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

Abstract

Abstract--Consideration of XRD, TEM, AEM, and analytical data reported in the literature indicates that dioctahedral aluminous smectite and illite form two separate solid solutions that differ chemically from one another primarily by the extent of A1 substitution for Si, the amount of interlayer K, and the presence of interlayer H20. The data indicate that limited dioetahedral-trioctahedral and dioctahedralvacancy compositional variations occur in both minerals. Excluding interlayer H20 and based on a half unit cell [i.e., Oto(OH)2], natural dioctahedral smectite and illite solid solutions fall within the compositional limits represented by Ao.3R~*Si40,o(OH)2-AR2+ R3+ Si40,o(OH)2-Ao2sR~3 R3178Alo.25Si3.7,Ojo(OH)2 for smectites and A~.sR3)`~d~5Sis.~'~(~H)2-A~5R~+5R3.~s~.'Si~9~'~(~H)2-~.9R2+R~9Si3.'~(~H)2 for illites, where A represents either monovalent cations or divalent cations expressed as their monovalent equivalent (e.g., Ca 2 § R-" + stands for the divalent cations Mg 2 § and Fe 2 § and R 3 + refers to the trivalent cations AP § and Fe -3 § Taking account of these compositional limits, smectite and illite solid solutions can be described in terms of nine and six thermodynamic components, respectively, all of which are consistent with both the law of definite proportions and the concept of a unit cell. Thermodynamic components that can be used to describe natural smectite solid solutions in terms of a half unit cell [i.e., Ojo(OH)2] can be expressed as NaA13Si30~o(OH)2, NaA13Si30~o(OH)2.4.5H20, A12Si40~o(OH)2 ,Cao.~A13Si3Oto(OH)2. Of these, NaAI3Si30~o(OH)2 " 4.5H20 provides explicitly for the presence of interlayer H20 in the mineral. Thermodynamic components representing illite solid solutions in natural systems can be written for a half unit cell as KA13Si30,o(OH)2, KMg3AISi30,o(OH),, KFe3AISi3OIo(OH)2 , AIzSi40~o(OH)~, KFe2AISi~O~o(OH)_,, and K3AISi40~o(OH)2. The calculations and observations summarized below indicate that neither smectite nor illite occur in nature as stoichiometric phases and that the two minerals do not form a mutual solid solution corresponding to mixed-layered illite/smectite.

Mineralogical and Rb-Sr Isotope Studies of Low-Temperature Diagenesis of Lower Cambrian Clays of the Baltic Paleobasin of north Estonia

“…The composition of these laths is similar to the high charge Fe, Mg-rich illite (highly illitic I-S) typically found in diagenetic environments (Meunier and Velde, 1989;Lanson and Champion, 1991;Srodofi et al, 1992;Freed and Peacor, 1992;Ransom and Helgeson, 1993;Hover et al, 1996). The Fe content in illite and illite-rich I-S can be overestimated owing to traces of Fe-rich chlorite in the same fraction, but since the amount is nearly the same for all samples, no attempt was made to correct for possible Fe-rich chlorite impurities.…”

Clay Mineral Thermometry—A Critical Perspective

“…It is unclear whether solubility measurements can be shown to have attained equilibrium, as clay minerals are by definition comprised of submicrometer crystallites with complex physical and chemical characters (May et al 1986). Summation techniques have also been used to estimate the thermodynamic properties of complex phases from those of simple end-members (e.g., Tardy and Garrels 1974, Nriagu 1975, Mattigod and Sposito 1978, Stoessell 1979, Tardy and Fritz 1981, Merino and Ransom 1982, Aagaard and Helgeson 1983, Sposito 1986, Chermak and Rimstidt 1989, Tardy and Duplay 1992, Ransom and Helgeson 1993. Summation approaches yield approximately correct results for solids because of the plague of small free energy differences (Fyfe et al 1958), but it is those same small differences that make all the difference in terms of reaction direction and whether a given phase is stable or metastable.…”

FY11 Development of Fully Coupled Repository THCM Simulation Tools Report: Thermodynamic Database Development, with Emphasis on Complex Clay Minerals

“…In a fully hydrated smectite, the interlayer is thought to contain two layers of water molecules, or 4.5 moles H 2 O per "O 10 (OH) 2 " in the common molar formula of smectite, where the "(OH) 2 " is considered as containing structural water (cf. Ransom and Helgeson, 1993). Cations in the interlayer are easily exchanged with aqueous solution.…”

“…Therefore, results from there procedure for say an idealized Na-beidellite of formula Na 0.33 Al 2 Al 0.33 Si 3.67 O 10 (OH) 2 would imply the interlayer water through the usage of the exchangeable oxide Na 2 O ex .However, this sort of treatment is not suficient if the loss or gain of interlayer water is sufficient to affect the local mass balance. Ransom and Helgeson (1993) suggest that a fully hydrated smectite would have about 4.5 H 2 O of interlayer water per O 10 (OH) 2 in the chemical formula (corresponding to ½ unit cell), equivalent to about 2 water layers in the interlayer.…”

“…The goal here would be do develop data for end-member compositions such as Na 0.33 Al 2 Al 0.33 Si 3.67 O 10 (OH) 2 .n H 2 O, where n is likely to have a maximum value between 4.5 and 7. Ransom and Helgeson (1993, 1994ab, 1995 develop an approach to dealing with variable hydration. It has some excellent features.…”