ON A PRESUMED FIND OF MOISSANITE IN THE LOWER PRECAMBRIAN ROCKS OF THE ALDAN SHIELD

Abstract

In detailed petrographic studies of the high-alumina rocks (garnet-sillimanite-cordierite-biotite-plagioclase-quartz schists and gneisses) that are widely developed in the oldest (more than 3.7 Ba), Upper Aldan group of the Aldan shield, individual and, as a rule, very large quartz grains were found to contain very small tabular, ideally euhedral microcrysts, which turned out to be impossible to identify by the usual petrographic methods. For diagnostic purposes, the microcrysts were studied on the MBX ('Cameca') scanning electron microscope-analyzer. The microcrysts are compounds of silicon, whose content in them is higher than in quartz. A compound satisfying this condition may be silicon quartz [sic!] (moissanite). The possibility that the microcrysts may be moissanite is also supported by certain of their physical properties, such as their high refractive index, their hexagonal crystal symmetry, their green color, their tabular form and also their high microhardness. The microhardness of the microcrysts and the quartz surrounding them was determined by the pressure method with a diamond pyramid on the PMG-3 microhardness analyzer. The microhardness values of the microcrysts exceed those of the quartz and approach the microhardness of corundum. In view of the fact that their microhardness was determined through a layer of quartz, the true values would be somewhat higher.