On the structures and orientations of hydrogen defects in natural and synthetic quartz crystals

Polarization ratios A = ε ⟂/ε ∥ for the narrow absorption bands in the 3 μm region are measured in natural clear quartz, citrine, and amethyst as well as synthetic quartz. Except for natural clear quartz they approach the limit of one at high hydrogen concentrations. This apparent variation is caused by randomly oriented OH groups with broad and isotropic absorption. In natural amethyst the same band positions are observed as in synthetic quartz, but the polarization ratios are considerably lower. No absorption bands specific for the FeH defects, characterized earlier by EPR measurements, are found, and with thermal destruction of these centres the absorption bands did not shift or decrease. These facts are better compatible with the model of SiH groups and localized modes associated with them, than with SiOH groups that are apparently present in the AlH centers.