Development of a recrystallized grain size piezometer for hematite based on high-temperature torsion experiments

Abstract

A series of hematite samples (>98 % Fe 2 O 3, Sishen Mine, South Africa) with different initial grain sizes of −5 s −1 and 4.7 × 10 −5 s −1 , resulting in finite shear strains between 0.4 and 4.7 and finite shear strengths in the range of 32–326 MPa. The data obtained at high shear strain (>3) fit to a power law creep equation with a pre-exponential constant of lnA = −5.03 ± 0.61 MPa −2.5 s −1 , an activation energy of Q = 249 ± 48 kJ mol −1 , and a stress exponent of n = 2.5 ± 0.5, indicating dislocation creep partially assisted by grain boundary sliding as the main deformation mechanism. Irrespective of the initial grain size, the final grain size ( D ) of fully recrystallized samples decreases continuously with increasing (steady state) equivalent stress (σ), yielding a piezometric relationship of the form: D = 1055 × σ 1.03 ± 0.10 . The piezometer is applied to itabiritic hematite ores of the Iron Quadrangle, Minas Gerais, Brazil, consisting mainly of layered hematite and quartz. Compared to hematite the measured recrystallized grain size of quartz is always larger, which is in agreement with the grain size range predicted by recrystallized grain size-based piezometers for hematite and quartz if deformed at similar stress below about 100 MPa.