Shock Compression of Feldspars

Hugoniot data for oligoclase and microcline to 670 and 580 kb and release adiabat data for oligoclase were obtained by means of the inclined mirror and immersed-foil-reflected-light techniques, respectively. Oligoclase and microcline have Hugoniot elastic limits in the range of 40-55 and 80-85 kb. These limits increase slightly with increasing driving shock pressure. Above the elastic limit, extending to •300 and •400 kb, transition regions of anomalously high compression are observed for microcline and oligoclase. These data and the data of McQueen, Marsh, and Fritz for albitite and anorthosite indicate .that at successively higher shock pressures within this region, the feldspars gradually transform to a high-pressure, highdensity polymorph. This polymorph probably corresponds to the rutfie-like hollanditc structure obtained in high-pressure quenching experiments by Kume, Matsumoto, and Koizumi (in KA1Ge•Os) and by Ringwood, Reid, and Wadsley (in KA1So80s, microcline). In the hollandite structure germanium or silicon is in octahedral coordination with oxygen. The zero-pressure density and the Birch-Murnaghan equation of state parameters for the adiabat and isotherm are calculated for the high-pressure polymorphs of oligoclase, microcline, anorthosite, and albitite. The release adiabat centered at 180 kb indicates that at this shock pressure some (•15%) of the hollanditc phase forms but apparently reverts to a lower-density phase on pressure release. Release adiabat curves centered at 272 and 417 kb and calculated postshock temperatures indicate that material of feldspar composition recovered from meteorite and laboratory impacts is converted to the hollanditc structure upon shock compression; upon pressure release this material probably reverts to the low-density maskelynite form.