Andrew J. Gratz

Solution-transfer compaction of quartzites: Progress toward a rate law

Data on dissolution, diffusion, and growth in the quartz-water system show that diffusion through grain-boundary films is rate limiting for solution transfer and too slow to permit significant compaction in geologic time, but that rates are more reasonable if the rough texture of natural grain boundaries is considered. On the basis of scanning electron microscope observations, a model is proposed that uses a hierarchical plumbing system in which grain boundaries consist of islands capped by grain-boundary films, separated by microcapillaries, and connected to intergranular porosity. A rate law based on such a model has an inverse grain-size dependence up to a critical grain size that is large (>>10 cm) under typical compaction conditions.

Shock metamorphism of quartz with initial temperatures −170 to + 1000° C

Shock experiments on quartz single crystals with initial temperatures −170 to +1000°C 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 transformation of quartz is temperature-activated with an apparent activation energy of <60 kJ/ mol. 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 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.

Quartz dissolution: Negative crystal experiments and a rate law

The negative crystal method is used to extend the range of measured quartz dissolution rates as a function of temperature and pOH, extent of saturation, and ionic strength. The activation energy is constant for pOH = 1 to 4, and rate decreases linearly with saturation, indicating a precipitation reaction that is first order in silicic acid. Dissolution is slowed by Ca and Mg ions; the effect of Ca is strongest on the unit rhombohedra. A simple rate law based on surface charging is proposed and shown to fit the data. Only a reaction in which the activated complex is negatively charged is consistent with rate measurements. Some constraints on fundamental kinetic parameters are inferred. Data for all materials (single crystals and powders) can be fit by the single rate law 〈Rdiss〉 = G·A0·T·S·exp(−Ediss/RT), S = I0.3 · {0.095 · aoH−−0.5+ 1}−1, G = 1 for rough faces while G = Y · exp(−ΔEinter/RT) for smooth faces, A0 = 6.64 · 10−3ms s−1K−1, where 〈Rdiss〉 = limiting dissolution rate in m/s, S is the surface charge, aOH− is the molal hydroxide activity relative to an infinitely dilute solution at STP, I is the ionic strength, T is the temperature, ΔEinter is the energy of interaction (~6 kJ/mol) causing step formation on the crystal surface,Ediss is the activation energy of dissolution of ~78.6 kJ/mol, and Y is a face constant equal to 1 for the prism and 1.63 or 1.87 for the positive and negative unit rhombs, respectively. The available data in the literature suggests G = 1 to 9.5 for various other crystal directions, G = 2–4 for powders with optically estimated areas, while G = 0.1–0.3 describes powders with BET-determined areas. This equation applies over a wide range of conditions: 25–300°C, ionic strength 0.003–0.1 m, and any pOH below the point of zero charge of quartz; a similar equation applies to growth. Dissolution mechanisms for defective crystals are discussed and may result in dissolution rate enhancements of ~2X. The observation that these enhancements are associated with generalized etching suggests that background dissolution follows the proposed rate law, but accompanied by surface roughening and rapid removal of defect volumes.

Dissolution of quartz in aqueous basic solution, 106–236°C: Surface kinetics of “perfect” crystallographic faces

Past work on dissolution kinetics of minerals has focused on particulates and the total transfer of material to solution. We look instead at local topography of single crystals. Etching of quartz in alkali hydroxide solutions produces four types of features: flat-bottomed negative crystals with facets for walls, believed to nucleate on microfractures; jumbo pits, giant etch pits cored by dislocations and often with hollow cores; small pits, also believed to be dislocation-cored; and etch tunnels extending from some jumbo pits. A transition from selective etching (revelation of intrinsic defects) to nonselective etching (most intrinsic defects dissolving at the same rate as bulk crystal) occurs as a function of ionic strength, with small pits forming only at ionic strengths below ~ 0.005 m. This transition is attributed to the decrease in Debye-Huckel length with increasing solution strength. We introduce the negative crystal method, whereby dissolution rates of nominally perfect crystal faces are obtained by measuring the size of individual negative crystals during a sequence of dissolution steps. Rates are obtained to ~4% accuracy, with temperature control of ±1.25°C. Using this method, we find apparent activation energies for dissolution of the prism and for an average of the rhombs to be 86.39 and 90.19 kJ/mol, respectively, with corresponding pre-exponential factors of 2.99 × 109 and 1.43 × 1010° μm/h. The closeness of these two activation energies argues for a single atomic site controlling the rate, while site density varies with the crystal orientation. At a constant ionic strength of 0.01 m, the rates vary in a complex manner with hydroxide activity, increasing as activity to the ~0.5 power up to ~0.005 m, above which the rate is nearly independent of the alkalinity. Very similar etching behavior is observed in all KOH-KCl mixtures, as well as in LiOH, NaOH, RbOH, and CsOH, although the detailed morphology of small pits is controlled by the chemistry of the etchant. The lower atomic weight hydroxides have nearly identical etch rates; RbOH and CsOH dissolve quartz roughly twice as quickly. We also note a factor of ~ 1.6 enhancement in dissolution rate in certain synthetic over natural samples.

Shock metamorphism of deformed quartz

AbstractDeformed, synthetic quartz containing a dislocation density of 2.9 ± 1.9 × 108/cm2 and abundant bubbles and small inclusions was shocked to peak pressures of 12 and 24 GPa. The resultant material was inhomogeneously deformed and extremely fractured. The 12 GPa sample contained large regions lacking apparent shock deformation, suggesting that the original microstructure of a quartz target may be distinguished in low-stress shocks with minimal annealing. No change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered.Generally the same types of microstructures were induced by shock of deformed quartz as by shock of relatively defect-free as-grown crystals. Glass-filled veins were abundant, especially at lower stresses, and contained angular fragments of quartz welded together. Microfaults formed on

A theory for buckling of the mantle lithosphere and Moho during compressive detachments in continents

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Distinguishing Between Shock and Tectonic Lamellae with the SEM

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