FORMATION MECHANISMS OF PLANAR DEFORMATION FEATURES IN NATURALLY SHOCKED QUARTZ

Shock waves induce peculiar defects in quartz: shock mosaics, high-pressure polymorphs (coesite and stishovite), diaplectic glass and 'planar deformation features' (PDFs). Together, these features are the indices of 'shock metamorphism'. PDFs appear under the optical microscope as straight and narrow defects parallel to the {101nBAR} rhombohedral planes with n = 3 and 2 as most frequent values, although n = 1, 4 and infinity (i.e. basal plane) are also found. We report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of PDFs in shocked quartz grains from a variety of sites (Slate Islands, La Malbaie and Manson in North America; the Vredefort complex in South Africa; the Ries Crater in Germany and the Cretaceous-Tertiary boundary layer at Raton Basin, Colorado). We distinguish four PDF fine structures: (1) bands of dislocations; (2) lamellae of mixtures in various proportions of amorphous silica and small crystallites; (3) thin Brazil twin lamellae; (4) short, parallel lamellae forming serrated ladder structures. The possible formation mechanisms of these PDFs are discussed in the second part of the paper. It is suggested that the dislocation bands are probably not original PDF structures, but stem from a later recrystallization stage. The Brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave. The other PDF structures, which consist of partial amorphization in the {101nBAR} planes, must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes. Theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 GPa. The growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave.