CLEAVAGE MECHANISM AND SURFACE CHEMICAL CHARACTERIZATION OF PHENGITICMUSCOVITE AND MUSCOVITE AS CONSTRAINED BY X-RAY PHOTOELECTRON-SPECTROSCOPY

Freshly cleaved (001) natural muscovite was chemically characterized b y X-ray Photoelectron Spectroscopy (XPS). The investigated muscovite c rystals have composition ranging from pure end member to phengitic mus covite. XPS provides experimental evidence that the chemical compositi on of the near-surface region differs from the bulk. This difference i s not due to analytical problems, but to extreme surface sensitivity o f XPS. Depth profiling was also carried out after tilting of 60 degree s or 70 degrees of the sample along an axis perpendicular to [001]. Af ter tilting of phengitic muscovite, the concentration of Al increases and of K decreases, and therefore Al is the topmost monolayer. Repulsi on between the octahedral and tetrahedral sheets, stacking faults or m ore probably interlayered phases (gibbsite-like) are responsible for t he cleavage. Possible perturbations of surface chemistry due to X-ray radiation and Ar+ ion sputtering have also been investigated. X-ray ra diation is negligeable, but Ar+ ion sputtering introduces important st ructural modifications, perturbation of the chemical environment of el ements and minor chemical variations.