Shock experiments on feldspar crystals

Abstract

Single crystals of sanidine, orthoclase, microcline, oligoclase, and labradorite as well as polycrystalline bytownite were shocked from 10.5 to 45 GPa and recovered. The direction of shock wave propagation in single crystals was always parallel to the crystallographic a‐axis. Optical observations reveal a continuous sequence of shock effects: fracturing starts to develop at <10.5 GPa, planar elements develop at 10.5–14 GPa, mosaicism at 18–26 GPa, transformation of the crystal into diaplectic glass begins at 26–34 GPa, and formation of melt glass is observed at ∼42 GPa. Microcline, however, remains weakly birefringent even at 45 GPa. Refractive indices of diaplectic glass drop sharply at shock pressures between 30 and 40 GPa. Xray investigations reveal only a minor expansion of the unit cell of sanidine, whereas the other feldspars display no systematic change of lattice constants. The structural state (Al,Si‐distribution) of feldspars is not affected by shock. EPR analyses reveal the disordering of the crystal structure and the formation of a short‐range order phase in feldpars at pressures as low as 10.5–14 GPa. The beginning of the formation of diaplectic glass in the range from 18–22 GPa is evident from IR‐spectra of shocked samples. It is assumed that diaplectic glass is an arrangement of grossly unchanged crystalline material and a glassy phase that is structurally indistinguishable from fusion‐formed glass. The influence of the chemical composition and initial structural state of feldspars on the development of shock effects is believed to be less than the influence of exsolution lamellae and alteration products. Based on the results of these investigations, pressure ranges are given for the lower and upper boundary of the mixed‐phase regime of the pV Hugoniot (p = shock pressure; V = specific volume) for all samples. Two independent methods for the determination of shock pressures from residual shock effects are proposed based on optical and IR‐spectroscopic analyses.