Iron: A Significant Determinant of Colours in Gem Tourmalines from Southwestern Nigeria

ABSTRACT; Gem tourmalines from southwestern Nigeria vary highly in composition and display limitless plethora of colours. This study intended to link the chemistry of these tourmalines with their colours. Fifteen differently coloured samples of tourmaline from threelocalities; Komu, Ijero-Ekiti and Okerimi-Oro were pulverized and analysed through the inductively coupled plasma-mass spectrometry (ICP-MS) method to characterize the chemical compositions. Results showed wide variation among the major oxides, SiO2and Al2O3with standard deviations of 11.74 and 9.52 respectively and narrow variation for Fe2O3with standard deviation of 2.74, while the remaining oxides remained depleted in the studied samples. Fe2O3was significantly high in samples R32(4.01%), R35(7.25%), R33(7.65%) and R34(9.22%), which are blue-green, dark-green, dark-blue and dark-blue coloured tourmalines respectively, relatively high in samples R048(2.58%) and R049(2.61%), a green and grey tourmaline respectively and of very low concentrations in samples R046 (0.15%) and R052(1.82%), a pink and red tourmaline respectively. This showed that the blues and the darker greens had the most iron, becoming less in the lighter shades of green and completely depleted in the lightest pink.TiO2 and MnO values onthe other hand have remained virtually very low and fairly constant throughout the analyzed samples despite the varied colours among them. This study showediron to be chiefly responsible for colour variation among the studied southwestern Nigerian tourmalines.