HR: 0800h
AN: MR11A-0924    [Abstracts]
TI: Direct measurements of the dissolution of silica-mica interfaces and its implication for pressure solution mechanisms
AU: Goles, J R
EM: boles@geol.ucsb.edu
AF: Dept of Geol Sci UCSB, 1006 Webb Hall, Santa Barbara, CA 93106 United States
AU: * Israelachvili, J
EM: jacob@engineering.ucsb.edu
AF: Dept of Chem Engrg UCSB, 3357 Engineering II, Santa Barbara, CA 93106 United States
AU: Alcantar, N
EM: alcantar@eng.usf.edu
AF: Dept of Chem Engrg Univ of So Florida, 4202 E. Fowler Avenue, ENB 118, Tampa`, FL 33620 United States
AU: Anzalone, A
AF: Dept of Chem Engrg Univ of So Florida, 4202 E. Fowler Avenue, ENB 118, Tampa`, FL 33620 United States
AU: Meyer, E
EM: emmo@engineering.ucsb.edu
AF: Dept of Chem Engrg UCSB, 3357 Engineering II, Santa Barbara, CA 93106 United States
AU: Greene, W
EM: wren@engineering.ucsb.edu
AF: Dept of Chem Engrg UCSB, 3357 Engineering II, Santa Barbara, CA 93106 United States
AB: Pressure solution is a rock deformation process defined by many authors as the dissolution of materials under high stress at grain to grain contacts, and precipitation at interfaces under low stress. Despite the fact that this dissolution-transport-precipitation mechanism has been under scrutiny for many years, the controls and kinetics of this process are still poorly discerned, in part because of the large time-scales involved. Using the Surface Forces Apparatus (SFA) technique for measuring the forces (or pressures) between two solid surfaces pressed together in liquid, and an optical interference technique for in situ visualization of the nano-scale deformations and dissolution of the surfaces at the junction, we report on our studies of mica-mica, amorphous silica-silica and quartz-mica interactions in various aqueous electrolyte solutions. Dissolution rates as slow as 1 nm per day could be recorded in real time, corresponding to geological rates as slow as 1 m per 3 million years. In the `symmetric case' of two mica surfaces, our results show that two muscovite mica surfaces in high NaCl salt solutions and normal pH experience a repulsive electrostatic and `structural hydration' force at short-range (<2-4 nm) that prevents the surfaces from coming into contact, but that does not appear to prevent water and ions from diffusing into or out of the junction (the nanometer thin water film between the two mica surfaces). In addition, no dissolution of the mica surfaces (the basal plane) was observed under any conditions. However, in the presence of calcium ions (CaCl2) calcite crystals were seen to grow on the mica surfaces as well as in the gap between two surfaces, which is similar to the observed occurrence of carbonate minerals preferentially growing between mica cleavages. In the `symmetric case' of two amorphous silica surfaces, the short-range forces were similar to those measured between mica surfaces, and again no dissolution of the (amorphous) silica surfaces was observed. In the `asymmetric' case of a quartz crystal surface against mica, dissolution of the quartz surfaces was observed that depended on the solution conditions, the externally applied `lithostatic' pressure, and on which crystal face was exposed to the mica surface. Quantitatively, our experiments show that there is an initial stage after fresh solution or acidic calcium solution is added in which the spacing between the surfaces increases. However, the thickness decreased continually after approximately four hours of exposure. For a particular set of conditions the process eventually slows down and reaches equilibrium after some time, but increasing the pressure at this point increases the rate of dissolution again. Addition of fresh solution later in the experiment has a similar effect. We discuss how our results may provide a better understanding of the molecular processes associated with pressure solution, and also relate these processes to other, apparently related, phenomena in materials science such as corrosion and Chemical Mechanical Polishing (CMP). These results are consistent with the observation that pressure solution of quartz is greatly enhanced when in contact with mica.
DE: 8099 General or miscellaneous
DE: 5199 General or miscellaneous
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting