Compositional and Solubility Relationships Between Aluminum-hydroxyinterlayered Soil-Smectites and Vermiculites
Coarse clay fractions (2–0.2 µm) of selected soil samples containing variable quantities (10–52%) of Al-hydroxyinterlayered smectite (HISM) or vermiculite (HIV) were investigated by XRD, thermal, elemental and solution composition analyses. The HISM vs. HIV nature of the sample was determined from the smectitic or vermiculitic origin of the sample and by alkylammonium ion exchange, expansion with glycerol and resistance to collapse after K-saturation and heating at 300° and 550 °C. X-ray diffraction and thermal analysis of tetradecylammonium exchanged clays and structural formulae calculations indicated all hydroxyinterlayered minerals, except for one HISM, to have high layer charge deficiencies (X > 0.5 equivalents per 0.5 unit cell). A greater portion of the total charge deficiency was satisfied by [Al(OH)x]3-x in HIV minerals which resulted in generally higher free energies of formation (ΔG°f). Hydroxyinterlayered smectites on the other hand, exhibited higher cation exchange capacities and higher OH/Al ratios in the hydroxyinterlayer material. Compositions of soil solutions in contact with these minerals supported a metastable HISM region in relatively Si-enriched solutions and a metastable or stable HIV region in somewhat Si-deficient solutions comparable to those supporting the stability of kaolinite. Structural formulae- and soil solution- compositions of the studied samples appeared to be consistent with the dynamic nature of these minerals in soil environments in terms of mineralogical distribution, physicochemical behavior, and stability.
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