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.
Characterization of quartz in some potential high purity quartz deposits in the Damara orogenic belt
A mini thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science (Applied Geology)
The kinetics of colour centre formation in quartz
Abstract It is shown that when free carriers are generated in a solid containing two or more types of defect capable of trapping electrons or holes, the occupancy of one type of trap is not, in general, a simple irradiation time dependent function of the form 1-exp (-t/τ) but is dependent on the parameters of the other traps and at saturation can be significantly lees than unity. Using a model for crystalline quartz it is possible to explain radiation bleaching by both x-rays and neutrons, by considering the dependence of the occupancy of one trap on other traps. With the same model an explanation of the effect of heat treatment on the kinetics of colouration observed by Forman (1961) is given. From a quantitative comparison between the predicted and observed rate curves of an x-ray irradiated crystal, f-values and defect concentration are calculated. Less detailed comments are made on the coloration of fused quartz.