CaO > 8,5 wt.%. The exsolution temperatures were estimated to 980-800°C. The main reasons for the appearance of the Ca-Fe olivine in the parabasalts are the composition of the initial melt enriched in FeO and CaO, fractional crystallisation resulting in further enrichment in iron of the residual low-silica melt, and reducing conditions during olivine crystallisation and exsolution.