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An experiment study of quartz-coesite transition at differential stress

机译:石英-柯氏体相变应力的实验研究

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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 Eanian spectra show that fine-grained coesite was present in the region of samples adjacent to the pistons at temperatures of 950-1000 deg C, confining pressure of 1.3 GPa, differential stress of 1.5-1,67 GPa, and total strain of 75 percent-81 percent. 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.
机译:为了研究在差应力条件下的石英-柯氏体转变,使用具有Griggs型固体中压容器的三轴测试系统进行了石英岩变形实验。在光学显微镜和Ean光谱下对塑性变形样品进行的分析表明,在温度为950-1000摄氏度,限制压力为1.3 GPa,应力差为1.5-1.5的条件下,与活塞相邻的样品区域中存在细粒堇青石。 1,67 GPa,总应变为75%-81%。显然,石英-柯氏体在差应力和强应变条件下的转变压力远低于等静压下用于石英岩稳定的压力。换句话说,堇青石发生的应力条件不是唯一的。差分应力条件下石英-柯氏体转变的围压降低是由提供高应力环境的最大主应力和导致样品变形的差分应力共同作用控制的。在这项研究中,塑性变形样品中产生了柯氏体,但它既可以在半脆性变形中也可以在塑性变形中发生,如先前的研究所示。半脆性变形状态下的相变是由剪切变形引起的局部机械不稳定性引起的,塑性流动状态下的相变是由于强烈变形的石英晶体中存在高位错密度而引起的应变不稳定性引起的。

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