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.
Amethyst: Optical Properties and Paramagnetic Resonance
Color centers in amethyst are produced by the action of ionizing radiation on precursor centers that arise from substitution of Fe+3 for Si+4 in the α-quartz structure. These Fe+3 centers provide the dominant features of the electron paramagnetic resonance spectrum of amethyst. The three equivalent Si+4 sites are unequally occupied by Fe+3, a circumstance that explains the optical biaxiality of amethyst.
Anisotropic Color Centers in α Quartz. Part I. Smoky Quartz
The color center at 460 mμ produced by x‐irradiation of naturally colorless or bleached smoky quartz is anisotropic. The absorption maximum of this color center for the extraordinary ray is 460 mμ and for the ordinary ray is 485 mμ. A second anisotropic peak with maximum in the vicinity of 625 mμ for the extraordinary ray is also present. The relative anisotropy of these color centers in a natural, a Bell synthetic and a Brush synthetic quartz are compared and found to be nearly identical in the case of the synthetic quartzes studied. The absorption coefficients of these color centers x‐rayed to saturation are compared with total aluminum content for five specimens.