Hydrous species in zircon

Abstract

A survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis. Stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed. The region of IR spectra of OH-bearing crystalline zircon from 4000 to 2500 cm-^(1), exhibits anisotropic OH bands resulting from at least three OH sites, one at vacant tetrahedra and two that may be at Si-occupied tetrahedra. One site associated with Si-occupied tetrahedra is the most important, producing IR absorption bands at ~3420cm^(-1)(E‖c)and ~3385cm^(-1)(E⊥c). OH maybe introduced during crystal growth by a coupled substitution of the form [M^(3+) + H^4] ↔ [Zr^(4+)]. The sites at vacant tetrahedra account for considerably less OH, producing relatively weak IR absorption bands at ~3510 cm^(-1)(E‖c > E⊥c). OH may be introduced during crystal growth by the hydrogrossular substitution of the form [4(OH)-] ↔ [(SiO_4)^(4-). An additional OH band at ~3270 cm^(-1) appears to be associated with an occupied tetrahedron. Heating experiments produced two more OH environments and indicate that H can migrate among the sites at high temperatures. Some crystalline samples are entirely free of OH. IR spectra of metamict zircon exhibit broad, asymmetric, isotropic OH bands similar to H_2O bands, but the absence of the H_2O bending-mode band indicates that OH is the dominant hydrous species present. IR spectra of partially metamict zircon samples retain some anisotropy. The breadth of their OH bands is a function of degree of metamictizatron. The maximum OH observed in any metamict or partially metamict zircon sample was approximately 0.1wt%, expressed as H_2O. SuchO H is probably secondary. The maximum primary OH observed is ~0.05 wt% partially metamict zircon from a hydrothermal vug. Crystalline crustal zircon contains more than an order of magnitude less OH. The maximum OH content observed in crystalline zircon, ~0.01 wt%,i s in a mantle zircon sample from a kimberlite, suggesting high activity of H_2O in the mantle. Additional OH is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite. Fluid inclusions containing H_2O occur in some samples. This may be a partial explanation of the high H_2O contents reported for some radiation-damaged zircon based on loss-on-heating measurements.

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