Gemmological Characteristics of Green Opals from Brazil

The gemmological characteristics of green opals found in Brazil, are quite different from those of traditional opals, and are investigated by using conventional gemmological testing methods and modern analytical techniques including electron microprobe (EPMA), Xray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR) and ultraviolet-visible absorption spectrometer (UV-Vis).The chemical compositions, mineral phase, microstructure, spectrum characteristics and colouring mechanism are also preliminarily researched.The results show that refractive index of the green opal samples from Brazil is 1.42-1.45, and density is 2.10-2.28g/cm³.The polarizing microscope investigations reveal that the green opal samples from Brazil are of irregular micelle structure and microwave-like texture, which indicate the heterogeneity of mineral component and distribution.The XRD analyses show that the green opals from Brazil are mainly composed of amorphous SiO₂, α-cristobalite andα-tridymite.The SEM images show random accumulation of SiO₂ nanoparticles about 10-30nm in diameter, most of which are assembled to form aggregates with diameters from 100nm to 400nm.It is then inferred that the uniform size, irregular shape and disordered arrangement of SiO₂ aggregates may result in unobvious play-of-color effect in green opals.The FT-MIR spectra characterize the vibration areas of Si-O groups.The asymmetry and symmetry stretching modes of Si-O mainly appear at 1 100, 795cm^-1 respectively.The bending vibration mode of Si-O appears at 475cm^-1. The FT-NIR spectra defines the vibration areas of different water species in green opals.The first overtone of H₂O and M-OH stretching modes mainly appear at 7 000cm^-1, and the H₂O combination mode appears at 5 200, 5 612cm^-1.The absorption bands relating to the combination modes of M-OH andSiO₄are between 4 100cm^-1 and 4 600cm^-1, which indicate that the water in green opals may occur in the form of planar water and structural water.The UV-Vis spectra and EPMA results show that the colour of green opals is mainly attributed to Fe³⁺, Fe²⁺ and Cr³⁺, and part Cr³⁺ impure ions tend to combine with H₂O intoCr (H₂O) 63+hydrated ions.The author further divides green opals form Brazil into Fe bearing ones and Cr bearing ones according to their UV-Vis characteristics and chemical compositions.The Fe³⁺ with ⁶A₁ →⁴E (⁴D), ⁶A₁ →⁴E+⁴A₁ (⁴D) and ⁶A₁ →⁴T₂ (⁴G) d-delectron transition absorption bands in Fe bearing opals appear at 373, 450and 635nm.The Cr³⁺ with ⁴A_2g →⁴T_1g (⁴F), ⁴A_2g →⁴T₂g (⁴F) and ⁴A_2g →²E_g (²G) d-delectron transition absorption bands in Cr bearing ones appear at 420, 608-620and 684nm respectively.