Growth, Morphology, and Compositional Variations in Uvite (Tourmaline) Crystals

Abstract

Surface microtopographs, internal textures and associated composition-al variations of uvite single crystals from Burumado, Bahia, Brazil, probably of conta ct metamorphic origin, have been investigated. The crystals are doubly terminated, bounded by well developed positive and negative trigonal pyramidal {1011} faces, prism {1120} and basal {0001} faces, associated by much smaller {1010} and other faces. The polarity was identified by the Kundt's method. From the surface microtopographic investigations, it was concluded with a fair certainty that the crystal grew by the spiral growth mechanism on positive and negative trigonal pyramidal and prism faces and that the growth rate in positive direction was more than two times higher than in the negative direction. The observed internal textures were essentially similar as those commonly observed in elbite crystals of pegmatitic origin, the central portion showing triangular outline, and as going outward the outline changing to hexagonal and further twelve sided. The internal morphological changes, i. e. morphology changes as a crystal growing larger, are associated with color changes, which were related to compositional changes. In the early stage of growth, uvite component is high, and dravite component increases at a later stage, reflecting compositional changes associated with the reaction process between original carbonate rocks and reacted granitic components. Based on these results, how the uvite crystals grew in contact metamorphic environment, and their morphologies varied during the growth history, were analyszed.