Synthesis and Paragenesis of Na-Beidellite as a Function of Temperature, Water Pressure, and Sodium Activity*

Synthesis and Paragenesis of Na-Beidellite as a Function of Temperature, Water Pressure, and Sodium Activity^*

J. Theo Kloprogge^{1, **}, A. M. J. van der Eerden¹, J. Ben H. Jansen^{1, ***}, John W. Geus² and Roelof D. Schuiling¹

¹ Institute of Earth Sciences, Department of Geochemistry, University of Utrecht, Budapestlaan 4, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
² Department of Inorganic Chemistry, University of Utrecht, P.O. Box 80.083, 3508 TB Utrecht, The Netherlands

^* Publication of the Debye Institute, University of Utrecht.
^** Present address: Plastics and Rubber Institute TNO, P.O. Box 108, 3700 AC Zeist, The Netherlands.
^*** Present address: Bowagemi, Prinses Beatrixlaan 20, 3972 AN Driebergen, The Netherlands.

Abstract: In the chemical system Na₂O-Al₂O₃-SiO₂-H₂O, the stability field of Na-beidellite is presented as a function of pressure, temperature, and Na- and Si-activity. Na_0.7-beidellite was hydrothermally synthesized using a stoichiometric gel composition in the temperature range from 275° to 475°C and at pressures from 0.2 to 5 kbar. Below 275°C kaolinite was the only crystalline phase, and above about 500°C paragonite and quartz developed instead of beidellite. An optimum yield of 95% of the Na_0.7- beidellite was obtained at 400°C and 1 kbar after 20 days. Gels with a Na-content equivalent to a layer charge lower than 0.3 per O₂₀(OH)₄ did not produce beidellite. They yielded kaolinite below 325°C and pyrophyllite above 325°C. With gels of a Na-content equivalent to a layer charge of 1.5, the Na-beidellite field shifted to a minimum between temperatures of 275° and 200°C. This procedure offers the potential to synthesize beidellite at low temperatures. Beidellite synthesized from Na_1.0-gel approach a Na_1.35 composition and those from Na_1.5- and Na_2.0-gels a Na_1.8 composition.