Diaplectic Quartz Research Articles

Coesite and stishovite in shocked crystalline rocks

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.

Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters

The principle of progressive shock metamorphism in nonporous silicate rocks is discussed on the basis of petrographic observations and experimental data. The p-T conditions and the nature of the basic types of shock effects observed in rock-forming minerals are considered as far as they are indicative of the degree of shock metamorphism of the source rock. Particular shock effects that are related to the main regimes of the Hugoniot curves of quartz and feldspar, as well as the shock-melting and vaporization behavior of the whole rock, characterize and define six stages or zones of increasing shock metamorphism. Each stage or zone represents a certain range of peak pressure and temperature. Rocks of stage O are shocked to shock states below the Hugoniot elastic limit of quartz and feldspar that are irregularly fractured. Shock stage I is characterized by diaplectic quartz and feldspar that are released from shock states in the ‘two-phase regime’ of the Hugoniot. Diaplectic glass of quartz and/or feldspar composition occurs within stage II, which is related to the lowest part of the ‘high-pressure-phase regime’ of the Hugoniot. Stage III is characterized by selective melting of feldspar minerals at sufficiently high postshock temperature. Postshock temperatures exceeding the liquidus of the whole rock produce rock glasses on pressure and temperature release (stage IV). Stage V is represented by condensation products of shockvaporized rock material. On the basis of experimental data of several authors, the proposed stages of metamorphism are tentatively correlated with a pressure-temperature scale. It is concluded that the proposed classification is generally valid for shock processes in nonporous rocks and can well be applied to the recognition and distinction of the different kinds of impact breccias of terrestrial or extraterrestrial origin.