Crystal structure model development for soil clay minerals – II. Quantification and characterization of hydroxy-interlayered smectite (HIS) using the Rietveld refinement technique

Abstract

The acidic weathering process of soils is a worldwide problem. In this process, Al3+ is dissolved from silicates and intercalated into swellable clay minerals by replacing exchangeable cations such as Na+ or Ca2+. Al3+ polymerizes in the interlayer regions of the clay minerals to form fixed oligomers, which significantly reduces the clay minerals swellability and cation exchange, and hence, the soil fertility. The layer silicates with fixed (Al) oligomers in the interlayer are known as hydroxy-interlayered minerals, such as hydroxy-interlayered smectite (HIS), or hydroxy-interlayered vermiculite (HIV). These phases have been recognized for many years, but it is still problematic to adequately characterize the details of their structure and to determine their proportion in soils, based on X-ray diffraction (XRD), which is the most common method for mineral quantification. In the present study, two crystal structure models are used to allow characterization and quantitative determination of HIS in artificial and natural soils, using the Rietveld refinement technique based on XRD. Both models are identical regarding the 2:1 layer and the description of the hydroxy-interlayering, but differ in the effect imposed by environmental conditions. The first model describes the smectite in the ethylene glycol saturated state (≈17 Å), commonly used by clay scientists as a diagnostic intercalate for swellable smectitic or vermiculitic layers in pure smectite or vermiculite or interstratifications. The second model describes the smectite in the air-dried (AD) state including the collapsed state (≈10 Å), one-water layer state (≈12.5 Å), and two-water layer state (≈15 Å). Both models are used to fit XRD patterns obtained from samples with preferred orientation and from XRD random powder specimens, each including structural disorder effects. The models provide plausible refinement results with respect to crystallographic parameters and quantitative proportions. The degree of hydroxy-interlayering gives an estimate of how many exchangeable cations of the smectite were replaced by fixed Al oligomers that inhibit expandability. This value corresponds to calculated values within a range of precision of ≤15% mol/mol. Physical mixtures of HIS with clinochlore, muscovite, quartz, and albite were quantified using the proposed crystal structure model. Results showed that the HIS model delivers reliable quantification results (deviations from actual values ≤3% w/w) which may be applied also for natural soil.