Tridymite
Tridymite is a high-temperature, low-pressure form of silica common in eucrites and mesosiderites.
Tridymite is another form of silicon dioxide, like quartz and cristobalite. It is stable between roughly 870 °C and 1,470 °C at low pressure, and it usually forms thin, plate-like crystals that are often twinned in groups of three, which gave the mineral its name from the Greek for 'triplet'.
Tridymite is the most common silica mineral in eucrites from Vesta and in mesosiderites, where it appears with pyroxene and plagioclase in the stony part of the rock. It also occurs in some lunar rocks, chondrites and Martian meteorites. In 2016, NASA's Curiosity rover found surprising amounts of tridymite in a mudstone in Gale Crater on Mars, which suggested that silica-rich volcanism once occurred there.
Because tridymite forms only at high temperatures and low pressures, it gives clear information about how and where a rock crystallised. It is a reminder that a single chemical formula can produce several minerals.
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