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Study of adsorbed water: Electric potential calculation and molecular orientation in the two layer hydrate of a Mg vermiculiteThe well characterized two-layer hydrate of Na-Llano vermiculite has been studied by continous wave (CW) wide-line NMR and by pulse NMR of IH nucleus in a wide temperature range. The absorption spectrum consists of a doublet and of a central line. The doublet splitting is orientation dependent between o and SO·C and orientation independent below -60·C. Below 6O·C, the symmetry of the hydration shell may be destroyed. The linewidth is compatible with rapidly rotating water molecules. An octahedral distribution of water molecules around the Na + cations fits the experimental data, the six rotation axes around which the water molecules are spinning rapidly being tilted by about 65· with respect to the C'" axis. The cation hydration shell is characterized by a rotational diffusion motion with an activation energy of 8.5 kcal, the diffusion coefficient at room temperature being about 0.5 X 10-8 cm 2 sec-I. The diffusion coefficient of free water or protons is about 0.2X 10-6 cm 2 sec-I. From +50 to -IOO·C, the observed T \1 is contributed successively by the diffusion of the cation hydration shell, the diffusion of free water or proton with respect to the paramagnetic centers, and finally by the paramagnetic contribution. Water molecules within the hydration shell and/or water molecules "between" the hydration shells exchange protons with a frequency between 10-4 and 10-5 sec-I at room temperature. This exchange occurs within the hydration shell or it is relayed by the "free" water molecules between the hydration shells.

In soil carbon dynamics, the role of physicochemical interactions between organic matter and minerals is not well understood nor quantified. This paper examines the interactions between soil organic matter and poorly crystalline aluminosilicates in a volcanic ash soil on La Re´union in the southern tropics. The soil examined is a profile composed of a surface soil (L-Ao-E-Bh) overlying four buried horizons (horizons 2Bw, 3Bw, 4Bw, 5Bw) that have all developed from successive tephra deposits. Non-destructive spectroscopy (XRD, FTIR and NMR of Si and Al) showed that the mineralogical composition varies from one buried horizon to another. Further, we show that buried horizons characterized by large amounts of crystalline minerals (feldspars, gibbsite) have the least capacity to store organic matter and the fastest carbon turnover. In contrast, buried horizons containing much poorly crystalline material (protoimogolite and proto-imogolite allophane, denoted LP-ITM) store large amounts of organic matter which turns over very slowly. To understand the mechanism of interactions between LP-ITM and organic matter better, we focused on a horizon formed exclusively of LP-ITM. We demonstrate, using Á 14 C and 13 C values, that even though LP-ITM is extraordinarily effective at stabilizing organic matter, C linked to LP-ITM is still in dynamic equilibrium with its environment and cycles slowly. Based on Á 14 C values, we estimated the residence time of organic C as $ 163 000 years for the most stabilized subhorizon, a value that is comparable to that for organic carbon stabilized in Hawaiian volcanic soils. However, this calculation is likely to be biased by the presence of microcharcoal. We characterized the organo-mineral binding between organic matter and LP-ITM by 27 Al NMR, and found that the organic matter is not only chelated to LP-ITM, but it may also limit the polymerization of mineral phases to a stage between proto-imogolite and proto-imogolite allophane. Our results demonstrate the important role of poorly crystalline minerals in the storage of organic C, and show that mineral and organic compounds have to be studied simultaneously to understand the dynamics of organic C in the soil.

This work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5, 7, and 10 with Si/Fe molar ratios in the range 0.25-4). By coupling the results of two spectroscopic techniques, FTIR and 29 Si NMR, interesting structural information emerges. We show that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si-O-Fe bonds hinders the formation of Fe oxyhydroxides. The ratio Si/Fe ) 1 constitutes a transition point between Si-O-Fe and Si-O-Si bond formation at pH ) 3 and 5, where Si-clusters appear once the maximum amount of Si-O-Fe bonds are formed. This is confirmed by 29 Si NMR which demonstrates the presence of Si pockets in three of the eight examined samples. The return to equilibrium of the 29 Si magnetization leads to a value for both dimensionality of the silica-rich pockets (D ) 2.2) and length over which dimensionality is observed (2 nm). By using both FTIR and 29 Si solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous study conducted by EXAFS.