THE DEGREE OF METAMICTIZATION IN ZIRCON - A RAMAN-SPECTROSCOPIC STUDY

A series of natural zircon samples representing various degrees of met amictization were investigated by Raman microprobe (RMP) analysis. We found systematic changes in wavenumbers and half-widths of the Raman b ands, caused by increasing irregularities of bond-lengths and bond-ang les and a general breaking-up of the structure as a result of metamict ization. Therefore, Raman spectroscopy can be used to measure the crys tallinity of zircons. The half-width of the antisymmetric stretching v ibration band (B1g) of the SiO4 tetrahedra, which has a frequency of a bout 1007 cm-1 in well-crystallized and 1000 to 955 cm-1 in metamict z ircons, is most suitable for estimating the degree of metamictization: Its value increases from about 5 cm-1 in well crystallized samples up to 30-55 cm-1 in highly metamict, X-ray-amorphous zircons, and is str ongly dependent on the degree of lattice destruction by metamictizatio n. In contrast, the Raman parameters seem to be almost uninfluenced by chemical variations. The potential value and advantages of such RMP m easurements, especially in radiometric age determinations, are discuss ed in the light of other methods.