Microstructure of quartz in partially inverted coesite from high-pressure metamorphic rocks

Coesite, a high-pressure polymorph of SiO2, was firstly synthesized by Coes. Naturally occurring coesite has been found in shock metamorphosed rocks, xenoliths in kimberlites, eclogites and metasediments. Coesite in metasediments indicates subduction of slabs of continental crust to depths of at least 90 km. Coesite can only survive obduction (decrease of P,T) as an inclusion in a high strength mineral like garnet (pyrope). Depending on the elastic behavior of the coesite-includmg garnets, coesite is always partially inverted into quartz. Stress associated with the volume increase due to the coesite inversion causes a radiating crack pattern around the including mineral. Complete failure of the pressure vessel results in a replacement of coesite by pseudomorphs of quartz after coesite. The present study intends to find characteristic microstructural features as reliable indicators for quartz pseudomorphs having been pseudomorphs after coesite. Four different types of quartz can be distinguished with respect to the fabric, due to the coesite inversion: