Correlation of Chlorite Frictional Strength with Composition

Abstract:

Chlorite is a common phyllosilicate mineral in crustal fault zones, and it is generally considered to contribute to fault-zone weakening. However, very little is known about its frictional properties, in part because of the difficulty in obtaining suitable material for study. Synthetic gouges (<90 µm grain size) have been prepared from four chlorite-rich separates whose compositions range between 18.3 and 33.6 wt% MgO, 1.3-21.6 wt% FeO (total Fe content), and 25.0-33.0 wt% SiO₂ (electron microprobe analysis). Magnesium contents vary directly with Si and inversely with Fe. Frictional strengths were measured in the temperature range 25-300°C and shearing rates between 0.00115 and 1.15 µm/s, with fluid pressure and effective normal stress held constant at 60 and 100 MPa, respectively. The chlorites are velocity strengthening at all tested conditions, promoting stable slip. No obvious trends in strength relative to either temperature or velocity were discernible for a given chlorite. In contrast, the frictional strength at any given set of physical conditions is a function of chlorite composition, the most Fe-rich chlorite being the strongest (coefficient of friction, µ in the range 0.26-0.36) and the most Mg- and Si-rich sample the weakest (µ = 0.16-0.22). This is one of the first reports of a compositional influence on the frictional properties of a mineral. The results confirm that chlorite will contribute to strength reduction in fault gouge, but the Mg-rich chlorites associated with ultramafic rocks will be more effective weakening agents than the relatively high-Fe chlorites more typical of quartzofeldspathic crustal rocks.