HR: 0800h
AN: T21B-0484    [Abstracts]
TI: Solution-Transfer Processes and the Frictional Strength of Heated Brucite
AU: * Moore, D E
EM: dmoore@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: The mineral brucite, magnesium hydroxide, has a layered crystal structure and a platy morphology, and in some respects its frictional behavior is consistent with that of the sheet silicate minerals. These characteristics include relatively low frictional strength that makes brucite an exception to Byerlee's Law, and the localization of shear in brucite gouge onto slickensided boundary and Riedel shear planes. However, the frictional strength of brucite also differs in some ways from that of sheet silicates such as the serpentine minerals with which brucite is commonly associated in nature. Under water-saturated conditions the serpentine minerals are characterized by increases in coefficient of friction with increasing temperature and effective normal stress. In contrast, the coefficient of friction of water-saturated brucite decreases overall with increasing effective stress and increasing temperature to 300 degrees C. At 100 MPa effective normal stress, water-saturated brucite has a coefficient of friction of about 0.3 at room temperature and 0.25 at 300 degrees C, whereas dry brucite has an essentially constant coefficient of friction of about 0.5 between 25 and 300 degrees C. We have initiated scanning electron microscope examinations of laboratory samples of sheared brucite gouge, to investigate the source of this discrepancy in frictional behavior between dry and water-saturated conditions. Preliminary observations suggest that solution transfer facilitates the shear of heated, water-saturated brucite. Brucite grains on boundary and Riedel shear surfaces are completely recrystallized to platelets with hexagonal outlines that are typically less than 0.5 micrometers in diameter. Adjoining small crystals coalesce to form wider, but still very thin, platy aggregates. Score marks on the shear surfaces that are aligned in the direction of shear may form as a result of dissolution in front of asperities that may themselves dissolve over time. Such a predominance of recrystallization textures has not been found to date in comparable laboratory samples of sheet-silicate gouges. The distinctive frictional behavior of brucite is consistent with geochemical data that show that it has high solubility and dissolution/precipitation rates that are several orders of magnitude greater than those of silicate minerals.
DE: 5112 Microstructure
DE: 8030 Microstructures
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8100 TECTONOPHYSICS
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005