Quartzine

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Quartz is a microcrystalline, fibrous structure variety of the mineral quartz (more precisely deep quartz ). The fibers are aligned along the crystallographic c-axis and have a thickness of less than one µm. From a mineralogical point of view, quartzine is often assigned as a sub-variety to the very similar chalcedony . In contrast to this, however, the higher refractive index is aligned parallel to the longitudinal direction of the fibers and the optical character of the fiber direction is thus "length-slow". Furthermore, quartzine shows no twisting of the fibers and no wrinkles .

Quartzine is translucent and cloudy and, like chalcedony in its pure form, is colorless to bluish-gray. Due to various foreign admixtures , however, it usually takes on a brownish, reddish or greenish color. The surfaces of the quartz aggregates show a waxy sheen . Due to the fibrous structure, its Mohs hardness of 6.5 to 7 is just below that of quartz (7).

Education and Locations

Quartzine forms together with chalcedony and mogánite near the surface in crevices and cavities of acidic and basic igneous rocks . In the formation of agate going from a crystallization of a SiO 2 - gel or SiO 2 -übersättigten solutions at temperatures between 25 and 200 ° C from.

Furthermore, quartzine forms in sedimentary form during the solidification of sandstone as one of the cement phases in the pore spaces of the sandstone.

morphology

Quartzine forms both spherulitic and parabolic aggregates.

Of nodular structure is when the fibers from a seed crystal to grow radialstrahlig and form spherical aggregates. The diameter of these spherulites is usually 0.1 to 0.2 millimeters. Under the polarization microscope , the spherulites show a characteristic extinction image when the polarizers are crossed, the “Bertrand's cross”.

A parabolic structure is formed when the mineral fibers grow radially on the surface of a rock cavity. Neighboring tufts of fibers hinder each other in their growth. Even at a short distance from the growth nucleus, only almost parallel fiber bundles grow into the cavity. Parabolic quartzine is found alternating with chalcedony layers in the concentric bands of the agates.

Crystal structure

Structurally, quartzine hardly differs from chalcedony and deep quartz. Electron microscopic examinations revealed a very high density of lattice defects for all microcrystalline quartz varieties . A dense sequence of twinning according to the “Brazilian law” is characteristic. However, quartzine does not show a smooth transition from twinned deep quartz to mogánite , as is typical for chalcedony. In quartz, areas with deep quartz structure are clearly delimited from areas with mogánite structure.

literature

  • Flörke, OW, Graetsch, H., Martin, B., Röller, K., Wirth, R. (1991): Nomenclature of micro- and noncrystalline silica minerals, based on structure and microstructure; New yearbook miner. Abh., 163: 19-42
  • Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , fine crystalline varieties. Chalcedony in the broader sense , p. 526 (first edition: 1891).

Web links

Individual evidence

  1. ↑ Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  440 .
  2. ^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin / New York 1981, ISBN 3-11-006823-0 , p. 423 .