A shock-induced phase change in orthoclase

Abstract

New shock compression data to 340 kb for single-crystal orthoclase (along (001)) demonstrate the onset of a shock-induced phase change at ~115 kb. Along the Hugoniot a mixedphase region extends to ~300 kb, above which the data are believed to correspond to the properties of a high-pressure phase having the hollanditc structure (zero pressure density of 3.84 g/cm ) reported by Ringwood et al. If the hollanditc value for the zero pressure density is used, the zero pressure bulk modulus of this phase is approximately 2.8 q-0.2 Mb. The very high pressure equation of state of orthoclase is of importance both in describing the effects of intense shock waves on potassiumfeldspar-bearing rocks on the earth and the moon [Hubbard et al., 1971; Drake et al., 1970; Chao, 1967; Kleeraan, 1971; yon Engelhardt and Stoffier, 1968] and in studying the earth's mantle. The latter is important because the orthoclase structure provides a model of the response of feldspar-bearing rocks to the high pressures of the earth's interior. Previously reported Hugoniot data for microcline [Ahre.ns et al., 1969a] demonstrated that this mineral, like plagioclase [McQueen et al., 1967], begins to transform to a new phase or phases at about 120 kb (along the Hugoniot). This phase change appears to go to completion for shock states above ~300 kb. Above this level the limited Hugoniot data for microcline suggested that the properties of a denser high-pressure phase were being sampled. The zero pressure density for the high-pressure phase inferred by Ahrens et al. [1969b] and Davies and Anderson [1971] of ~3.5 g/cm ' compares unfavorably and inconclusively with densities of 3.2 and 3.84 g/cm expected for the possible high-pressure phases in the structures jadeitc plus stishovite and hollanditc. Because previous static high-pressure quenching experiments on both silicate and germanate (analog) potassium feldspar