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.
On the pleochroism of amethyst quartz and its absorption spectra
9. SummaryAmethyst is optically biaxial, and does not conform to the trigonal symmetry of colourless quartz, though thec-axis appears as the acute bisectrix in its biaxial figure. The pleochroic and biaxial properties taken together conform only to a symmetry of the monoclinic class, the axis of symmetry being coincident with one of the electrical axes of the colourless quartz in which it appears. For vibrations along this axis, the colour of the transmitted light is light orange (absorption maximum near 5000 Å); while for the two vibrations in the perpendicular plane which make approximately 45° on either side with thec-axis, the colours are respectively, blue (absorption maximum near 5750 Å), and a deep reddish-purple (absorption maximum near 5250 Å).Which of thea-axes of quartz is the symmetry axis of amethyst is determined—in relation to the particular rhombohedral face with which the colour-sector is associated—by the right- or left-handedness of the segment; while the disposition of the other two colour axes with reference to the associated face depends on whether this belongs to the primary or the secondary rhombohedron. Hence, the presence of any twinning naturally complicates the pleochroism.Photographs are reproduced illustrating (a) the absorption spectra for the trichroic colours; (b) the idiophanous and biaxial figures seen through dense sectors in convergent light (the optic axial plane containing the symmetry axis); and (c) the anomalous dispersion of the birefringence.
Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas
The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.