Crystal structure and growth fabric of length-fast chalcedony |
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Authors: | G Miehe H Graetsch O W Flörke |
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Institution: | 1. Institut für Kristallographie der Universit?t Frankfurt, Senckenberg-Anlage 30, 6000, Frankfurt, West Germany 2. Institut für Mineralogie der Ruhr-Universit?t Bochum, Universit?tsstr. 150, 4630, Bochum, West Germany
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Abstract: | Chalcedony from Brazilian agates, has been investigated by using transmission-electron microscopy, X-ray-diffraction, thermogravimetry and optical techniques. The quartz fibers of length-fast chalcedony are composed of submicroscopical polysynthetic, lamellar-twinned right- and lefthanded crystals, according to the Brazil law. This very narrow twinning causes 3 systems of diffuse diffraction streaks (corresponding to the three-fold symmetry) parallel to 〈10.1〉, very frequently possessing an intensity maximum at h±1/2, k, l±1/2. These extra reflections were detected both in electron- and X-ray-diffraction patterns. Wall-lining chalcedony is parallel fibrous consisting of smaller crystallites with a higher total water content (0.06±0.01 μm and 1.2±0.1 wt %) than spherulitic chalcedony in horizontal agate bands (ca. 0.1 μm and 0.7±0.1 wt%). |
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