Isothermal Treatments of Regularly Interstratified Montmorillonite-beidellite at Hydrothermal Conditions

-A glass in the intermediate composition of montmorillonite and beidellite, 50/50 in mole ratio, was treated under a hydrothermal pressure of 100 MPa, in the temperature range from 250 to 500~ and durations from 2 to 129 days. The phases identified in the products were plotted in a time-temperaturetransformation (TTT) diagram. The TTT diagram showed that the regularly interstratified montmorillonite-beidellite (r.i.M-B) was a metastable phase above the temperature of 350~ and changed to the assemblage of Na-rectorite + saponite + quartz, through the intermediate assemblage of beidellite + saponite + quartz. The TTT diagram suggested also that the r.i.M-B might be a stable clay mineral below the temperature of 300~ in the middle region of montmorillonite-beidellite pseudo-binary system, although the laboratory confirmation of the mineral stability was not easy for the sluggish reaction. Key Words--Hydrothermal condition, Isothermal treatments, Regularly stratified montmorillonite-beidellite. I N T R O D U C T I O N It is an open question in the crystal-chemistry of clay minerals what the smectite with the intermediate composition between montmoril lonite and beidellite is. It might be a two-phase mixture of montmoril lonite and beidellite, or a phase of complete solid solution between them. Recently another state, the regularly interstratified montmorillonite-beidellite (r.i.M-B) has been found in the pseudo-binary system of rnontmorillonite-beidellite at a hydrothermal pressure of 100 MPa and below the temperature of 375~ (Yamada et aL, 199 lb). Because of the short duration of 7 days in that synthetic experiment, the r.i.M-B and other phases that appeared might be far from their equilibrium. The confirmation of reaction sequence and the stability relations among the phases was thus necessary by a longduration experiment. In the present paper, a kinetic study ofhydrothermal experiments at 100 MPa is performed changing the run duration and temperature. EXPERIMENTAL MATERIALS A N D METHODS The starting material was the same glass used in portions of the previous study (Yamada et al., 199 lb). The chemical composition of the glass was determined to be SiO2:AI203:MgO:Na20 = 66.80:28.16:1.93:3.22 wt. %, using an electron microprobe analyzer (SHIMADZU, EPMA SM-7). The composition was expressed as M48B52 , where M and B denote the composition of ideal anhydrous Na-montmori l loni te and Na-beidellite, respectively. The glass was crushed and size-fractionaled to < 10/zm by hydraulic elutriation. The hydrothermal experiments were performed using a rapid-quench hydrothermal apparatus (Yamada Copyright 9 1993, The Clay Minerals Society et al., 1988). The mixture of glass and water in the ratio of 1:1 by weight was sealed in Au-tube, then treated at a pressure of 100 MPa and at temperatures of 250~176 The experimental durations ranged from 2-129 days for the kinetic study. The run products were examined by powder X-ray diffraction (XRD) after quenching. Three samples were prepared for X R D from a run product by the same method as used in the previous study (Yamada et aL, 1991b): the compression-free (CF) samples, the ethylene-glycolated (EG) samples, and those treated by the so called Greene-Kelly test (GK) (Greene-Kelly, 1953; Lim and Jackson, 1986). The CF sample was prepared by crushing the quenched product and mounting on a quartz plate of an off-Bragg cut without compression to avoid the influence of preferred orientation. The EG sample was prepared as follows: The product oriented by water on a slide glass was placed in a sealed container with ethylene glycol, then heated overnight at 60~ The GK sample was obtained in the following manner: A product was ion-exchanged by washing three times with aqueous 1 M LiCI and then three times with 80% ethanol in a test tube. An Liexchanged sample was oriented on a silica glass slide, heated overnight at 300~ put in a sealed glass container with glycerol, then heated overnight at 90~ Some products were examined by an in situ X R D method under various relative humidity conditions in the range of 0-100% (Watanabe and Sato, 1988).