Montmorillonite phase diagrams

The phase diagrams developed from available chemical data on natural dioctahedral minerals permits the distinction of different parageneses where the chemical components are inert, i.e., where their relative masses determine the phases produced. Other systems are possible which involve dioctahedral montmorillonites. These, or some of these, will be discussed later in conjunction with other clay minerals. 

Figure 41. Phase diagram for the extensive variables R -R -Si combining the data for synthetic magnesian chlorites and the compositional series of natural sepiolites and palygorskites. Numbers represent the major three-phase assemblages related to sepiolite-palygorskite occurrence in sediments. Chi = chlorite M03 = trioctahedral montmorillonites M02 = dioctahedral montmorillonite Sep = sepiolite Pa = palygorskite Kaol = kaolinite T = talc.

G = gibbsite, Kaol = kaolinite, Q = amorphous or crystalline SiO, Mi = potassic mica, Mo = K-beidellite, ML = mixed layered mica-montmorillonite minerals, F = potassium feldspar, Py = pyrophyllite. This is necessary to simplify portions of the diagrams where our knowledge of phase relations is not sufficient to judge the roles which each individual mineral will play. 

In order to construct the activity diagrams in a rigorous fashion, a certain amount of information must be available. Some experimental data for the mica-feldspar-kaolinite-gibbsite-montmorillonite relations are available. Data for the other minerals are often inferred from measurements of natural chemical parameters (K+, SiC, H+ concentrations in solutions) in situations where the different minerals are assumed to be stable. The relations between minerals can also be calculated as a function of K+, SiO and pH using thermochemical data for the participating phase (Hess, 1966) when they are known with precision. Frequently it is... 

Figure 50. "Facies diagram" for phyllosilicates in pelitic rocks and sediments. Zones I to VI are discussed in the text. M02 = dioctahedral montmorillonites M03 = trioctahedral, fully expandable phases ML =...

If we refer to the MR -2R -3R diagram, the pole MR (feldspar) would become sodic as temperature increases while the mixed layered phase becomes potassic. If we consider calcium in this system, it will not form a feldspar as does sodium and must enter into solution or be precipitated as carbonate when montmorillonite layers decrease in the mixed layered phase. In either event the net effect is to reduce the potassium part... 

Figure 9.11 Log([K+]/[H" ]) versus log[H4Si04] diagram at 25°C with plotted chemical analyses of waters in contact with clays, as reported by Aagaard and Helgeson (1983). Phase boundaries are drawn consistent with the analyses and with the illite and montmorillonite compositions proposed in the text. Agreement of the data and boundaries suggest equilibrium between the phases and support the idea that illite and montmorillonite behave as two discrete phases.AfterR.M.Garrels in C/oystfe Clay Minerals, 32 161-66, Copyright 1984.

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