A study on the characteristics of some C- and CT-opals from Brazil

摘要

Physical properties of blue and fire opals found in Piaui state, Brazil, have been characterized through optical analysis specific gravity measurements, XRPD and 1R spectroscopy, whereas chemical composition has been determined by LA-ICP-MS and SEM-EDS. Values obtained for refractive index and specific gravity are n = 1.430-1.461 and G = 1.98-2.28, in agreement with literature. The results of the XRPD and IR-spectroscopy show the cristobalite presence (C and -CT type). Trace quantities of chromophore elements are correlated with the variations of colour. The particular "coralloid islands" structure, built up by micro-spherules of amorphous silica, observed by SEM analyses could explain the particular effects of iridescence shown by some blue specimens.