Boron Measurement in Tourmaline from Pegmatite Veins, Simdega Area, Chhotanagpur Gneissic Complex, Eastern India using Electron Probe Microanalysis

Tourmaline group of minerals, the primary source of boron, are cyclosilicates which are widespread in the earth’s crust. Earlier studies involving the nomenclature and classification of tourmaline were based on the measurement of its common elements (Al, Mn, Fe, Mg, etc.). In all such studies, boron was assumed to be fixed in the composition and restricted only to the triangular structural site. However, recent discovery of the presence of boron in the tetrahedral structural site as well, necessitates the measurement of boron content. Much of the earlier attempts to measure boron were based on solution methods, and electron microprobe analysis (EPMA) was the least used due to low levels of detection of its analytical crystals. In the present study, we quantify boron – particularly along with fluorine and other major elements – in tourmaline grains using high-sensitivity PC3 analytical crystal. We found that the measured boron content slightly exceeds that of the stoichiometrically calculated boron. Also, the studied tourmalines come under the alkali group in general and belong to the schorl– dravite solid solution series in particular.