Soft X-ray spectroscopic characterization of montmorillonite
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1124-Q05-03
Soft X-ray spectroscopic characterization of montmorillonite J-E Rubensson1, F Hennies2, L O Werme1, 3, O Karnland4 1 Department of Physics and Materials Science, Uppsala University, Box 530, SE-75121 Uppsala, Sweden 2 MAX-lab, Lund University, Box 118, SE-22100 Lund, Sweden 3 Swedish Nuclear Fuel and Waste Management Co, Box 250, SE-10124 Stockholm, Sweden 4 Clay Technology AB, IDEON Research Center, SE-22370 Lund, Sweden
ABSTRACT Soft X-ray spectroscopy was applied to study a calcium bentonite from the Kutch area in India. We recorded the X-ray absorption spectra from the L-edge of calcium, silicon, and aluminum, and from K-edge of oxygen. The Ca absorption spectrum shows a quasi-atomic behavior, while the Si spectrum closely simulates the absorption spectrum of a pure silicon oxide. The O K spectrum shows a pre-peak, which is absent in the spectra of both the pure, bulk aluminum and silicon oxides. The Al L spectrum is complex and shows almost no resemblance to the absorption spectrum of aluminum oxides. The chemical state of the Al atoms (in octahedral coordination) must, thus, be quite different from what is common in the oxides. The obtained data show that soft X-ray spectroscopy is a promising technique for studying clay minerals. It is capable of supplying unique information that is complementary to information accessible using other techniques; especially, it can be used to determine the local electronic structure at various atomic sites in the complex samples. INTRODUCTION Bentonite is proposed in many countries, including Sweden, to be used as a buffer material in nuclear waste repositories. Bentonite is a geological term for soil materials with a high content of a swelling mineral, which usually is montmorillonite. The montmorillonite belongs to the smectite group, in which all members have an articulated layer structure. The desirable physical bentonite properties of the buffer, mainly swelling pressure and low hydraulic conductivity, are due to interaction between water and the montmorillonite in the bentonite. This interaction is affected by changes in the ion concentration in the groundwater and by changes in the mineral structure of the montmorillonite. The mineralogical stability of the montmorillonite is, therefore, of crucial importance for the performance of the buffer. Montmorillonite can be stable for hundreds of millions of years in its formation environment, but changes in the geochemical environment can lead to a relatively rapid change of the mineral structure. Montmorillonite consists of nanostructures of silicon and aluminum oxide sheets, where typically one monolayer of octahedrally coordinated aluminum atoms with oxygen as ligands, is sandwiched between two monolayers of silicon oxide. The aluminum in the octahedral sheet is partly substituted, principally by Mg. The silicon in the tetrahedral sheet may partly be substituted, principally by aluminum. The substitutions result in a net negative charge of the montmorillonite layer in the range of 0.4 to 1.2 elementary charge
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