Study of cations and charge distribution in swelling clay sheets by XPEEM
D. Vantelona, R. Belkhoua, I. Bihannicb, E. Montargès-Pelletierb, L.J. Michotb,
S. Maddib, J.L. Robertc, O. Mentesd, L. Aballed, A. Locatellid
aSynchrotron SOLEIL, L’Orme des Merisiers, F-91192 Gif-sur-Yvetteb
LEM, UMR7569, 15, av. du Charmois, F-54500 Vandoeuvrec
IMPMC, UMR 7590, 140, rue de Lourmel, F-75000 ParisdSincrotrone ELETTRA, Srada Stratale 14, I-34012 Basovizza

Swelling clay minerals, such as montmorillonites, are lamellar alumino-silicates
formed with two silica sheets sandwiching an aluminum or magnesium hydroxide sheet.
Isomorphous substitutions by lower charge metal cations in the crystal lattice result inpermanent negative charge that is compensated by electrostatically attracted cations(exchangeable). The lateral extension of individual layers ranges from 25 to 1000 nm while
they are 10 Å thick. They exhibit a natural heterogeneity at different organization levels:
substitutions in the crystalline network, size and shape of the sheets, structures resulting fromsheets interactions, and macroscopic behavior (aggregation, rheology). Understanding thesestructures remains an open question as well as the links existing between them. As distributionof structural cations and charge is the lowest level of heterogeneity, it might control at least partof the other organization levels and macroscopic behavior. Our objective was then to investigate
this level of organization at the individual layer scale taking advantage of the performances of
the X-PEEM installed on the ELETTRA Nanospectroscopy beamline (Trieste, Italy). Highresolution X-PEEM imaging using core level spectroscopy performed on clay monolayer
deposits enabled us to single out the different clay sheets and to study their local compositionand charge distribution. From the intensity variations of the photoemission core level spectra,
one can deduce the local distribution of the relevant species. Meanwhile, their binding energy
provides information on their electronic and chemical environment. Indeed, substituted
cations in the layer influence the electronic environment of the neighboring atoms which
results in variations in the core level binding energy (BE) of the atom of interest.
We studied two montmorillonites from different ore bodies, Wyoming and Milos. They
exhibit different macroscopic properties (i.e. rheology, sol-gel transitions) and octahedral irondistributions (ordered in Wyoming, randomized in Milos) although their lattice formulae are very
similar. X-PEEM measurements (Images, local XPS and XPS dispersive plane spectra) were
performed recording the Si2p, Al2p and Mg2p core levels spectra from the clay sheets. A major
result comes from the Si2p photoemission core level spectra which position shifts to the lower
BE as substitutions in the silica layer, thus tetrahedral charge, increase. Our preliminary results
clearly demonstrate that, for a given clay ore body, charge is not homogeneously distributed
from one sheet to an other. Furthermore, a large layer can exhibit two different silica
environments and at least one of these two environments varies with the ore bodies. This lastresult is of peculiar interest as it may be a new clue to understand the variations in macroscopic
behavior exhibited by different ore bodies.