The variation range of optical constants and distribution characteristics of shock lamellae in shock-metamorphosed quartz

Two moderately shocked rock samples collected from the Ries Crater, West Germany (granite—gneiss sample RC-647-29 and biotite-granite sample RP-627-55) and two weakly shocked pegmatite samples (Lj-711-12 and Lj-711-5) taken from Lake Lappajarvi, Finland, have been optically studied to establish the variation range of optical constants and distribution characteristics of shock lamellae in shocked quartz. It has been found that sample RC-647-29 contains shocked quartz grains with the average refractive index ranging from 1.4612 to 1.5331, and sample RP-627-55 from 1.5002 to 1.4669, i.e., they cover a wide range of shock pressures. As for the larger quartz grains in samples Lj-711-12 and Lj-711-5, the variation range of the average refractive indices are smaller than those of samples from the Ries Crater. Hence the estimation of degree of shock must est with the investigation of a set of representative shocked quartz crystals from a single shocked rock sample. The optical data on shocked quartz indicate that the degree of shock is highly independent of the number of shock lamellae sets and their orientations; the most sensitive optical indicator is the index of refraction. On the basis of TEM investigations of single crystal grains of shocked quartz differing in refractive index, three mechanisms of formation of shock lamellae have been established: host quartz crystals with lamellae having closely spaced dislocations; host quartz crystals with lamellae of randomly oriented fine grains of quartz; and host quartz crystals or their residual fragments with lamellae of silica glass.