Relationship between bulk and surface structure of muscovite

The aim of this project is to study the relationships between bulk and surface structure of a natural muscovite crystal.The muscovite studied [(K0.92Na0.08) (Al1.86Fe3+0.01Mg.0.07 Fe2+0.06Ti0.02) (Si3.03Al0.97) O10 F0.09 (OH)1.91] is monoclinic, 2M1 polytype, with symmetry C2/c and unit cell parameters a = 5.19690(10) b = 9.0138(3) c = 20.0835(7) (Å) and β = 95.763(2) (°).The muscovite structure was refined by single crystal X-ray diffraction and the final refinement yielded the following agreement factor R = 0.0299. The difference Fourier map shows a significant residual electron density (2.2-) close to the position related to M1 site, thus indicating that the excess of octahedral cations is located at M1. M1-O mean bond distance is 2.247(9) and M2-O distance is 1.929.Grazing incidence X-ray diffraction (GIXRD) was recently performed at SpLine at ESRF. This type of analysis is very useful to identify typical structural features of the surface, such as possible imperfections, relaxations or reconstructions of the surface [1]. Preliminary data collected by GIXRD could indicate that on muscovite surface a reconstruction could occur, but we need more reflections to be certain of this assumption. Some indirect evidence of surface modification of mica crystals may be suggested by a lower K content on mica surface than observed in the bulk, which is a rather common observation when subjecting micas to chemical analyses. K release can also be attributed to a variation of interlayer cavity topology at surface, as well as to a breaking of K-O bonding. This effect can play a significant role in determining surface charge in micas, either completely negative, or partly positive and partly negative, depending on K release.