Wetting-induced layer contraction in illite and mica-family relatives

• Published in: Applied clay science Vol. 135; pp. 226 - 233
• Main Authors: Kühnel, Radko A., Van der Gaast, Sjierk J., Broekmans, Maarten A.T.M, Theng, Benny K.G.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2017
• Subjects: Contraction/expansion; Humidity-controlled XRD; Hydronium; Illite; Mica; Protonation/deprotonation; Protonation/deprotonation; Humidity-controlled XRD; Contraction/expansion; Mica; Hydronium; Illite
• ISSN: 0169-1317; 1872-9053
• DOI: 10.1016/j.clay.2016.09.027

When dry illite is wetted, its layer structure contracts along the c-axis by up to 0.2Å. 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. •RH-controlled XRD of selected illites reveals d(100) contraction upon wetting.•Related phyllosilicates like smectite, chlorite, and celadonite show expansion.•Muscovite and phengite contract negligibly; chlorite has the greatest expansion.•The contrasting behavior is attributed to different interlayer occupancy and content.•The presence of hydrous species in the interlayer is a crucial factor.