Chemical and structural studies on natural spinels, MgAl2O4, of Sri Lanka

Abstract

Natural Spinels from Sri Lanka (Ratnapura and Bibile) have been found to be mainly in the form, MgAl204 , with minor substitutions of Fe (0.7-3.5 wt% FeO) and Zn (0.07-4.4 wt% ZnO). Concentrations of other elements such as Cr, Co, Ni, V, and Mn were very low and, less than 0.1 wt% oxide. The natural spinels were found to have a complete normal cation distribution, A II[ ,(B III2)O4, suggesting an origin from a slow cooling rock forming process of the igneous and metamorphic rocks. Available experimental data of the Sri Lankan natural spinels indicate that complete ordering occurs at 280 ±175oC. The lattice constants were in the range of 8.087-8.091 A. Heating of the natural spinel results in disordering which may lead to a decreasing cell edge. Disordering of Al at 1000oC, as determined from AI-NMR spectroscopy, shows 13% of the Al occupying the tetrahedral sites. Optical spectra in the visible region are dominated %y spin forbidden transitions in Fe2+ and in some Cr-rich samples by spin allowed transitions in chromium. These are the main features that determine the colour of natural spinels investigated in the present study. The study indicates that the natural spinels have ordered cation distribution and the cation distribution in tetrahedral and octahedral sites depends on the thermal history of the crystal. The presence of transition elements, mainly Fe and Cr upto about 0.01 wt% oxide is sufficient to produce intense colour in spinels.