The incorporation of hydroxyl groups and molecular water in natural zircon (ZrSiO4)

The nature of "water" incorporated in natural zircon has been studied by polarized infrared absorption spectroscopy combined with heating and deuteration experiments and Raman, thermogravimetric, chemical, and X-ray analyses, and previously published results have been re-evaluated. Hydroxyl groups are probably the only hydrous species primarily incorporated in zircon during crystal growth, with a saturation level in the 102 ppm range or even below. We present three crystallographic models for probable locations of such "simple" (OH)- groups in crystalline zircon, most probably associated with Si-vacant tetrahedrons (O-H // c) or occupied SiO4 tetrahedrons without or with a closely neighbored zirconium vacancy (O-H ? c). The majority of "water" present in natural zircons, reaching from some tenth (this work) up to > 10 wt percent (literature), seems to be secondary in nature and includes both hydroxyl groups and water molecules. The incorporation of such secondary water is an indirect result of metamictization. However, hydrogen incorporation is not necessary for the transformation of crystalline into radiation-damaged structures. Hydrogen incorporation is rather an accompanying effect of the metamictization process, favored by the enhanced presence of structural defects and local charge imbalance in metamict zircon. Critical discussion of the entire data set available seems to raise more problems than it solves. Results of the different analytical techniques reveal apparent contradictions. The use of infrared absorption, normally very sensitive for detecting hydrogen in nominally anhydrous minerals, is widely limited in the case of natural zircons. Detailed interpretation about hydrogen locations in metamict zircons fails at the present stage because of insufficient knowledge about such structures.