Compositional Gap in Dioctahedral-Trioctahedral Smectite System: Beidellite-Saponite Pseudo-Binary Join

Hirohisa Yamada1, Katsuaki Yoshioka2, Kenji Tamura3, Kazuko Fujii1 and Hiromoto Nakazawa1
1 National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
2 Nippon Paint Co. Ltd., 4-1-15 Minamishinagawa, Shinagawa-ku, Tokyo 140-0004, Japan
3 Central Laboratory, Kawasaki, Showa Denko K.K., 3-2 Chidori-cho, Kawasaki-ku, Kawasaki 210-0865, Japan
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Abstract: A series of hydrothermal experiments were performed to determine the phase relations on the beidellite-saponite pseudo-binary join. Quenched glasses with stoichiometric dehydrated compositions of an Na-rich smectite on the join were heated at 250–500°C for durations of 1–151 d at 100 MPa. Time-temperature diagrams showed that immiscibility occurs between dioctahedral smectite (beidellite) and trioctahedral smectite (saponite) below 400°C. Thus, smectite with intermediate chemical composition was considered as metastable in this system. Above 400°C the assemblage of regularly interstratified saponite-chlorite, quartz, and feldspar was recognized in the intermediate chemical compositional region of this join. On the beidellite side of this join, beidellite and mixed-layer phases of smectite and a regular interstratification of montmorillonite-beidellite, are possible phases that occur at <300°C. They readily reacted to form a mixture of dioctahedral rectorite plus quartz at 300°C. This assemblage then reacted to a dioctahedral “mica”, which can expand with glycol and quartz. On the saponite side of this join, a single phase, saponite, existed at <400°C, and transformed to saponite plus trioctahedral rectorite with aging and increasing temperature of synthesis. The alteration was affected strongly by the chemical composition of the binary system.

Key Words: Beidellite-Saponite Join • Compositional Gap • Hydrothermal Experiment • Mixed-Layer Phases

Clays and Clay Minerals; December 1999 v. 47; no. 6; p. 803-810; DOI: 10.1346/CCMN.1999.0470616
© 1999, The Clay Minerals Society
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