摘要
In order to study quartz-coesite transition under the conditions of differential stress, experiments of quartzite deformation were carried out using a triaxial testing system with a Griggs type solid medium pressure vessel. Analyses on the plastically-deformed samples under optical microscope and Raman spectra show that fine-grained coesite was present in the region of samples adjacent to the pistons at temperatures of 950-1000℃, confining pressure of 1.3 GPa, differential stress of 1.5-1.67 GPa, and total strain of 75%-81%. It is evident that the transition pressure of quartz-coesite at differential stress and intensely-strained conditions is far lower than the pressure for coesite stability at isostatic pressure. In other words, the stress condition of coesite occurrence is not unique. The decrease in confining pressure for quartz-coesite transition under differential stress conditions is controlled by a combined effect of the maximum principal stress that provides a high stress environment, and differential stress that causes sample deformation. Coesite was produced in the plastically-deformed samples in this study, but it can occur in both semi-brittle and plastic deformation regimes as seen in previous studies. Phase transition in semi-brittle deformation regime is caused by local mechanical instability induced by shear deformation, and phase transition in plastic flow regime is due to strain instability induced by the presence of a high dislocation density within intensely-deformed quartz crystals.
In order to study quartz-coesite transition under the conditions ofdifferential stress, experiments of quartzite deformation were carried out using a triaxial testingsystem with a Griggs type solid medium pressure vessel. Analyses on the plastically-deformed samplesunder optical microscope and Eanian spectra show that fine-grained coesite was present in theregion of samples adjacent to the pistons at temperatures of 950-1000 deg C, confining pressure of1.3 GPa, differential stress of 1.5-1,67 GPa, and total strain of 75 percent-81 percent. It isevident that the transition pressure of quartz-coesite at differential stress and intensely-strainedconditions is far lower than the pressure for coesite stability at isostatic pressure. In otherwords, the stress condition of coesite occurrence is not unique. The decrease in confining pressurefor quartz-coesite transition under differential stress conditions is controlled by a combinedeffect of the maximum principal stress that provides a high stress environment, and differentialstress that causes sample deformation. Coesite was produced in the plastically-deformed samples inthis study, but it can occur in both semi-brittle and plastic deformation regimes as seen inprevious studies. Phase transition in semi-brittle deformation regime is caused by local mechanicalinstability induced by shear deformation, and phase transition in plastic flow regime is due tostrain instability induced by the presence of a high dislocation density within intensely-deformedquartz crystals.
基金
supported by the National Natural Science Foundation of China(Grant No.40002020)
the Institute of Geology,China Earthquake Administration(Grant No.2005B0010).
