Hydrothermal reactivity of smectite

Hydrothermal experiments were conducted to explore the effects of changing chemistry, temperature, and pressure on the reaction of smectite, run I : I with pure water, to mixed-layer clay. At high temperatures trioctahedral smectites are more stable than dioctahedral smectites' For dioctahedral smectites, hydrothermal stabil ity is enhanced by saturation with interlayer cations of greater hydration energy than potassium, and by increased water pressute. Introduction Many of the unusual physical and chemical properties of the smectite group of clay minerals are related to the expandable interlayer region. These properties, which include swell ing, cation exchange capacity, catalytic activity, and thixotropy, are sometimes diminished or lost during hydrothermal treatment as the smectite reacts to form mixed-layer clay. It is of interest, therefore, from both a geologic and an industrial point of view, to determine the conditions under which smectites are least l ikely to react. Our experiments explore the effects of chemical composition, temperature, and pressure on the hydrothermal alteration of smectite to mixed-layer clay. Experimental techniques A direct comparison between the reactivity of dioctahedral and trioctahedral smectites was made by saturating each type of clay with various interlayer cations and then subjecting them to identical hydrothermal conditions. Trioctahedral starting materials included a natural saponite from Karolihof, Switzerland (<2 micron size fraction) saturated with K, Na, Ca, or Mg; u natural saponite from the Amargosa Valley, Nevada ((2 micron fraction); and gels of potassium and magnesium saponi te composi t ion, Mg.Sir. rAlo3aOr0(OH)rxilrr, where X+ is the exchangeable cation. Dioctahedral starting materials were the Wyoming bentonite ((2 micron fraction, Mol l e l a l . , 1975) saturated wi th K, Na, Ca, or Mg, and a gel of potassium montmor i l lon i te composi t ion, Alrsia 6? 4.10 $Oro(OH)rKo.re. Gels were prepared by the method of Hami l ton and Henderson (1968). oo03-004x/78/0304-0401$02 00 401 Hydrothermal runs were prepared by introducing 30 mg of clay or gel and 30 pl of pure water into gold tubes (20 mm long, 2.5 mm I.D.) which were then welded shut. These charges were heated in large hotseal autoclave reaction vessels or in small cold-seal reaction vessels. The former generated maximum pressures of approximately 0.3 kbar, and the latter were maintained at 0.5 or 2 kbar by equil ibration with a large reservoir (see Eberl and Hower, 1976). All vessels were heated in resistance furnaces, and temperatures were controlled by on-off regulators attached to thermocouples located in wells near or in the base of the vessels. Run products were oriented on glass slides, glycolated, and X-rayed with a Norelco diffractometer, using Ni-fi l tered CuKa radiation. The expandabil ity of mixed-layer i l l i te/smectite was determined by comparison with the calculated patterns of Reynolds and Hower (1970). Experimental results The difference in reactivity between dioctahedral and trioctahedral smectite is clearly shown in Tables I and 2 and in F igures 1,2, and 3. At 400'C and autoclave pressure, the K-montmoril lonite gel (run I ) and the natural montmoril lonites saturated with K, Na, Ca, and Mg (runs 2-5) reacted extensively to lbrm regularly interstratif ied mixed-layer clay. The sraponites run under equivalent conditions did not react (runs l0-18). The effect of interlayer chemistry on the reaction of dioctahedral clays is also shown in Table l. Montmoril lonite with interlayer potassium (run 6) reacted at a lower temDerature than did mont402 EBERL ET AL.' REACTIVITY OF SMECTITE Table I Montmor i l loni te autoclave runs Run n o . Start ing mater i . a l Ternp. ( oc ) Time (days ) Run produc ts