FAAS, FT-IR and XRD Identification of Natural and Heat Treated Opals Located in Wadla Woreda, North Wello, Ethiopia

Opals are naturally occurring hydrous silica materials (SiO2*nH2O), and have been largely used in jewelry and as decorative elements in artworks due to their optical properties. In this study, we present some gemological properties, a combined spectroscopic (FAAS, FTIR) and X-ray powder diffraction (XRD) identification of natural and heat treated opals obtained from the main deposits in North Wollo, Ethiopia. The gemological measurements, FTIR and XRD spectra for natural and heat treated samples are nearly identical, FAAS shows clear differences on their color this may be due to the concentration differences of metals. Both of these opals showed spectra and diffraction patterns typical of Opal-CT, with clearly defined patterns and main peaks in the 2θ range of cristobalite and tridymite, called microcrystalline opals.

Phase transformation of hydrous ringwoodite to the lower-mantle phases and the formation of dense hydrous silica

To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.

Density and loss on ignition as indicators of the fossilization of silicified wood

Measuring density of silicified wood and determining weight loss after 450°C heating provides useful data for interpreting the process of permineralization. These simple gravimetric methods do not replace X-ray diffraction, electron microscopy, polarized light microscopy, Raman spectroscopy, and other specialized techniques for studying fossil wood, but they can be performed rapidly, and require minimal laboratory facilities. Woods mineralized with opal have densities of 1.9–2.1 g/cm3, compared to 2.3–2.6 g/cm3 for wood mineralized with chalcedony or quartz. Weight loss after 450°C heating, commonly described as “loss on ignition” can be used to roughly estimate the % of original organic matter that remains in chalcedony or quartz-mineralized wood, using the density of extant taxa for comparison. For opalized wood, 450°C weight loss mostly represents dehydration of the hydrous silica. Data from specimens from 20 localities reveal two characteristics: 1) silicified woods typically consist either of opal or chalcedony/quartz, not an intermediate mixture of the two silica polymorphs; 2) the percentage of organic matter that remains after petrifaction is usually very small.