Crystal structure and growth fabric of length-fast chalcedony (original) (raw)

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%).