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Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. It is very rare on the earth's surface. It may however be a predominant form of silicon dioxide in the earth, especially in the lower mantle.〔Dmitry L. Lakshtanov et al. "The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth" PNAS 2007 104 (34) 13588-13590; 〕 Stishovite was named after Sergey M. Stishov, a Russian high-pressure physicist who first synthesized the mineral in 1961. It was discovered in Meteor Crater in 1962 by Edward C. T. Chao. Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile (TiO2). The silicon in stishovite adopts an octahedral coordination geometry, being bound to six oxides. Similarly the oxides are three-connected, unlike low pressure forms of SiO2. In most silicates, silicon is tetrahedral, being bound to four oxides. It was long considered the hardest known oxide (~30 GPa Vickers〔); however, boron suboxide has been discovered in 2002 to be much harder. At normal temperature and pressure, stishovite is metastable. Stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.〔 ==Appearance== Large natural crystals of stishovite are extremely rare, and are usually found as clasts of 1 to 2 mm in length. When found, they can be difficult to distinguish from regular quartz without laboratory analysis. It has a vitreous luster, is transparent (or translucent), and is extremely hard. Stishovite usually sits as small rounded gravels in a matrix of other minerals. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Stishovite」の詳細全文を読む スポンサード リンク
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