Examination of Maskelynite through Static Recompression and Dynamic Compression

This is an experimental study that aims to clarify the possible formation mechanisms of maskelynite. Maskelynite is a diaplectic glass, that forms during shock compression of feldspar far below the melting point, and without fusion. Maskelynite also paramorphises precursor feldspar grains. Maskelynite is an important probe of shock-pressures at terrestrial impact sites and in many meteorites. Two mechanisms of formation of maskelynite are examined here: 1) maskelynite is result of a pressure-induced amorphization of feldspar compressed beyond its mechanical stability where the formation of thermodynamically stable phases is kinetically inhibited [1, 2]. 2) Feldspar transforms upon dynamic compression into a high-pressure polymorph. Upon release from the peak shock pressure, this crystalline polymorph transforms back either into a dense glass or a highly disordered solid that appears amorphous in common probes (optical microscope, optical spectroscopy, diffraction). The latter scenario avails for diaplectic silica that formed in shock-experiments on quartz. Upon static compression of synthetic diaplectic silica at 300 K the material resumes the crystalline structure of stishovite [3], a high-pressure polymorph of silica which has also been observed in situ during shock compression of fused quartz [4]. Hence, the second scenario implies a memory effect of the high-pressure crystalline structure in the diaplectic glass. In the present study, we test this hypothesis for maskelynite by a) X-ray diffraction of maskelynite similar to the study of diaplectic silica in [3], b) synchrotron X-ray diffraction analysis of synthetic maskelynite at ambient pressure, both with the goal of identifying possible crystalline states. If no crystalline state is observed, the second proposed mechanism of maskelynite formation is not supported and the first mechanism appears more likely. In the static compression experiment I find indications of a change in middle-range order of maskelynite but no transition to long-range crystalline order upon compression to 19 GPa. In the shock-recovered maskelynite I observe crystalline material, even in material recovered from 38.5 GPa. The crystalline material is disseminated in an amorphous matrix and has feldspar-like structure rather than a structure related to a high-pressure polymorph of feldspar. Hence this crystalline material is remnant crystalline feldspar rather than a phase that formed upon shock-compression