Montmorillonite/Illite Stability Diagrams

Abstract

Abstract-Chemical activity diagraoas , prepared to illustrate the properties expected if mixed-layer montmorillonite/illite is regarded as a solid solution, are compared to those derived from a treatment of these materials as a mixture of two phases. If the system is a solid solution, the coexisting aqueous solution should range from higher dissolved silica contents in the presence of kaolinite and a montmorillonite end member to lower dissolved silica in the presence of kaolinite and an illitic end member. Silica concentration in the aqueous solution might vary by a factor of as much as six. If the system is two phase, the silica content of a solution in equilibrium with kaolinite and both phases would be fixed at a given T and P, as would a solution equilibrated with both phases and K-feldspar. In the absence of a third phase, silica in equilibrium with both phases should be nearly constant, but increase with increasing ratio of K+/H + in solution. Available data on coexisting aqueous solutions apparently are more nearly consistent with two phases than with a solid solution.

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

“…The boundaries between kaolinite and beidellite defined by Reaction 4 and between bayerite and beidellite (Reaction 7) become less steep upon increasing sodium content of the beidellite. For low #Na20 and #sJo2 values this diagram is in good agreement with the /z(K,Na)-#(Si) diagram reported by Velde (1985) and the ion activity diagrams reported by Garrels (1984), although they report gibbsite (3,-AI[OH]3) instead of bayerite (a-Al[OH]3). This difference may be explained by the relatively high pH of approximately 8-9 in our experiments, favoring the formation of bayerite rather than gibbsite.…”

Compositional End Members and Thermodynamic Components of Illite and Dioctahedral Aluminous Smectite Solid Solutions

“…Structural considerations (Newman and Brown, 1987) and thermodynamic analysis of aqueous solution compositions coexisting with smectites and illites in natural systems (Garrels, 1984) suggest that the number of cations occupying the octahedral sites of illites and smectites ranges from ~ 1.9-2.1 [per OIo(OH)z]. The site occupancies in Tables 1 and 2 support this conclusion.…”

“…It has been shown by Garrels (1984) that illites in equilibrium with natural waters can be described in terms of an end member with an octahedral occupancy of 1.9 (AL-ILL) and a generalized chemical formula corresponding to For example, the end-member formula for chlorite in pelitic rocks saturated with At and Si corresponds to Fe454A12~2Si2540)o(OH)~ (Holdaway, 1980). Table 4 (see text and caption of Figure 7).…”

“…As in the case of illite, the smectite end member adopted in the present study to represent Ooc = 1.9 (AL-SMEC) corresponds to the smectite end member proposed by Garrels (1984). This end member was selected because it is consistent with water compositions reported to be in equilibrium with smectites in natural systems (Aagaard and Helgeson, 1983;Garrels, 1984). The generalized chemical formula of the end member is A0.3All.gSi40~0(OH)2.…”

Some Comments on the Chemistry and Stability of Interstratified Illite-Smectite and the Role of Ostwald-Type Processes

“…Notwithstanding the attempts of numerous authors to develop a singular, self-consistent mineralogical description of the species (compare the 'MacEwan crystallite' approach of Altaner et al, 1988 with the fundamental particle model devised by Nadeau et al, 1985a), considerable effort has also been made to define the stability of I-S in terms of equilibrium thermodynamics (Aagaard & Helgeson, 1983;Garrels, 1984;Sass etal., 1987;Aja etal., 1991). In summarizing the work of Sass etal.…”