Railsback's Some Fundamentals of Mineralogy and Geochemistry The many forms of silica (SiO2) I: minerals and solutes Melt Silica minerals (more or less) ~80 kb Stishovite High-temperature silica found in Cristobalite meteorite impacts Lechatelierite highest-temperature form of SiO2, Coesite G = 4.35 with ! and " polymorphs, found in fused silica (silica glass) found in fulgurites found in meteorite siliceous volcanic rocks Siefertite (lightning-strike features) ~20 kb impacts and found in G = 2.32 (!) 2.20 (") and meteorite impacts. eclogite Martian meteorites G = 3.01 Six-fold Gcoordination = 4.3 Cooling Shocked quartz: Four-fold coordination ~1470°C Tridymite quartz with planar deformation features Impact a high-temperature form of resulting from SiO2 with multiple intense pressure polymorphs, found in Moganite siliceous volcanic rocks Quartz low-T low-P form of low-T low-P form of SiO G = 2.22-2.26 2 SiO found with quartz found in igneous, sedi- 2 High-pressure silica in microcrystalline silica mentary, and metamorphic ~870°C deposits such as rocks and other Earth- chalcedony and chert. Dis- surface materials. Of its solved two polymorphs, ! and ", ! Chalcedony: silica Varieties of quartz is the one stable at most fibrous microcrystalline Lussatite: crustal and all surficial quartz with lesser length-slow fibrous distinguished by color: moganite. Orthosilicic acid Rock crystal (clear) conditions. opal-CT Milky quartz (white) G = 2.65 (!) H4SiO4 or Si(OH)4 Rose quartz (pink) 2.53 (") Four-fold coordination Citrine (yellow) Monomer is most Amethyst (purple) Opal Three-fold coordination abundant in dilute Dehydration & Cairngorm (brown) SiO · nH O solutions; dimer is recrystallization 2 2 increasingly abundant Smoky quartz (black) Opal CT G = 2.0-2.3 in concentrated opalline silica yielding solutions. Herkimer diamonds: weak cristobalite and doubly-terminated tridymite XRD peaks Silica precipitated Meta- quartz crystals Opal A H Si O or from very super- silicic Acid 6 2 7 amorphous saturated solutions Si2O(OH)6 opalline silica H2SiO3 or SiO(OH)2 or by organisms Sources of information include Heaney, P.J. et al., eds., 1994, Silica: physical behavior, geochemistry, and materials applications: LBR SFMGFormsofSilica08 10/2009 Reviews in Mineralogy v. 29; Britten, W.G.A., and Books, J.R.V., 1972, The Penguin Dictionary of Geology; Hurlburt, C.S., Jr., and Klein, C., 1978, Manual of Mineralogy (19th edn.); Clark, A.M., 1993, Hey's Mineral Index.; El Goresy et al., 2008, Eur. J. Mineral. 20: 523-528..