Biaxial Color Centers in Amethyst Quartz
Anisotropy of the characteristic absorption bands in a natural amethyst crystal from Brazil is demonstrated by the dependence of the value of the absorption coefficient upo,n the direction of the electric vector of incident light within the 4.13-0.95 eV region, for differently oriented sections of the crystal. The absorption indicatrix is used for interpreting the observed optical absorptions. A new absorption band in the 6.20 eV region is reported. A complex absorption band in the 5.60 eV region, usually attributed to a transition related to ligand-cation charge transfer in Fe"* compounds, is clearly resolved into at least three bands. The biaxiality and pleochroism of amethyst is explained as due to the existence of strongly anisotropic color centers, related to iron, of orthorhombic or lower symmetry. Upon bleaching the amethyst color, either optically or thermally, the biaxiality disappears. This anomalous biaxiality and pleochroism observed in amethyst quartz is a property oi the color center itself and not a property of the c-quartz Crystal structure.
Iron in Synthetic Quartz: Heat and Radiation Induced Changes
As part of a study of the possible changes in silicate material on the lunar surface produced by solar radiation, effects of ionizing radiation, of heating, and of heating followed by ionizing radiation on iron-doped synthetic quartzes have been studied as models. The optical changes are mainly related to valence changes in ferric or ferrous ions.
Geological, mineralogical and chemical characterization of Devonian kaolinite-bearing sediments for further applications in the ceramic (tiles) industry in La Paz, Bolivia
Clay minerals are widely distributed in Bolivia; among them, illitic clays are most common in the Altiplano where they are mined for use in the ceramic industry that has been growing in the last few years. In addition to illitic sediments, kaolinitic sediments have been recently discovered in sedimentary units in the Bolivian Altiplano. Residual ball clay occurrences in Devonian sedimentary units were studied as part of this work. Geological mapping and geophysical studies (ERT and GPR) were done for better understanding the origin of the deposits and were part of a preliminary study of the mineral potential to define the relationship with the host rock. Chemical and mineralogical techniques such as X-Ray Diffraction, Scanning Electron Microscopy and Inductively Coupled Plasma analyses were performed in samples from the studied area to verify the presence of kaolinite. Atterberg limits and behaviour of the raw material in ceramic specimens supported by chemical analyses show that this material is suitable for manufacturing tile ceramics. This study provides fundamental knowledge for deposit exploitation and future generation of an alternative source of employment for the inhabitants of Micaya.