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Coesite, formerly known only from impact craters, has been found in rocks that were once at the surface (first found in 1984) .

This means that in continent-continent collision zones (like the Himalayas and Alps), rocks somehow get from the surface down to 95km and back fast enough to preserve coesite

Coesite Index
Indicator Minerals

Coesite (SiO2) is a high pressure variant of the mineral quartz. It is stable at pressures above 28 kbar and was first discovered in highpressure experimental deformation of silica.

However, it was not found in naturally occurring rocks until the 1950s, when it was identified in shocked sandstone in the Arizona Meteorite Crater, and then in the 1980s in metamorphic rocks in the Alps. This dramaticdiscovery changed many Alpine geological theories as these rocks now had to have come to the surface from depths of over 90km.

In the last 20 years coesite inclusions have been found in many high pressure rocks across the world, generally enclosed within gar-net grains.

As a rock is buried to great depths, quartz transforms to coesite; when the rock is uplifted to the surface, the coesite reverts back tothe stable quartz. However, if the coesite is enclosed within a garnet grain (as an inclusion), there is sometimes insufficient room forthe coesite to invert to the less-dense quartz: the rigid garnet acts like a pressure-vessel. This causes some of the coesite to remaininside garnet grains, and results in cracking and deformation of the surrounding garnet.

This application note shows how EBSD mapping can be used to identify coesite/quartz inclusions within garnet grains in an eclogitefrom the Dabie Shan in E. China, and to analyse the effect of the transformation on the surrounding garnet.