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The Role of H3O+ in the Crystal Structure of Illite

Published online by Cambridge University Press:  01 January 2024

Fernando Nieto ,
Marcello Mellini  and
Isabel Abad
Fernando Nieto*
Affiliation:
Departamento de Mineralogía y Petrología and IACT, Universidad de Granada, CSIC, Av. Fuentenueva, 18002 Granada, Spain
Marcello Mellini
Affiliation:
Dipartimento di Scienze della Terra, Università di Siena, Via Laterina 8, 53100 Siena, Italy
Isabel Abad
Affiliation:
Departamento de Geología, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain
*
* E-mail address of corresponding author: nieto@ugr.es
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Abstract

In spite of decades of research on the subject, the crystal structure of illite is still poorly understood. The purpose of this study was to address this problem by investigating the nature of the interlayer content in illite IMt-2 from Silver Hill, Montana, using analytical transmission electron microscopy (ATEM), thermogravimetry (TG), and X-ray powder diffraction (XRPD) analyses. The ATEM data, together with literature and TG results, yielded the formula K0.70a0.01(H2O)0.42 (Al1.53Fe2+0.06Fe3+0.19Mg0.28)Σ−2.06(Si3.44Al0.56)O10(OH)2 or, assuming the presence of H3O+, K0.69Na0.01(H3O)+0.28(Al1.47Fe2+0.06Fe3+0.19Mg0.28)Σ−1.99(Si3.40Al0.60)O10(OH)2. The first formula indicates surplus interlayer and octahedral species, whereas the second shows no excess. The XRPD data were refined by Rietveld techniques, down to an Rp factor of 10.48–13.8%. The mineral composition consists largely of illite-2M1, illite-1M, and minor quartz. Although the refinement accuracy is limited by the intrinsic poor quality diffraction of the illites, the partially refined model is consistent with the chemical composition; in particular, attempts to introduce octahedral cations in excess of 2 were fruitless. All the results support the simple structural model, by which the illite structure strictly corresponds to a dioctahedral mica with H3O+ replacing K. As a consequence, the crystalchemical formula of illites should be calculated on the basis of six tetrahedral plus octahedral cations.

Keywords

CompositionHydroniumIlliteWater

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Type
Article
Information
Clays and Clay Minerals , Volume 58 , Issue 2 , April 2010 , pp. 238 - 246
Copyright
Copyright © Clays and Clay Minerals 2010

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