New insights on the distribution of interlayer water in bi-hydrated smectite from X-ray diffraction profile modeling of 00l reflections

The interlayer configuration proposed by Moore and Reynolds (X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: New York, 1997) and commonly used to reproduce the 001 reflections of bi-hydrated smectite is shown to be inconsistent with experimental X-ray diffraction data. The alternative configuration of interlayer species with cations located in the mid-plane of the interlayer and one sheet of H2O molecules on each side of this plane is also shown to imperfectly describe the actual structure of bi-hydrated smectites. Specifically, the thermal fluctuation of atomic positions (Debye-Waller factor) used to describe the positional disorder of interlayer H2O molecules has to be increased to unrealistic values to satisfactorily reproduce experimental X-ray diffraction data when using this model. A new configuration is thus proposed for the interlayer structure of bi-hydrated smectite. Cations are located in the mid-plane of the interlayer, whereas H2O molecules are scattered about two main positions according to Gaussian-shaped distributions. This configuration allows reproduction of all 001 reflections with a high precision, with only one new variable parameter (width of the Gaussian function). The proposed configuration is consistent with those derived from Monte Carlo calculations and allows matching more closely the amount of interlayer water that can be determined independently from water vapor adsorption/desorption isotherm experiments. In addition, the proposed configuration of interlayer species appears valid for both dioctahedral and trioctahedral smectites exhibiting octahedral and tetrahedral substitutions, thus not allowing differentiation of these expandable 2:1 phyllosilicates from their respective interlayer configurations.