How brucite may affect the frictional properties of serpentinite

The frictional strength of brucite gouge has been measured at hydrothermal conditions to 450C. At room temperature, brucite has a coefficient of friction, 0.30, making it one of the weakest minerals identified to date. With increasing temperature at a constant effective normal stress, the coefficient of friction of brucite decreases to a minimum of 0.20 near 300C, and 0.22-0.24 in the temperature range 350-450C. Brucite has a sheeted crystal structure, and its low frictional strength may be attributed to the relatively weak bonds between the layers. In addition, the temperature dependence of to 300C can be explained in terms of the anomalously large coefficient of thermal expansion of brucite, which will further weaken the interlayer bonds. Brucite is a common constituent of serpentinite, and at 300C, where brucite is weakest, all the major serpentine minerals have 0.5. The maximum expected brucite content of a serpentinite is close to 20% by weight or volume. That amount of disseminated brucite will lower the coefficient of friction of serpentinite by 10-15% in the deeper parts of the seismogenic zone. However, the effect will be much greater if shear can be concentrated along brucite-lined slip surfaces in the serpentinite body.