Tridümiiti

Tridümiiti is a polymorph of silica, meaning it is a mineral composed of silicon dioxide (SiO2) but with a different crystal structure than common quartz. It is a high-temperature and high-pressure polymorph, typically forming under conditions not found at the Earth's surface. Tridümiiti is often found in volcanic rocks, particularly in those that have undergone rapid cooling after eruption. It can also occur in meteorites and in industrial materials like refractory bricks.

The crystal structure of tridümiiti is characterized by a hexagonal lattice. At lower temperatures, it undergoes a series of phase transitions to various other structural forms, including alpha-tridümiiti, beta-tridümiiti, and gamma-tridümiiti, each with slightly different arrangements of the SiO2 tetrahedra. These transitions are often displacive, meaning the atoms shift positions without breaking bonds.

While quartz is the most common silica polymorph, tridümiiti is less stable under typical Earth surface conditions and tends to invert to quartz over geological time. However, its presence in certain rocks can provide valuable clues about the thermal and pressure history of those rocks. It is generally considered a metastable phase when found in surface environments. Its formation requires temperatures above 870 °C and pressures below 2 kbar.