tridymiitti

Tridymiitti (Finnish for tridymite) is a high-temperature polymorph of silicon dioxide (SiO2). It is one of several crystalline forms silica can take and occurs relatively rarely in nature compared with quartz.

In nature there are two main crystalline forms of tridymite: α-tridymite and β-tridymite. They differ in crystal symmetry and are stable at different temperature ranges, with transitions between them occurring at high temperatures. Upon cooling, tridymite can transform into other silica polymorphs such as cristobalite or quartz, depending on the cooling rate and pressure conditions.

Natural occurrence is primarily in silica-rich, high-temperature environments. Tridymite is found in certain volcanic rocks and glasses, especially rhyolites and obsidian, often as minute crystals within vesicles or dispersed in the glassy matrix. Because it forms under specific high-temperature conditions, its presence can provide information about the thermal history of the host rock.

Formation and stability are tied to high-temperature crystallization of silica-rich melts or vapors, typically at low pressures. Tridymite is metastable at ambient conditions and often does not persist once rocks cool slowly, where quartz or cristobalite may form instead. It is therefore commonly encountered as microscopic crystals or as inclusions in volcanic glass.

Identification relies on mineralogical techniques such as X-ray diffraction and electron microscopy. Tridymite has limited industrial use compared with other silica polymorphs, but it remains of interest to geologists and mineralogists for understanding high-temperature silica processes and volcanic histories.