Cristobalite

High temperature polymorph of silicon dioxide (SiO₂). Has the same chemical composition as coesiteHigh-pressure polymorph of silicon dioxide (SiO2). Has the same chemical composition as cristobalite, stishovite, seifertite and tridymite but possesses a different crystal structure. Coesite forms at intense pressures of above about 2.5 GPa (25 kbar) and temperature above about 700 °C, and was first found naturally on Earth in impact Click on Term to Read More, stishoviteDense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized in 1961 before it was discovered at Meteor Crater, Arizona. Its structure consists of parallel chains of single SiO6 octahedra. The octahedra are on their sides, sharing opposing edges. Image Click on Term to Read More, seifertite and tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. but possesses a different crystal structureMutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal. Click on Term to Read More. This silica groupMminerals formed exclusively from silica. There are ten known silica polymorphs, two of which are synthetic. Five of the naturall polymorphs are related by reconstructive transformations and can exist metastably: stishovite, coesite, quartz, tridymite, and cristobalite. Conditions to form coesite and stishovite are attained only during meteoroid impacts where there mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More occurs in terrestrial volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid. rocks, martian and lunar meteoritesAchondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman and others were found in the cold desert of Antarctica, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because, Click on Term to Read More, chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More and impact glasses like Libyan Desert GlassHigh-silica glass (SiO2) found in western Egypt near the Libyan border. The glass is often worn smooth due to aeolian sandblasting and ranges in color from milky white to the gem quality translucent yellow-green. The glass often contains small bubbles and lechatelierite. The origin of the glass is still disputed, Click on Term to Read More. Cristobalite has a very open structure consisting of sheets of tetrahedra in 6-fold rings with tetrahedra pointing alternately up and down. The α to β inversion occurs at 268 °C. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures.