SILICA MINERAL

any one of the forms of silicon dioxide (SiO2), including quartz, tridymite, cristobalite, coesite, stishovite, melanophlogite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically, among which are keatite and silicalite. Except for stishovite, all silica minerals are made up of tetrahedral groups comprised of four oxygen atoms surrounding a central silicon. Each tetrahedral group shares an oxygen atom with another tetrahedral group, forming a three-dimensional structure. The principal difference among the various silica minerals is the detailed geometry of the arrangement of tetrahedra, which gives rise to different crystal structures and thus physical properties. Quartz has a relatively dense packing of tetrahedra compared to tridymite and cristobalite, which exhibit relatively large open cavities. This packing difference is reflected in their densities: 2.65, 2.32, 2.26 grams per cubic centimetre for quartz, cristobalite, and tridymite, respectively. Each of these three polymorphs of silica has a field of stability under equilibrium conditions, but because transformation from one structure to another is sluggish, tridymite and cristobalite are found within the stability field of quartz. Each of the polymorphs also has high- and low-temperature modifications that are only slightly different structurally. Therefore, under low-pressure conditions, low-quartz is stable until 573 C (1,063 F), at which point high-quartz becomes stable. At 867 C (1,593 F), high-quartz transforms to tridymite. The high-low transformations require only slight displacements of the tetrahedral groups and occur rapidly. Compositionally, quartz is usually quite pure, with only traces of other elements. In contrast, tridymite and cristobalite may contain up to about one percent by weight of impurities because of the open nature of their framework which easily accommodates other atoms, especially those of aluminum, sodium, potassium, and lithium. Quartz is the second most abundant mineral in the Earth's crust and is present in many igneous rocks ( e.g., granite and syenite) and in hydrothermal vein deposits. Because it is both chemically and mechanically stable at low temperature, quartz persists in clastic sedimentary rocks. Tridymite and cristobalite occur in volcanic rocks such as rhyolite and slowly invert to quartz polymorphs. Of the other silica phases, coesite and stishovite are high-pressure polymorphs found where quartz was shocked by meteorite impact. Coesite also occurs in some eclogite xenoliths from the Earth's upper mantle. Stishovite is unlike other silica phases in that silicon is in octahedral rather than tetrahedral coordination, resulting in a high density of 4.3 g/cc. Keatite is a high-pressure polymorph, but it has not been found in nature. Chalcedony is cryptocrystalline silica consisting of minute quartz crystals and submicroscopic pores. Melanophlogite has an open structure large enough to occlude sulfur species. Lechatelierite is silica glass (amorphous) and is only rarely found in nature. any of the forms of silicon dioxide (SiO2), including quartz, tridymite, cristobalite, coesite, stishovite, melanophlogite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically, among which are keatite and silicalite. Additional reading W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); J.J. Papike and M. Cameron, Crystal Chemistry of Silicate Minerals of Geophysical Interest, Reviews of Geophysics and Space Physics, 14(1):3780 (1976). William B. Simmons