Experimental deformation of flint in axial compression

Abstract

AbstractThe experimental deformation of flint, a water-rich (1–2 wt%) fine-grained (1 μm) microcrystalline quartz, has been studied using a gas-confining medium apparatus at 300 MPa confining pressure in the temperature range 500–1000 °C. In constant strain-rate axial compression tests a mechanically unstable behaviour with peaked and undulating stress-strain curves, especially at the faster strain rates, is manifest. The rheology of these tests can be approximately described by the traditional power-law equation with a stress exponent (n) of between 3 and 5, and an apparent activation energy (Q) of 108 kJ mol−1. However, the strain-independent (‘steady-state’) equation is only a partial description of the data. The mechanical properties were studied at lower stresses using stress relaxation testing, this method has the advantage of recording specimen response over a very small strain interval (c. 1%), i.e. nearly constant structure. These tests revealed that in a high stress regime above 100 MPa the rheology was only weakly dependent on strain and very similar to the constant strain-rate behaviour. Below 100 MPa the rheology can be described by a power law with Q = 64 kJ mol−1 and n = 1. The low-stress behaviour is extremely sensitive to specimen strain, the strain rate decreasing with increasing strain for a given stress.The optical microstructure reveals that the mechanical instabilities are related to localized displacement zones, sometimes en echelon, consisting of relatively large grains (10 μm). The important reduction of the integral infrared absorption of deformed specimens, a 98% reduction of the initial water content, suggests that a pore fluid was developed by specimen dehydration in the sealed (undrained) assembly. The presence of pores or bubbles decorating grain boundaries and microfractures normal to the piston-specimen interface both testify to the presence of the pore fluid during deformation. The pore fluid pressure may have approached the confining pressure resulting in a near-zero effective pressure. Transmission electron microscopy revealed the presence of many Brazil micro-twins parallel to grain boundaries. The twins are thought to be growth defects indicating rapid grain growth during deformation. Dislocations in the basal plane were observed with a 1/3 〈a〉-type Burgers vector. Dislocations were also observed in the grain boundaries, possibly caused by grain-boundary sliding in the low stress regime.The complete lattice preferred orientation (LPO) of homogeneously deformed specimens has been determined by X-ray texture goniometry. The inverse pole figures have a strong concentration of compression axes parallel, and a weaker concentration normal, to the c-axis. The local c-axis pole figures in heterogeneously deformed specimens were characterized by the photometric technique. The c-axis pole figures in the bulk are identical to that determined by X-rays in homogeneously deformed specimens. In the displacement zones the fabrics are asymmetric with respect to the zone boundaries. The asymmetry is not consistent with the shear sense imposed by the deformation geometry and the usually accepted dislocation slip models. Fabric development by a grain growth mechanism in which the mechanically nucleated micro-twins (Brazil law) favour growth of grains with low-energy twin grain-boundary orientation relationships with their neighbours is proposed. The mechanism is consistent with the microstructure (twins parallel to grain boundaries, low density of dislocations on glide planes) and the c-axis pole figures. In natural deformations under similar conditions (e.g. subsolidus granites in the β-quartz field) we suggest caution in using fabric asymmetry as it may not be a dependable indicator of shear sense, as grain growth and dislocation slip will give rise to different interpretations.