The frictional strength of brucite gouge has been measured at hydrothermal conditions to 450 °C. 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 µ 0.20 near 300 °C, and µ 0.22-0.24 in the temperature range 350-450 °C. 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 µ to 300 °C 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 300 °C, where brucite is weakest, all the major serpentine minerals have µ 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.

Temperature-fluid pressure conditions of the experiments of this study relative to the brucite-stability relations determined by Kennedy [1956], with data at 400 °C from Weber and Roy [1965]. (from Figure 1).