Coesite
Coesite is a dense, high-pressure form of silica (SiO2) that, in surface rocks, is usually a fingerprint of a meteorite impact.
Coesite has the same chemical formula as quartz, SiO2, but its atoms are packed into a denser, monoclinic framework with a density of about 2.9 g/cm³. It is stable only at pressures of roughly 2–3 gigapascals (GPa) and above. Chemists first made it in the laboratory in 1953, and it is named after the American chemist Loring Coes Jr. In 1960 it was discovered in nature for the first time, in shocked Coconino Sandstone at Barringer Crater (Meteor Crater), Arizona, a find that helped prove the crater was made by an impact.
Why does it matter? When an impact shock wave passes through quartz-rich rock, it briefly reaches pressures that crustal processes cannot, and some quartz converts to coesite before the pressure drops. Coesite has since been confirmed at many impact structures, including the Nördlinger Ries in Germany. It is not unique to impacts, though: it also occurs in ultra-high-pressure metamorphic rocks that were carried deep into the mantle and brought back up, so the geological setting must always be considered.
Coesite cannot be identified by eye. It forms tiny grains, often in glassy or granular patches within shocked quartz, and is confirmed with Raman spectroscopy or X-ray diffraction.
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