Static amorphization of anorthite at 300 K and comparison with diaplectic glass

Abstract

DIAPLECTIC glass, which is produced without fusion, is characteristic of meteorite impact sites in feldspar- and quartz-rich sites on the Earth and Moon; it has also been produced in laboratory shock experiments^1–3. Its presence in nature is thus considered to be strong evidence of exposure to a high-pressure shock wave generated by impact. The formation of such glasses has been universally interpreted in terms of the brief, high-temperature and high-pressure conditions characteristic of shock loading. Here we report that crystalline anorthite (Ca Al₂Si₂O₈) becomes amorphous at static pressures of between 22 and 28 GPa at 300 K. The amorphization is associated with a pressure-induced increase in the coordination of silicon and aluminium by oxygen, from fourfold to five- and sixfold. The increase in coordination appears as the sample vitrifies at pressure; on decompression, the coordination reverts to fourfold. The vitrification is spatially heterogeneous, and may be crystallographically controlled. The formation of glass from crystalline material under static conditions at 300 K indicates that the high strain rates and temperatures associated with shock compression are not required for the creation of glasses without fusion.