Unusual shape of pyrrhotite inclusions in scapolite of igneous rocks from the southernern Urals

Abstract

The unique igneous rock (scapolite–diopside gabbro) from the Ilmeny Mountains in the southern Urals is described. Gabbro fills a segment of dike 1.3 m thick that cuts through calcite–dolomite carbonatite. Medium-grain pyroxenite with scapolite that occurs at selvages gradually passes to scapolite-bearing gabbro in the central part of the dike. Scapolite crystals display surfaces of concurrent growth, which are evidence of their magmatic origin. Scapolite (Me 63–70%) contains numerous pyrrhotite inclusions as platelets 0.001 mm thick oriented parallel to the cleavage plane {100}. The calculated pyrrhotite formula is consistent with its stoichiometry (Fe_1–xS). The morphology of the platelets (hexagonal sections) and their optical properties indicate a hexagonal symmetry of pyrrhotite. As follows from the insignificant difference between scapolite grains with and without pyrrhotite inclusions, scapolite and pyrrhotite should be regarded as products of synchronous magmatic melt crystallization.