Coesite and stishovite in shocked crystalline rocks

Abstract

Quantitative determination of the coesite and stishovite content of nonporous crystalline rocks of variable composition and degree of shock metamorphism taken mainly from the Ries crater was made by chemical concentration and X-ray powder diffractometry. The stishovite abundances range between 4 and 0.005% of the primordial quartz. The coesite abundances are between 7 and 40% of the primordial quartz. Coesite occurs as very fine grained aggregates embedded in diaplectic quartz or quartz glass with mean refractive indices below 1.48. Stishovite is included within the planar deformation structures of diaplectic quartz having mean refractive indices between 1.546 and 1.472. Traces of stishovite are also present within diaplectic glass. On the basis of these data, of microscopical observations, and of experimental data on the shock compression of quartz, as well as the thermal stability of coesite and stishovite, it is concluded that stishovite and coesite are formed and metastably preserved in the peak pressure ranges from about 120 kb to 450 kb and from about 300 kb to 550 kb, respectively. Stishovite is considered to crystallize during shock compression from a high-pressure silica phase with silicon in sixfold coordination. Coesite obviously crystallizes behind the shock front, that is, during pressure release, from a stishovite-like high-pressure phase and/or after pressure release from a silica phase of short-range order with fourfold coordination of silicon. It is only formed by shocks transforming the primary quartz completely or almost completely to the high-pressure phase within the period of shock compression. The influence of ‘secondary’ thermal metamorphism of shocked rocks on the metastable persistence of coesite and stishovite in various kinds of impact breccias shows that both high-pressure polymorphs may be used as very sensitive temperature indicators for any annealing processes.