Nanoparticles Observed in a Shear Fracture of Dolomite and a Probable Formation Mechanism

Nanoparticles have been extensively found in brittle faults or ductile shear zones, and their formation is closely related to shear movement along the fault plane. However, the formation mechanisms of these nanoparticles are not yet clear. In this study, dolomite samples were triaxially compressed, at a confining pressure of 200-300 MPa, a temperature between 27 °C and 900 °C and a strain rate of approximately 10^-5s^-1, with a Paterson designed gas medium high-temperature and high-pressure deformation apparatus (HTPDA). Samples deformed at room temperature were characterized by universal microcracks and undulatory extinctions in some grains; when at a temperature between 300 °C and 500 °C, well-developed mechanical twins dominated the microstructure, while at a temperature ≥800 °C, displacements of twin lamellae along a cleavage and a well-developed fracture zone could be seen. Nanoparticles of different shapes were discovered on the slip surfaces of a shear fracture or in microcracks by field emission scanning electron microscopy (FESEM). Nanoparticles on deformed samples under low differential stress were usually of sporadic spherical shapes and uneven distribution; while deformed samples under high differential stress had more dense distributions that were identified. Moreover, grain-overlap and nanofine granulation could be recognized in high strain samples. Based on a mechanical data analysis and microstructural observations, it was suggested that the initial formation of nanoparticles was macroscopically determined by the differential stress subjected to the host rocks, and had nothing to do with temperature; whereas the aggregation morphology of the nanoparticles was related to the temperature during the formation and evolution processes of the nanoparticles.