a number of mineral species that are polymorphic modifications of silicon dioxide. The minerals are stable within certain temperature intervals as a function of the pressure (see Figure 1 and Table 1).
The crystal structure of silica minerals is derived from the three-dimensional lattice of tetrahedrons, which are interconnected by the common oxygen atoms (SiO4). However, they differ in the symmetry of their arrangement, packing density, and mutual orientation, which is reflected in the crystal symmetry of the various minerals and in their physcial properties. Stishovite is an exception, consisting of octahedrons (SiO6) that form a structure resembling rutile. All silica minerals, with the exception of some varieties of quartz, are usually colorless. Their hardness on the mineralogical scale varies from 5.5 (α-tridymite) to 8–8.5 (stishovite).
Silica minerals are found as small grains, as cryptocrystalline fibrous formations (α-cristobalite, lussatite), and, sometimes, as spheroidal formations. Silica minerals are more rarely found in the form of small tabular or lamellar crystals (tridymite), octahedral and bipyramidal crystals (β- and β-cristobalite), and acicular crystals (coesite, stishovite). Most silica minerals (except quartz) are very rare and unstable under the conditions prevailing in the surface strata of the earth’s crust. High-temperature modifications of SiO2,, such as β-tridymite and β-cristobalite, are formed in small cavities of young effusive rocks (dacites, basalts, liparites). Low-temperature α-cristobalite, along with α-tridymite, is a component of agate, chalcedony, and opal, which are deposited from hot aqueous solutions and sometimes from colloidal SiO2. Stishovite and coesite have been found in the sandstones of Meteor Crater in Devil’s Canyon (Arizona, USA), where these minerals were formed from quartz at the instantaneous superhigh pressures and high temperatures accompanying the impact of the meteorite. The following minerals are also found in nature: quartz glass (lechatelierite), which is formed by the fusion of quartz sand by lightning, and melanophlogite, consisting of small cubic crystals and crusts (pseudomorphs consisting of opal- and chalcedony-type quartz), which were deposited onto native sulfur in formations in Sicily (Italy). Keatite has not been found in nature.
REFERENCESDana, J., E. S. Dana, and C. Frondel. Mineraly kremnezioma. Moscow, 1966. (Sistema mineralogii, vol. 3. Translated from English.)
Mineraly, vol. 2, 2nd ed. Moscow, 1965.
Kostov, I. Mineralogiia. Moscow, 1971. (Translated from English.)
|Table 1. Polymorphs of silica|
|Mineral||System||Pressure (at)°||Temperature (°C)||Density kg/m3|
|* 1 at = 1 kgf/cm2 = 0.1 MN/m2|
|α-tridymite (metastable)||below 117||2,260†|