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Wetting-induced layer contraction in illite and mica-family relatives
Kühnel, R.A.; Van der Gaast, S.J.; Broekmans, M.A.T.M.; Theng, B.K.G. (2017). Wetting-induced layer contraction in illite and mica-family relatives. Applied Clay Science 135: 226–233. dx.doi.org/10.1016/j.clay.2016.09.027
In: Applied Clay Science. Elsevier: Amsterdam. ISSN 0169-1317; e-ISSN 1872-9053, more
Peer reviewed article  

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Author keywords
    Contraction/expansion; Humidity-controlled XRD; Hydronium; Illite; Mica; Protonation/deprotonation

Authors  Top 
  • Kühnel, R.A.
  • Van der Gaast, S.J.
  • Broekmans, M.A.T.M.
  • Theng, B.K.G.

Abstract
    When dry illite is wetted, its layer structure contracts along the c-axis by up to 0.2 Å. This behavior contrasts with that of smectite which shows interlayer swelling on wetting. Layer contraction has also been found to occur in glauconite and celadonite that are structurally more disordered than illite, as well as with phengite and artificially degraded hydromuscovite. In contrast, muscovite proper does not show contraction. The contraction is ascribed to the deprotonation of hydronium ions (H3O+ → H2O + H+) occupying interlayer K+ positions. The hydronium ion is approximately 5% larger than the neutral water molecule. This difference in size is proportional to the magnitude of contraction. The change in molecular volume and interlayer separation of illite particles may explain the anomalous decrease in density after dehydration under pressure. This paper reports on wetting-induced contraction in illite and related layer silicates, using humidity-controlled X-ray diffraction.

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