Shock experiments on pre-heated α- and β-quartz: II. X-ray and TEM investigations

Abstract

Quartz single crystals pre-heated to different temperatures (20°, 275°, 540° and 630°C) were experimentally shocked to pressures ranging from 20 to 40 GPa, with the shock front propagating parallel to (10 1 0) or (0001). Shocked quartz pre-heated to 630°C is nearly X-ray amorphous at 26 GPa, while quartz shocked at room temperature still displays several X-ray diffraction lines up to 30 GPa. All samples that were still crystalline when shocked above 25 GPa exhibit lattice expansion, internal strain and reduced crystallite sizes. Lattice expansion was up to 1% for the constants a o and c o and up to 3% for the cell volume, V o. Quartz shocked parallel to (10 1 0) always shows larger lattice constants and is more strained than quartz shocked parallel to (0001), indicating that shock effects are controlled by the structural anisotropy. Internal strain is anisotropic, with minimum strain in the planes belonging to {10 1 2}. The dominant shock defect revealed by TEM are sets of parallel lamellae consisting of pure SiO 2 glass, so-called planar deformation features (PDFs). Below 25 GPa, PDFs are extremely rare and thin (≈ 30 nm). At pressures above 25 GPa, however, PDFs occur frequently and are about ten times thicker. This qualitative observation of increasing PDF frequency and thickness with increasing pressure is quantified by calculations of the amount of glass using X-ray and density data. The results of this study substantiate the crucial role of PDFs for the lattice expansion of quartz and the formation of diaplectic glass and lechatelierite. Lattice expansion is ascribed to the enormous expansion of PDFs during decompression, leading to elastic stretching in adjacent crystalline areas. The observation that PDFs occur predominantly parallel to {10 1 2}, where minimum strain is present, supports this hypothesis. The increasing amount of PDFs above 25 GPa is interpreted as the cause for the formation of diaplectic glass. Shock temperature calculations support the assumption that these PDFs consist, under compression, of superheated melt. The heat is possibly sufficient to melt crystalline areas adjacent to the superheated PDFs. At enhanced pressures, such molten bands coalesce to a total melt, which is quenched upon decompression. Thus, diaplectic glass represents a quenched high-pressure melt, while lechatelierite, SiO 2 glass with a fluidal texture, is interpreted as quenched low-pressure melt.