Gibbsite montmorillonite

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... 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

The major aluminous clay minerals, alkali zeolites and feldspars which are most commonly associated in nature can be considered as the phases present in a simplified chemical system. Zeolites can be chemiographically aligned between natrolite (Na) and phillipsite (K) at the silica-poor, and mordenite-clinoptilolite at the silica-rich end of the compositional series. Potassium mica (illite), montmorillonite, kaolinite, gibbsite and opal or amorphous silica are the other phases which can be expected in... 

Figure 36. Representation of the zeolite-clay mineral assemblages found in a systeirf at 25°C and atmospheric pressure where Na is an intensive variable (perfectly mobile component) whereas A1 and Si are extensive variables or inert components of the system. G = gibbsite kaol = kaolinite Mo = montmorillonite Si = amorphous silica Anal = analcite.

However, before considering such a complex system of four independent variables, which is represented in planar perspective, let us first take the variables as they can be represented in a sequence of change from inert components which, one by one, become "perfectly mobile" or intensive variables of a thermodynamic system. We will first assume that the phases which will be present in some portion of the system are gibbsite, kaolinite, crystalline or amorphous silica, mica, illite, mixed layered illite-montmorillonite (beidellite), K-feldspar (no pure potassium zeolite is present). Initially we will simplify the mineralogy in the following way ... 

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. 

Figure 46a. Development of a portion of the system K-Al-Si as an increasing number of the chemical components become intensive variables of a given system. F = feldspar Mi = mica G = gibbsite Kaol = kaolinite Q -quartz Mo = montmorillonite ML = mixed layered mineral. All chemical components are extensive variables.

Figure 4. Silicate stability. KF, KM, G, K, and Q are K+-feld-spar, K+-mica, gibbsite, kaolinite, and amorphous silica, respectively. M and AB are montmorillonite and albite. W, S, FW, and SW represent areas of winter lake data, summer lake data, extracted fresh water sediments, and extracted sea water sediments, respectively...

Apparently winter lake waters are stable with respect to gibbsite relative to kaolinite, the micas, montmorillonite, and the feldspars whereas bottom sediments seem to be stable with respect to kaolinite. Kaolinite has been determined in bottom sediments of the Great Lakes as a major phase, but gibbsite has not been found so far. 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

Fit . 3.5. Activity-ratio diagram for smectite (montmorillonite), kaolinite, and gibbsite based on Eq. 3.22. Solubility windows for kaolinite and gibbsite are shown bounded from below by dashed lines. The range of silicic acid activity expected in soils is indicated by the two short vertical lines labeled Si(),(am)" and "quart/."... 

Lowering the water activity stabilizes montmorillonite and destabilizes kaolinite and gibbsite, as can be inferred from Eqs. 3.18-3.20.)... 

The most widespread fill material is reddish brown (2.5 YR 4/4, 5 YR 4/4) loam with a minor admixture of relatively large oolitic bauxite pebbles (derived from the Late Triassic - Camian - beds) and coarse clasts of black chert. Pilot X-ray diffraction analysis revealed mostly muscovite/illite, plus mixed-layer clay minerals of illite/montmorillonite type, chlorite plus mixed-layer clay minerals of chlorite/montmorillonite type, calcium montmorillonite, and diaspore plus gibbsite, or just traces of bauxite minerals (Misic, 2000). The mineral composition is not as uniform as might be expected, and further research, intended for application of factorial analysis, is in progress. A potential sediment source area in the present Cerkniscica River basin (Fig. 1) appears obvious at first glance, but similar outcrops of bauxite and chert do also appear at other sites that are not much more remote. 

Silicate mineral dissolution is usually incongruent, with precipitation of relatively amorphous metastable products that may crystallize with time to form minerals such as gibbsite, kaolinite, illite, and montmorillonite (Helgeson et al. 1984). The incongruency means that the net release rates of individual components from a silicate mineral into the water may not be equal (cf. White and Claassen 1979 Helgeson et al. 1984). 

Figure 3.1. Kinetics of Ni sorption (%) on pyrophyllite, kaolinite, gibbsite, and montmorillonite from a 3 mM Ni solution, an ionic strength I = 0.1 NaNOs, and a pH of 7.5. (From Scheidegger et al., 1997a.)...

XAFS data, showing the formation of Ni-Al LDH phases on soil components, are shown in Figure 3.5 and Table 3.1. Radial structure functions (RSFs), collected from XAFS analyses, for Ni sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite were compared to the spectra of crystalline Ni(OH)2 and takovite. All spectra showed a peak at R 0.18 nm, which represents the first... 

Gilbert, M. and R. van Bladel. 1970. Thermodynamics and thermochemistry of the exchange reaction between NHJ and Mn " in a montmorillonite clay. J. Soil Sci. 21 38-49. Kingston, F. J. 1970. Specific adsorption of anions on goethite and gibbsite. Ph.D. dissertation, University of Western Australia, Perth. 

Montmorillonite is a layered smectite clay. It has a central gibbsitic octahedral layer of alumina sandwiched between two tetrahedral layers of sUica. But usually isomorphous substitution occurs and natural clay has Al substituted by Mg and Fe leading to negative charge on the layers. The interlamellar space between the layers is occupied by hydrated cations usually Na", K Ca" etc. balancing the negative charge on the layCTS. 

---