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.
Trace elements and mineral chemistry of silicified wood from Thailand: colours and elemental distribution
ABSTRACT Thai silicified woods were examined using electron probe microanalysis, yielding chemical data that characterised the samples into two groups: low and high silica contents (82—94 wt% and 94—98 wt%). The elements analysed in order of abundance include Si > Fe > Ca > Na > Al > Ti > K > Mg > Mn > Zr. Iron plays a major role in the colour range (red, orange, yellow, brown, grey and black) of the samples. Calcium is associated with Fe in the darker colours of the wood. Pseudo-crystallochemistry has been used for the substitution of trace elements for Si4+ in silica polymorphs. The atomic channels that run parallel to the c-axis of silica polymorphs or lattice defects, or even the charge balance for trivalent-ion substitution for Si4+, can accommodate monovalent ions (K+ and Na+). Vacant and atomic cavities, which are charged balanced by trivalent ions [Al3+ or Fe3+ substituting for Si4+], are commonly occupied by divalent ions (Ca2+, Mg2+ and Mn2+). Quadrivalent ions, Ti4+ and Zr4+ are non-structurally incorporated but form clusters of mineral inclusions in the samples. Several other trace-element contents are also in the form of mineral/fluid inclusions hosted in the woods.
The EU Legal Framework for the Enforcement of Consumer Law
The European Union (EU) is special in that it is a multi-level system in which, principally, consumer law can be adopted at the EU level but it is enforced at the level of the Member States. Nevertheless, the EU has over time developed an ever denser legal framework for the enforcement of consumer law by the Member States. It is this legal framework that this contribution focuses upon.