Experimental deformation of muscovite

Abstract

The strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45°, 0° and 90° to the basal plane (001) at temperatures T from 20 to 400°C, confining pressures P c from 10 to 400 MPa, and strain rates ε from 2.5 × 10 −7 to 2.3 × 10 −4 s −1. Samples deformed as 45° to (001) exhibit smooth undulatory extinction in thin section associated with dislocation glide, with few well developed low-angle kink boundaries. Basal shear strengths depend on confining pressure at P c < 50 MPa with a coefficient of internal friction μ of ∼0.4, while shear strengths measured at 100–400 MPa appear to be insensitive to confining pressure. Results for samples shortened at P c ≥ 100 MPa at 45° to (001) and directions [001], [110], [310] and [010] within the basal plane suggest only a weak mechanical anisotropy within (001), corresponding to Burgers vectors 〈100〉, 1/2〈110〉 and 1/2〈110〉. The effects of temperature and strain rate on glide strengths are relatively small. Assuming an exponential relationship between differential stress σ and strain rate ε of the form ε ̇ = A exp (aσ) exp (-Q/ RT) , results from temperature- and strain rate-stepping experiments performed at P c = 200 MPa yield an activation energy Q of 47 ± 19 kJ mol −1 and an exponential constant a of 0.5 ± 0.2 MPa −1. Samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands. Strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45° to (001) and exhibit significant pressure dependence ( μ ≊ 0.8) up to confining pressures of 200 MPa. Kink bands are generated at the ends of samples where contact is made with the pistons. Samples loaded at 90° to (001) are much stronger than samples of all other orientations and deformation is accomplished by fracture. Fracture strengths exhibit a strong dependence on confining pressure ( μ ≊ 0.9), similar in magnitude to other silicates that are brittle at the same conditions. By comparison with biotite, muscovite is weaker in shear on (001) at the experimental conditions tested with an activation energy for glide, Q, that is significantly smaller than that for glide in biotite.