Growth and sector zoning in a beryl crystal

Abstract

The structural defects and minor element concentrations of a beryl crystal from Brumado (Bahia, Brazil) were studied by X-ray diffraction topography and electron probe microanalysis. Examination showed that crystal growth was divided into two main stages, with significant differences with regard to defect density and variations in minor element concentrations, particularly Mg and Na. In the first growth stage, compositional variations and low defect density were observed; in the second stage, there was a decrease in impurity content and a sharp rise in defect density. Reconstruction of the growth history allowed the growth-sector boundaries to be located and related to compositional variations. In the first growth stage, minor element concentrations varied sharply when crossing the boundaries between the pinacoidal-pyramidal and pyramidal-prismatic growth sectors, whereas they decreased smoothly in the prismatic sector. They were therefore interpreted to be sector and chemical zonings respectively. Only chemical zoning was detected in the second stage of crystallisation.