MOSSBAUER STUDY OF BRAZILIAN COLUMBITE

Columbite mineral from Lavra de Golconda in Governador Valadares, Minas Gerais, Brazil has been characterized by X-ray diffraction and electron microscopy. It has short prismatic nonconducting crystals with noninherent impurities of quartz, and exhibits one set of perfect cleavage. The reflectance is lowest normal to the cleavage surface, almost black. The surface is uneven and has small particles in a regular manner. On the other cleavage surface there is a regular pattern of white adherent impurity. The chemical composition of (Fe0.32Mn0.68)Nb2O6 was determined by mass spectrometry. The mineral has been studied by Mossbauer spectroscopy down to liquid nitrogen temperature. The isomer shift is 1.15 mm/sec and is almost temperature independent, but, the quadrupole splitting (1.55 mm/s at room temperature) is strongly temperature dependent (2.23 mm/s at liquid nitrogen temperature with large linewidth). This temperature dependence is explained in terms of relatively small crystal-field splitting of the t2g levels.