Infrared Spectroscopic Analyses on the Nature of Water in Montmorillonite

-Interlayer cations and moisture content greatly influence the molecular vibrations of H20 in montmoriUonite as shown through reflectance spectroscopy in the infrared. The absorptions due to H~O have been studied in montmorillonites exchanged with H, Na, Ca, Mg and Fe 3+ interlayer cations under variable moisture environments. Band assignments have been made for absorptions in the 3 ~tm region due to structural OH vibrations, symmetric and asymmetric 1-120 stretching vibrations and the H20 bending overtone. Changes in the energies of the absorptions due to H20 stretching vibrations were observed as the samples were dehydrated by reducing the atmospheric pressure. Absorptions near 3620 cm -1 and 3550 cm -1 have been assigned to water bound directly to cations (inner sphere) and surfacebonded H20 and absorptions near 3450 cm -1 and 3350 cm ~ have been assigned to additional adsorbed water molecules. Band assignments have been made for combination bands in the near-infrared as well. Absorptions near 1.41 ~m and 1.91/zm are assigned to bound H20 combination bands, while the shoulders near 1.46 izm and 1.97 ~*m are assigned to combinations of additional H20 molecules adsorbed in the interlayer regions and along grain surfaces. Key Words--Montmorillonite, Spectroscopy, Water. I N T R O D U C T I O N These experiments were initiated to develop an understanding of the spectroscopic properties ofsmectites under variable environmental conditions that is necessary for the near infrared (NIR) and mid-range infrared (Mid-IR) geologic mapping of the Earth and Mars. Remote sensing measurements are made on these planets under a range of environmental conditions, requiring laboratory studies to determine the effects of environmental conditions on spectroscopic features in minerals and soils. The spectroscopic features due to water in smectites are sensitive to the cations present in the interlayer sites, as well as to the relative humidity of the sample environment, and are examined in detail in this study. Parallel laboratory studies of the influence of atmospheric pressure on the spectroscopic properties of montmorillonites containing hydrated ferric oxides, oxyhydroxides and sulfates prepared under a variety of chemical conditions (Bishop et al 1994) and the influence of atmospheric pressure and temperature on a few representative soils and chemically altered montmorillonites (Bishop and Pieters 1994) have been performed as well. Once the effects of the chemical environment on the interlayer water in model clays are better understood, it will possible to use the hydration bands measured via reflectance spectroscopy to gain information about the chemical composition and moisture content of clays in real systems. Developing * Address correspondence to: Janice Bishop, DLR, Forchungszentrum Berlin Institute fuer Planetenerkundung Rudower Chaussee 5 12484 Berlin, Germany. Copyright 9 1994, The Clay Minerals Society a procedure for obtaining chemical and moisture information about surface soils from a remote perspective is especially useful in remote sensing of the Earth and Mars. Reflectance spectroscopy is an effective tool, commonly used to study and characterize clay minerals (e.g., Lyon and Green 1975; Clark et al 1990; Bishop et al 1993; Post and Noble 1994). Many researchers have studied isolated spectroscopic features or either the near-infrared region or the mid-infrared region of smectites. However, as mentioned by Clark et al (1990), differences in the spectrometers or sample preparation often make it difficult to compare the spectral features from one study to another. For this reason spectra are measured here in the near-infrared to mid-infrared regions allowing the study of the spectroscopic features due to fundamental, overtone and combination vibrations in the same samples under the same environmental conditions with the same spectrometer. Previous spectroscopic research on clays supports a complex system of interlayer water in montmoril lonites (Fripiat et a l 1960; Grim and Kulbicki 1961; Russell and Farmer 1964; Mortland et a l 1963; Poinsignon et al 1978; Sposito and Prost 1982), including water that is bound to the interlayer surface, bound to interlayer cations, bound to other water molecules and adsorbed on the grain surfaces. The purpose of the study presented here is to examine several montmoril lonite samples with different interlayer cations under variable moisture environments. The effects of these cations on the water molecules have been analyzed through infrared reflectance spectroscopy from 1-25 tzm (10,000400 cm-1). By measuring the H20 fundamental vibra-