An experimental study of hydroxyl in quartz using infrared spectroscopy and ion microprobe techniques

We have measured the concentrations of hydroxyl, deuterium, Al, Fe, Li, Na, K, and Rb in a natural quartz crystal before and after hydrothermal treatment at 1.5 GPa and 800°-1050°C. We employed microbeam infrared spectroscopy and ion probe techniques to avoid impurities trapped in healed cracks and fluid inclusions that might bias a normal bulk analysis. Our analyses indicate that more hydrogen was introduced into the treated samples than Al or Fe. Because one proton or alkali cation is needed to screen each Al or Fe atom substituted into a Si lattice site, we conclude that the hydrothermal treatment had produced new hydroxyl defects in the quartz that did not contain Al or Fe. Although the speciation of this excess hydroxyl is unknown, it is present in all varieties of quartz that show hydrolytic weakening: synthetic quartz, amethyst, hydrothermally treated natural quartz crystals, and natural quartz deformed in talc assemblies. In the absence of microcracking or solution-precipitation mechanisms that may mechanically trap OH or H₂O molecules, we suggest that hydrogen diffusion, and reaction with lattice oxygen, may introduce hydroxyl defects into quartz and contribute to hydrolytic weakening. -from Authors