tridymite

{{Infobox mineral | name = Tridymite | category = Tectosilicate minerals | group = Quartz group | boxbgcolor = | image = Tridymite tabulars - Ochtendung, Eifel, Germany.jpg | imagesize = 260px | caption = Tabular tridymite crystals from Ochtendung, Eifel, Germany | formula = SiO2 | molweight = 60.08 g/mol | strunz = 4.DA.10 | IMAsymbol = Trd | system = Orthorhombic (α-tridymite) | class = Disphenoidal (222) H–M symbol: (222) | symmetry = C2221 | color = Colorless, white | habit = Platy – sheet forms | cleavage = {0001} indistinct, {1010} imperfect | fracture = Brittle – conchoidal | mohs = 7 | luster = Vitreous | refractive = nα=1.468–1.482 nβ=1.470–1.484 nγ=1.474–1.486 | opticalprop = Biaxial (+), 2V = 40–86° | birefringence = δ 2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings (compound crystals comprising three twinned crystal components).

==Structure== 100px|thumb|left|Crystal structure of α-tridymite 100px|thumb|left|β-tridymite Tridymite can occur in seven crystalline forms. Two of the most common at standard pressure are known as α and β. The α-tridymite phase is favored at elevated temperatures (above 870 °C) and it converts to β-cristobalite at 1,470 °C. However, tridymite does usually not form from pure β-quartz, one needs to add trace amounts of certain compounds to achieve this. Otherwise the β-quartz-tridymite transition is skipped and β-quartz transitions directly to cristobalite at 1,050 °C without occurrence of the tridymite phase. {|class="wikitable" style="text-align:center" |+Crystal phases of tridymite !Name!!Symmetry!!Space group!! (°C) |- | HP (β) ||Hexagonal||P63/mmc|| 460 |- | LHP||Hexagonal||P6322|| 400 |- | OC (α)||Orthorhombic||C2221|| 220 |- |OS ||Orthorhombic|||| 100–200 |- |OP ||Orthorhombic||P212121|| 155 |- | MC||Monoclinic||Cc|| 22 |- | MX||Monoclinic||C1|| 22 |} In the table, M, O, H, C, P, L and S stand for monoclinic, orthorhombic, hexagonal, centered, primitive, low (temperature) and superlattice. T indicates the temperature, at which the corresponding phase is relatively stable, though the interconversions between phases are complex and sample dependent, and all these forms can coexist at ambient conditions. Mineralogy handbooks often arbitrarily assign tridymite to the triclinic crystal system, yet use hexagonal Miller indices because of the hexagonal crystal shape (see image). thumb|P-T Diagram for