Clays and Clay Minerals, Vol. 38, No. 4, 415-425, 1990. HYDROTHERMAL REACTIVITY OF MIXED-LAYER KAOLINITE/SMECTITE: EXPERIMENTAL TRANSFORMATION OF HIGH-CHARGE TO LOW-CHARGE SMECTITE D. PROUST 1, J. LECHELLE, 2 A. LAJUDIE, 2 AND A. MEUNIER 1 Laboratoire de P6trologie des Alter6rations Hydrotherrnales Universit6 de Poitiers, 86022 Poitiers Cedex, France 2 Commissariat ~ l'Energie Atomique, DRDD/SESD 92260 Fontenay-aux-Roses, France Abstract--A mixed-layer kaolinite/smectite (K/S) containing trace amounts of quartz, discrete kaolinite, goethite-hematite, and calcite was hydrothermally reacted with deionized water at 150", 200", and 250"C for 1 to 12 months. The starting K/S contained 50% smectite consisting of 15~ low-charge and 350 high-charge layers. The X-ray powder diffraction and chemical analyses of the reacted products indicated a progressive reaction from high-charge to low-charge smectite as a function of time and temperature. The reaction reached completion after 4 months at 250"C, at which point high-charge smectite layers entirely reacted to low-charge smectite layers, the latter maintaininga constant proportion of about 90% for longer run durations. For long reaction times, discrete kaolinite totally reacted, whereas quartz showed only partial dissolution and iron oxides remained stable. Thus, the reaction of high-charge to low-charge smectite layers may be expressed as: high-charge smectite + kaolinite (both interstratified and discrete component) + quartz ~ low-charge smectite. Key Words--Cation-exchange capacity, Hydrothermal transformation, Kaolinite/smectite, Layer charge, Smectite, X-ray powder diffraction. INTRODUCTION Many workers have observed that dioctahedral smectites are not stable at temperatures >75"--100"(2 under diagenetic conditions. The most detailed studies ofsmectite reactivity as a function of temperature have focused on the smectite-to-illite conversion in potas- sium-rich environments via intermediate mixed-layer illite/smectite (I/S). Studies of natural, low-tempera- ture geological environments (see, e.g., Hower et al., 1976; Inoue et al., 1978; Velde and Brusewitz, 1982; Inoue and Utada, 1983; Ramseyer and Boles, 1986) as well as laboratory experiments (Eberl and Hower, 1976; Eberl, 1978; Roberson and Lahann, 1981; How- ard and Roy, 1985), suggest that smectite iUitization operates through tetrahedral Al-for-Si and oetahedral Mg-for-A1 ionic substitutions within the 2:1 layer. These ionic substitutions create an increasing negative layer charge with the formation of high-charge smectite and K-fixation in the interlayer sites (Howard, 1981; How- ard and Roy, 1985). Illitization reactions have been formally expressed as: (1) Smectite + A13+ + K § -~ illite + Si4+, if solid- phase transformation is assumed (Hower et al., 1976); and (2) Smectite + K § ~ illite + Si 4+, if smectite dis- solution is assumed to supply A1 for illite crystallization (Boles and Franks, 1979). These two reactions lead to the release of Si to so- lution. A recent study of I/S stability in a natural, low- Copyright 9 1990, The Clay Minerals Society temperature hydrothermal environment (Bouchet et al., 1988) suggested that the high-charge smectite com- ponent of the I/S reacted to form: (1) illite, if external K was supplied (i.e., high-charge smectite + K + -~ illite) or (2) low-charge smectite, if external Si was supplied (i.e., high-charge smectite + Si4+ -~ low-charge smec- tite). The second reaction induces a marked increase in the cation-exchange capacity (CEC) of the clay ma- terial. The main purpose of this paper was therefore to study further the reaction of high-charge to low- charge smectite under experimental alteration condi- tions. To avoid uncontrolled smectite illitization, the following constraints were imposed on the experi- ments: (1) the starting expandable clay material was free of illite, either as a discrete clay mineral or as a component of a mixed-layer clay mineral, and (2) al- teration solutions were potassium-free. Hydrothermal runs were therefore conducted using a mixed-layer kaolinite/smectite (K/S) as the starting clay material, run durations of 1 to 12 months, tem- peratures of 150*, 200 ~ and 250~ and deionized water as the alteration solution. MATERIAL AND EXPERIMENTAL METHODS Material The starting clay for the hydrothermal runs was ob- tained from the Sparnacian Argiles Plastiques For- mation in the northwestern part of the Paris Basin (Vexin area). These clays occur in highly weathered 415