Physical and chemical properties of some italian opals

The physical and compositional properties of some opals from different parts of Italy have been investigated through several methodologies like optical analysis, specific gravity, refractive indices, XRPD, IR spectroscopy, LA-ICP-MS. The opals show different colors: white, brownish-white, yellowish-white, yellow-/greenish white and grey. Black and metallic inclusions, consisting of todorokite, are sometimes present. Play-of-color have not been observed but some opals show small iridescence zones; opals are inert to the long and short wavelength UV radiation (366 - 254 nm) with the exception of one sample; also phosphorescence is absent. Refractive index and specific gravity values are respectively 1.43 to 1.44 and 2.07 to 2.33 g/cm3 consistent with data available in the literature. XRPD analyses highlighted Italian opals are A, CT and C types, but most of them can be classified as opals- CT. IR spectroscopy data confirmed the opal classification. The most abundant impurities are Mg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (72-96 ppm) and Ni (20-65 ppm). Similarly to what observed in other opals worldwide, Fe appears to be the principal factor that determines the brown to red to orange to yellow colors. Chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and probably do not influence the physical properties of the Italian opals analyzed. Ni is present in more relevant amounts and is probably related to clay minerals. Mn is clearly detected only in two samples and is related to the presence of dendrites. The investigated Italian opals show a rather homogeneous trace element composition which is in the range of worldwide opal’s composition, except for their noticeable amount in Ni.