X-RAY PHOTOELECTRON-SPECTROSCOPY (XPS) EVIDENCE FOR INTERLAYER PHASESIN NATURAL MICAS - EFFECTS ON PHYSICOCHEMICAL PROPERTIES AND GEOCHRONOLOGICAL CONSEQUENCES

After cleaving, the chemistry of the (001) topmost layers of natural m icas was experimentally investigated by X-ray photoelectron spectrosco py (XPS). There is evidence that phlogopite cleaves along very thin ch lorite interlayers also including graphite-rich microinclusions. Musco vite cleavage exposes an aluminum-rich (gibbsite, kaolinite or pyrophy llite) interlayer as well as graphite-rich microinclusions planes. Cle avage occurs in these interlayered phases because they have the weakes t bonds. These interlayered phases are also responsible for changes of the mechanical properties of micas (i.e.:lattice slip is enhanced). T he investigated micas yield Rb-Sr, K-Ar and Ar-Ar ages that are unrela ted to metamorphic or igneous events. The formation of the interlayere d phases probably perturbed the Rb-Sr and K-Ar isotopic systems.