Surface morphology and reactivity of natural zircon

Zircon is a mineral known to be highly resistant to weathering. It may also survive to several geological cycles including metamorphism and anatexis. However, in some environments (hydrothermal fluids, tropical soils), zircon may loose its resistance and use of zirconium as an immobile element in weathering mass balance studies has been questioned. In addition, loss of crystal structure due to the accumulation of radiation induced defects, i.e. metamictization, strongly modifies the bulk thermodynamic properties of zircon.As surface properties are known to control the silicate weathering rates and may be different from bulk properties, a detailed study of the surface of zircons from tropical soils and sediments has been undertaken in order to better understand the resistance of zircon to dissolution. Surface chemistry of zircons was probed by Rutherford backscattering analysis whereas their surface morphology was analyzed by Scanning Electron Microscopy and Tapping Mode Atomic Force Microscopy. First results show contrasted surface morphologies depending on the crystallographic nature of the observed face. Concerning the well crystallized zircon, these contrasts can be interpreted from theoretical surface structures. In the case of variably metamict zircon, the metamictization degree can be probed using Raman spectroscopy and is shown to partly control the surface morphology of zircons originating from weathering environments. In both cases, no strong chemical modification was found near the surface of zircon. (authors)