Department of Geology, Occidental College, Los Angeles, California 90041, U.S.A.
George R. Rossman, Leon T. Silver
Division of Geological and Planetary Sciences, California
Institute of Technology
Pasadena, California 91125-2500, U.S.A.
A suite of gem zircon samples from Sri Lanka has been studied using infrared (IR) spectroscopy, X-ray diffraction, and chemical analysis. The degree of metamictization of the zircon, as indicated by unit-cell parameters, increases systematically with U-Th content up to the point of total metamictization. The appearance of IR spectra also varies as a function of metamictization; band widths increase and intensities decrease with increasing U-Th contents. Persistence of bands related to Si-O bonds and disappearance of bands related to Zr-O bonds indicates that the structure of metamict zircon consists of distorted and disoriented isolated silica tetrahedra with few if any undisplaced Zr cations. All spectroscopic indicators of crystalline order show that total metamictization is reached at an accumulated radiation dose of ~4.5 x 1015 alpha decay events per mg. Hydrous components enter the structure only after total metamictization, but the amounts are not correlated with U-Th content. In all cases OH was the only hydrous species detected.