No abstract

No abstract

No abstract

The processes involved in the dissolution and growth of crystals are closely related. Atomic force microscopy (AFM) of faceted pits (called negative crystals) formed during quartz dissolution reveals subtle details of these underlying physical mechanisms for silicates. In imaging these surfaces, the AFM detected ledges <1 nanometer (nm) high that were spaced 10 to 90 nm apart. A dislocation pit, invisible to optical and scanning electron microscopy measurements and serving as a ledge source, was also imaged. These observations confirm the applicability of ledge-motion models to dissolution and growth of silicates; coupled with measurements of dissolution rate on facets, these methods provide a powerful tool for probing mineral surface kinetics.

Abstract.Shock experiments on quartz single crystals with initial temperatures -170 to +1000~ showed that ambient temperature does not affect the type of defects formed but can lower the pressure of complete amorphization. The amount of glass recovered increases with both pressure and temperature, and the shock-induced phase transtbrmation of quartz is temperatureactivated with an apparent activation energy of < 60 kJ/ tool. The phase transformation is localized along three types of transformation lamellae (narrow, s-shaped, and wide) which contain fractured and/or high-pressure phases. Transformation lamellae are inferred to form by motion of linear collapse zones propagating near the shock front. Equilibrium phases, such as stishovite, were not recovered and are probably not formed at high shock pressures: the dominant transformation mechanism is inferred to be solid-state collapse to a dense, disordered phase. Melting occurs separately by friction along microfaults, but no high-pressure crystal phases are quenched in these zones. Shock of quartz thus produces two types of disordered material, quenched melt (along microfaults) and diaplectic glass (in transformation lamellae); the quenched melt expands during P-T release, leaving it with a density lower than quartz, while recovered diaplectic glass has a density closer to that of quartz. At low pressures (< 15 GPa), quartz transforms mostly by shear melting, while at higher pressures it converts mostly along transformation lamellae. We find that shock paleopiezometers using microstructures are nominally temperature-invariant, so that features observed at impact craters and the K/T boundary require in excess of 10 GPa to form, regardless of the target temperature. Shock comminution will be much more extensive for impacts on cold surfaces due to lack of cementation of fragments by melt glass; shock on hot surfaces could produce much more glass than estimated from room-temperature experiments. Because of the shock-impedance mismatch between quartz specimen * Present address: Lawrence Livermore National Laboratory, Livermore, CA 94550, USA and steel capsule, the incident shock wave reverberates up to a final pressure. The dynamic compression process is quasi-isentropic with high strain rates. Preheating and precooling achieves final shock pressures and temperatures representative of single-shock states of room temperature quartz and of quartz on known planetary surfaces. Stress histories were calculated by detailed 1-and 2-dimensional computer simulations. The stress history throughout the sample is relatively uniform, with minor variations during unloading. Significant differences between impact pressures calculated by the shock-impedance-match method and specimen pressures calculated by computer simulations indicate the importance of modeling shock recovery experiments computationally.