Tridymite

Tridymite is a low pressure, mostly high-temperature-stable polymorph of silica that can also form or persist metastably at low temperatures. The high-temperature form occurs most notably as vapour-deposited, platey crystals in vesicles in some volcanic rocks, also rarely as phenocrysts in some felsic volcanics, or as a contact metamorphic material in some hornfels.

Tridymite can occur in seven polytypes and the most common at standard atmospheric pressure are known as α and β; both commensurately and incommensurately modulated structure variants are known. Below 100 °C the triclinic form, α-tridymite is stable; there are also orthorhombic, monoclinic and hexagonal polytypes stable at higher temperatures. The orthorhombic β-tridymite polytype is most stable at elevated temperatures (>870 °C) and it converts to β-cristobalite above 1470 °C. However, tridymite does not usually form directly from pure β-quartz, and it is usually stabilised by alkali metals. Otherwise, β-quartz transitions directly to cristobalite at 1050 °C without the occurrence of the tridymite phase.

Some polytypes of tridymite (from Deer et al. 2004)

Name Symmetry Space group T (°C) HP (β) Hexagonal P63/mmm 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 above 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.

A low temperature form is commonly reported as a constituent of certain types of opal, intergrown with cristobalite (opal-CT), found in many environments including marine sedimentary rocks derived from biogenic opaline sediments, and low temperature cavity infillings, replacements, etc., including some precious opal (Sanders, 1975). Some wood opal is mostly tridymite (Mitchell & Tufts, 1973). However, some workers note that opal is hydrous and lacks any long-range ordering, the opal structure just mimicking cristobalite and tridymite, so may not contain true tridymite (Smith, 1998).

Large tridymite deposits have been detected on Mars, but their nature and origin are uncertain (Lakdawalla, 2015).