The shear strength of gypsum single crystals on three cleavage planes

Abstract

Single crystals of gypsum were strained to failure in simple shear at low confining pressures (1–10 bar). The crystals were sheared on the (010) principal cleavage plane in seven directions, and they showed both elastic and ductile behavior prior to failure. Mean critical resolved shear stresses ( τ c ) range from 0.20 to 0.52 MPa for shear on the (010) plane. On the two secondary cleavages, (100) and (011) the mean values for τ c are 14.0 MPa and 10.5 MPa, respectively. The ultimate shear strengths ( τ u ) in each of the seven directions on the (010) plane range from 0.58 to 1.36 MPa. The minimum ultimate shear strength is in the [103] direction which has been previously identified by other workers as the preferred translation glide direction. Gypsum crystals strained to failure on two secondary cleavages, (100) and (001), have much higher ultimate shear strength than any of the seven shear directions measured on (010). Strengths range from 18.5 MPa on (100) to 14.3 MPa on (011). The static modulus of rigidity on (010) ranges from 0.07 GPa to 0.16 GPa, and the values of dynamic rigidity obtained from ultrasonic pulse measurements range from 2.85 to 2.02 GPa. The range of values of static rigidity on the two secondary cleavages is 0.90 to 3.0 GPa, approximately one order of magnitude higher than the values on the (010) plane. The work of Fairbairn (1935) can be cited to demonstrate how the results can be applied to field studies of gypsum deformation.