THERMAL HISTORY OF HYDROTHERMAL QUARTZ CRYSTALS: INSIGHTS FROM MICRO-IR SPECTROSCOPY

Hydrothermal quartz crystals from the natural environment contain systematic variations in defect abundances. Using high-resolution FTIR, our group has shown that natural quartz crystals are composed of six sector zones characterized by distinct concentrations of hydrous species, including AlOH, HOH, and LiOH (Ihinger & Zink, 2000). The sector zones are regions within the crystal that correspond to crystallization on the six rhombohedral growth faces. Using micro-IR on samples from Mexico, China, and Brazil, we present measurements that define classic diffusion profiles (with decreasing abundance in impurities toward the six prism faces) superimposed upon otherwise homogeneous concentrations within individual sectors. In all crystals, LiOH exhibits greater diffusive loss compared to HOH, which in turn exhibits greater diffusive loss than AlOH, demonstrating relative mobility in the order LiOH > HOH > AlOH. Analyses from successively younger regions of crystals document progressively lesser diffusive loss, suggesting that diffusion occurs as the crystals are still growing. Quartz crystals from some environments, such as Mt. Ida, AR, contain negligible concentrations of hydrous impurities, perhaps reflecting prolonged exposures to high temperatures post crystallization. With accurate knowledge of the temperature dependence of the diffusivity of hydrous species, our measurements may constrain quantitatively crystal growth rates and the temperature evolution of hydrothermal systems.