Caracterização químico-mineralógica e espectroscopia mössbauer de água-marinha da região de Pedra Azul, nordeste de Minas Gerais

Abstract

Aquamarines from three pegmatites located the vicinities of the Pedra Azul city, Minas Gerais state, were investigated in terms of chemical composition, physical properties and Mossbauer spectroscopy. The small lenticular pegmatite bodies are usually less than 5m wide and exhibit a conspicuous mineralogical and textural zoning. The last thermal event in these pegmatites, determined by K-Ar method in muscovite, is of Neoproterozoic age, coincident with the late stages of the Brasiliano tectono- metamorphic cycle. Chemical analyses showed that sodium is the alkali with higher contents in the aquamarines, thus enabling their classification as sodic beryls. In zoned samples there is an increase of Fe as well as Mn from center to border, while no systematic variation could be detected for other elements. The specific gravity of 2.72 to 2.80 g/cm 3 is higher than the values determined for samples from other pegmatites of Minas Gerais. The refraction indices are ne= 1.569 - 1.579 and nw= 1.573 - 1.581 and the birefringence varies from 0.002 to 0.008. The refraction indices increase with the BeO content. The color of aquamarines varies from medium to light blue, sometimes greenish blue. Mossbauer spectra obtained at room temperature and at 80 K show that Fe 2+ is the main cromophore-ion and suggest that the iron is present in octahedral sites as well as in the structural channels. Mossbauer spectra also indicate that the incorporation of Fe 3+ may cause a shift from deep blue to light blue colors. Based on their aquamarine composition the pegmatites were classified as barren, poor in rare alkalis (Li, Rb, Cs) and therefore little differentiated. The relatively simple mineralogy and the lack of lithium minerals such as lepidolite and spodumene confirm this classification. Thus, the composition of beryl can be used as a tracer for the prospection of pegmatites with different degrees of differentiation and consequently with different types of mineralization.