Influence of stress and strain on dolomite rim growth: a comparative study

Abstract

Triaxial compression and torsion experiments were performed to investigate the influence of non-isostatic stress and strain on dolomite reaction rim growth using orientated natural calcite and magnesite single crystals at a temperature of 750 °C, 400 MPa confining pressure, stresses between 7 and 38 MPa, and test durations up to 171 h. Reaction products were composed of a polycrystalline magnesio-calcite layer, palisade-shaped dolomite, and granular dolomite grains. In all experiments, inelastic deformation was partitioned into calcite and reaction products, while magnesite remained undeformed. Calcite deformed by twinning and dislocation creep, where the activation of additional glide systems at high stress allowed high strain. Depending on grain size, magnesio-calcite deformed by diffusion creep and/or grain boundary sliding, twinning, and dislocation creep. Dolomite deformed mainly by diffusion creep, assisted by enhanced dislocation activity allowing Ca enrichment in the granular rim. A weak crystallographic preferred orientation of the reaction products was observed. In triaxial compression, dolomite rim growth was diffusion-controlled and showed no influence of axial stresses up to 38 MPa on the reaction kinetics. At high strain (>0.1), the magnesio-calcite layer is wider suggesting faster growth kinetics. This may be related to additional diffusion pathways provided by enhanced dislocation activity. At very high strain (>0.3–0.6), twisted samples showed a gradual decrease in layer thickness of dolomite and magnesio-calcite with increasing strain (-rate).