Polycrystalline pyrite, deformed in triaxial compression tests at a confining pressure of 300 MPa, strain rates of 10 (super -4) sec (super -1) to 10 (super -5) sec (super -1) , and temperatures above about 450 degrees C, shortens by dislocation flow mechanisms. Above 500 degrees C dynamic recovery and recrystallization are important, and steady state flow is attained after 10 to 20 percent shortening. The dynamically recrystallized grain size (d) (in mu m) is related to the flow stress (Sigma ) by the relationSigma / G = k (d/b) (super -0.9) ,where k = 16.2, G = 7.8 X 10 4 MPa, and b = 5.42 X 10 (super -4) mu m. Below about 450 degrees C polycrystalline pyrite deforms by brittle mechanisms after less than a few percent strain by dislocation flow.The strength of polycrystalline pyrite decreases significantly with increasing temperature and decreasing strain rate. Over the temperature interval 500 degrees to 700 degrees C, at strain rates of 2 X 10 (super -5) sec (super -1) , the steady state flow stress drops from about 500 to 70 MPa.Pyrite single crystals shortened in the <100> and <110> orientations at 600 degrees C, about 10 (super -5) sec (super -1) , and 300 MPa confining pressure exhibit three-stage work-hardening behavior in which the stage 2 work-hardening rates are nearly as high as the elastic modulus. For <100> shortening the yield stress (280 MPa) and the differential stress at the commencement of stage 3 (800 MPa) are several times larger than for <110> shortening. The strength during <110> shortening is comparable to that of the polycrystalline pyrite tested.The single crystal strength data, structural considerations, and transmission electron microscopy suggest that {100} <001> and possibly {100} <011> are major slip systems. The {110} dislocation glide may also be important but has a critical resolved shear stress several times higher than {100} glide.The present data indicate that pyrite can be considerably weaker and more ductile than indicated by previous studies, and that pyrite is likely to be deformed by dislocation flow mechanisms under a range of geologically realistic conditions.

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