Mineralogia e cristalografia do diamante do Triângulo Mineiro.

Mineralogia e cristalografia do diamante do Triângulo Mineiro.
The purpose of this work is to characterize the diamond from the "Triangulo Mineiro" region (Minas Gerais) according to its morphological features; surface structures and surface figures; spectrographc properties (IV absorption and UV fluorescence) and specially by the recognition of its mineral inclusions. These crystallographic data may be important to guide, with greater sucess, the future geological work searching the diamond matrix-rock, until unknown in that region. The morphological study revealed a predominance of the rhombododecahedral habit among the single forms presented by the diamond crystals. It is very frequent also the occurrence of diamond twinned crystals (33% of the observed samples). The predominance of the rhombododecahedral habit, with both rounded faces and edges, is explained as a result of a natural dissolution process in a oxidizing media. The rhombododecahedral form was deduced here as the equilibrium form during that process. It was considered in that assumption the dissolution velocities changes, according to the diamond crystal structure. The surface figures and surface structures are, on its general features, the same as the described ones on diamonds from other localities. Some of them, however, may be not still mentioned. The origin of some of those figures should be admitted as a result of crystal dissolution, by its presence on natural clivage surfaces. The infrared absorption diagrams showed a anomalous occurrence of the type lb diamonds. That kind of stone is very rare amongst the natural ones and that property may be the more remarkable characteristic of the diamond from the "Triangulo Mineiro". The mineral inclusions, identified by x-ray diffraction (precession method) - forsterite, pirope, magnesio-chromite and pentlandite - sugests a genetic process very close to the ultrabasic rocks. The secondary effects observed on the host crystal (anomalous birefringence and tension fractures) and the epitaxic development of the inclusions in the diamond, indicates, on the other hand, that those minerals were enclosed during the diamond crystal growing. The primary paragenesis, revealed by that syngenetic inclusions, sugests the equilibrium: forsterite + spinel + coesite pirope, that may be applied to estimate the pressure and temperature conditions in the time of the diamond crystallization.