Portal de Periódicos da CAPES

O BERILO GEMOLÓGICO DA PROVÍNCIA PEGMATÍTICA ORIENTAL DO BRASIL

2003; Volume: 17; Issue: 1 Linguagem: Inglês

10.21715/gb.v17i1.194

2358-2812

M. A. Kahwage, Júlio Cézar Mendes,

Geochemistry and Geologic Mapping

�GXULQJ neoproterozoic granitogenesis of the Brasiliano tectonothermal event that gave origin to the AracuaiWest Congo Belt. The granitic suites were responsible for several pegmatites generations, with distinct ages, mineral assemblages and differentiation degrees. The study of beryl from various pegmatites, located in the areas of Medina, Itinga, Padre Paraiso, Sao Jose da Safira and Galileia, revealed differences concerning physical properties as well as chemical composition. The content of minor elements, mainly alkaline metals (Li, Cs and Rb), have a direct influence on density and refraction indexes of beryl, besides being important indicators of generation and evolution processes of pegmatites. High concentrations of alkaline metals in beryl (Itinga and Sao Jose da Safira pegmatites) reflect the highly differentiated conditions of the pegmatites hosts. Li occurs in tetrahedral coordination in substitutions for Be and alkalis are concentrated in the open channels. The specimens from less differentiated pegmatites (Medina and Padre Paraiso) have low alkalis and high iron, indicating that octahedral substitutions of Fe for Al prevail over the tetrahedral substitutions. The presence of Fe in the octahedral site and filling out of the open channels is responsible for change transfer effect responsible for the blue, green and yellow colors. In specimens of less differentiated bodies related to calc-alkaline granitoids of the I-type (Medina and Padre Paraiso) colors are more intense than in beryl from differentiated bodies (Itinga, Sao Jose da Safira and Galileia) associated to S-type granites, due to the higher iron contents of the former. Mn is sufficient concentration to generate rosy color was detected only in samples from differentiated pegmatites. High gemological quality due to intense bluish, greenish and yellowish colors is more common for beryl from the less differentiated bodies related to post-tectonic I-type granitoids. In the other pegmatites only pink varieties are of gem-quality and the other color types must be submitted to color enhancement treatment whose success depends on the Fe-content, which has to be enough to enable the generation of yellowish tints.