We experimentally investigated changes to the titanium concentrations of quartz-rich rocks caused by grain-boundary migration. Synthesis experiments were performed to make high-Ti quartz starting materials for usage in recrystallization experiments. We isothermally recrystallized quartz starting material in separate experiments at 10–25 kbar to reduce the solubility of Ti in quartz. Microstructural and geochemical results indicate that the high-Ti quartz starting material recrystallized to form low-Ti quartz with acicular rutile inclusions. During recrystallization, high-Ti quartz was consumed to form low-Ti quartz and acicular rutile crystals precipitated behind moving grain boundaries via the discontinuous precipitation unmixing process. Experimentally imposed changes to the equilibrium solubility concentration of Ti in quartz by amounts as low as ~ 0.02 weight % (200 µg/g or ppm by weight) created a free energy driving force of 8.7 kJ/mol that was sufficient to drive large-scale recrystallization of the quartzites. Microstructures of relict grains of the high-Ti quartz starting materials are characterized by cuspate and irregular grain boundaries, whereas new quartz grains exhibit subhedral to euhedral grain morphologies and abundant three-grain junctions. Discontinuous precipitation can significantly change mineral compositions, and in the absence of a free fluid phase can operate much faster than other types of mass transfer processes commonly associated with solid-state reactions.

中文翻译：

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金红石石英的不连续沉淀：由钛在石英中的平衡溶解度变化引起的晶界迁移

我们通过实验研究了由晶界迁移引起的富含石英的岩石中钛浓度的变化。进行合成实验以制备用于再结晶实验的高钛石英起始材料。我们在 10-25 kbar 的单独实验中等温重结晶石英原材料，以降低 Ti 在石英中的溶解度。显微结构和地球化学结果表明，高钛石英起始材料再结晶形成具有针状金红石包裹体的低钛石英。在再结晶过程中，高钛石英被消耗形成低钛石英，针状金红石晶体通过不连续的沉淀分离过程沉淀在移动的晶界后面。通过实验对石英中 Ti 的平衡溶解度浓度施加低至约 0.02 重量%（200 µg/g 或 ppm 重量）的变化，产生了 8.7 kJ/mol 的自由能驱动力，足以驱动大规模石英岩的再结晶。高钛石英起始材料残余晶粒的显微结构以尖尖和不规则晶界为特征，而新的石英晶粒表现出亚自形到自形晶粒形态和丰富的三晶结。不连续沉淀可以显着改变矿物组成，并且在没有自由流体相的情况下，其运行速度比通常与固态反应相关的其他类型的传质过程快得多。7 kJ/mol 足以驱动石英岩的大规模再结晶。高钛石英起始材料残余晶粒的显微结构以尖尖和不规则晶界为特征，而新的石英晶粒表现出亚自形到自形晶粒形态和丰富的三晶结。不连续沉淀可以显着改变矿物组成，并且在没有自由流体相的情况下，其运行速度比通常与固态反应相关的其他类型的传质过程快得多。7 kJ/mol 足以驱动石英岩的大规模再结晶。高钛石英起始材料残余晶粒的显微结构以尖尖和不规则晶界为特征，而新的石英晶粒表现出亚自形到自形晶粒形态和丰富的三晶结。不连续沉淀可以显着改变矿物组成，并且在没有自由流体相的情况下，其运行速度比通常与固态反应相关的其他类型的传质过程快得多。而新的石英颗粒表现出亚自形到自形颗粒形态和丰富的三颗粒连接。不连续沉淀可以显着改变矿物组成，并且在没有自由流体相的情况下，其运行速度比通常与固态反应相关的其他类型的传质过程快得多。而新的石英颗粒表现出亚自形到自形颗粒形态和丰富的三颗粒连接。不连续沉淀可以显着改变矿物组成，并且在没有自由流体相的情况下，其运行速度比通常与固态反应相关的其他类型的传质过程快得多。