CORUNDUM- BRAZIL

This volume embodies a set of articles compiling the results of the study of the main Brazilian corundum occurrences distributed over the states of Minas Gerais, Bahia, Santa Catarina, Tocantins and Mato Grosso do Sul. Twenty-eight occurrences are reported but in only a few there is or there used to be some gem production. The most important deposits were sampled, investigated in terms of geology and detailed mineralogical characterization on corundum was performed using several analytical techniques such as optical microscopy, electronic microprobe, electronic scanning microscopy, neutronic activation, plasma, spectroscopy, thermogravimetric and thermodifferential analyses, X-ray diffraction, and conventional gemmological analyses. With the exception of Peixe, Tocantins, where corundum is found in alkaline pegmatite, and Catingal, Bahia, where corundum occurs in granulite-facies gneiss, the other occurrences are secondary sedimentary deposits where no fresh host rock is preserved. Information obtained from the two occurrences with corundum in the host rock helped to understand the origin of corundum from the other localities. Similar to Catingal, the occurrences of Indaiá, Palmeiras, Sapucaia, and Caputira, in eastern Minas Gerais, are all situated in the same geological framework of high-grade metamorphic rocks belonging to the neoproterozoic Araçuaí Fold Belt. The geological similarities of this area and the region where the important ruby and sapphire deposits from Sri Lanka are found suggest that new occurrences might be discovered in Minas Gerais. The study of the deposits in Minas Gerais also enabled to understand the modus operandi of the depositional system, which formed the alluvial sediments where corundum was trapped, thus establishing possible search controls for other areas such as in Barra Velha, Santa Catarina. Chemical analyses for trace elements, spectroscopy in the range of UV-visible and the identification of solid inclusions on corundum samples from eleven major occurrences supplied additional elements for the characterization and origin of corundum. Comparison of Cr, Fe, Ti, and Ga-contents enabled conclusions concerning the genesis of the corundum. In Coxim, Mato Grosso do Sul, there are indications of an origin associated with alkali-basalt. In Minas Gerais, Bahia and Santa Catarina chemistry corroborated the interpretation of crystallization in a metamorphic milieu, with the exception of Malacacheta in Minas Gerais, where a metassomatic origin can not be excluded. Anomalous high concentrations of elements such as REE were detected in a few deposits and could be associated with monazite inclusions not completely eliminated during the sample preparation procedures. Gemmological and physico-chemical analyses in samples from selected deposits indicate that, in some cases, an adequate thermal treatment may promote gem enhancement and turn the economic production of gem-quality corundum viable. Minas Gerais and Bahia are the states with the largest number of occurrences and the greater potential for the production of ruby and sapphires.