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ILLITE-SMECTITE MIXED-LAYER MINERALS IN THE HYDROTHERMAL ALTERATION OF VOLCANIC ROCKS: I. ONE-DIMENSIONAL XRD STRUCTURE ANALYSIS AND CHARACTERIZATION OF COMPONENT LAYERS

¹ Department of Earth Sciences, Chiba University, Chiba 263-8522, Japan
² LGIT-Maison des GéoScience, BP53, Université de J. Fourier, 38041 Grenoble Cedex 9, France
³ Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, 119017 Moscow, Russia
⁴ Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan
⁵ HydrASA-UMR 6532 CNRS, Université de Poitiers, 40 av. Recteur Pineau, 86022 Poitiers Cedex, France

^* E-mail address of corresponding author: atinoue{at}earth.s.chiba-u.ac.jp

For a series of mixed-layer illite-smectite (I-S) minerals from a drillhole near the Kakkonda geothermal field, one-dimensional structure analysis by X-ray diffraction (XRD) was performed using Ca-saturated specimens in both air-dried and ethylene glycol-solvated states. The expandability characteristics of component layers were also examined by means of alkylammonium exchange and Li saturation. The K content in the illite layers was 1.5–1.7/O₂₀(OH)₄ in the I-S series from 3 to 85% of I-layer content (% I). The layer charge of the smectite layer varied slightly within the range of 0.3–0.5/O₁₀(OH)₂ by alkylammonium exchange experiments and the expandability was independent of the beidellite content within a range of 0–0.5 by the Li-saturation test. The degree of long-range ordering represented by Reichweite (R) parameters varied from R0 to R3 via R1 and R2 with increase in % I. The I-S samples contained <10% vermiculite as the third component and the vermiculite content tended to decrease with progressive illitization.

In contrast to the smectitic R0 samples (<10% I), more illitic R0 (e.g. 35% I) and >R1 I-S samples showed complicated expandability with alkylammonium exchange. The XRD patterns of dodecylammonium-exchanged I-S samples can be interpreted by random interstratification of several types of sub-units such as layer-doublets, layer-triplets and layer-quartets present in the crystallites. This interpretation is consistent with the variation in the occurrence probabilities of layer-multiplets calculated from the junction probabilities and the proportions of layers. Because the interpretation indicates that I-S is a stack of various types of the sub-units, the smectite illitization can be described by a systematic change in the type and proportion of the sub-units constituting crystallites.

Key Words: Alkylammonium Exchange • Expandability • Hydrothermal System • Illite-smectite Mixed-layer Minerals • One-dimensional XRD Structure Analysis

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