On the pleochroism of amethyst quartz and its absorption spectra
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Amethyst 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.
KeywordsUnpolarized Light Secondary Sector Amethyst Trigonal Symmetry Optical Indicatrix
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