Cristobalite
Cristobalite is a high-temperature form of silica found in some eucrites, Martian and lunar rocks.
Cristobalite has the same chemistry as quartz, silicon dioxide, but a more open crystal structure that is stable at high temperatures and low pressures. It forms in volcanic rocks that crystallised at high temperature and cooled quickly enough to keep it.
In meteorites, cristobalite and its cousin tridymite are the usual silica minerals, while quartz is much rarer. They occur as small grains in eucrites, mesosiderites, some ordinary chondrites and Martian shergottites, and in lunar basalts.
Cristobalite often shows a distinctive cracked, tile-like pattern called curved fractures, formed when the crystal changed structure on cooling. Recognising which silica mineral is present helps petrologists judge the temperature and pressure history of a rock. On Earth, cristobalite dust can harm the lungs, which is one reason meteorites should never be ground or crushed at home.
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