Charge Heterogeneity in Smectites

Abstract

The heterogeneity of sites for Ca---~K exchange was examined by microcalorimetry in the <0.2-/zm fractions of some selected smectites. Six groups of sites, ranging in exothermic exchange enthalpy (-d(AHx)/dx) from 5.7 to 10.9 kJ/eq were identified. In Wyoming bentonites, only three groups with enthalpies of 5.7 to 7.5 kJ/eq were distinguished, although in the 0.2-1.0-p~m fraction, a 10.7-kJ/eq group was also observed. Redhill and Camp Berteau smectites contained, in addition, groups with enthalpies of 8.7-10.9 kJ/eq, but a New Mexico sample only had groups with the higher values. Thus, the exchange enthalpies of four main groups of sites (reclassified from those observed) appear to be inversely related to the extents of interlayer expansion in the samples by the adsorption of polar molecules. Consequently, a 'true montmorillonite,' such as a <0.2-/~m Wyoming bentonite, contains only fully expanding layers with -d(AHx)/ dx values between 5.7 and 7.5 kJ/eq. As such, it is less heterogeneous and should have a much greater swelling capability than Camp Berteau montmorillonite which has, in addition, groups with exchange enthalpies of 8.7 and 10.3 kJ/eq.

Apparent Charge Heterogeneity in Kaolins in Relation to Their 2:1 Phyllosilicate Content

“…The AGo and AHo values observed for these kaolins are more negative than those for the smectites reported earlier (Talibudeen and Goulding, 1983), especially the AGo values. Also, the AS0 values are more positive for the kaolins than for the smectites, which may signify that the entropic contribution of the mica component of the kaolins (reported for muscovite mica by Goulding and Talibudeen in 1980) is dominant.…”

“…As in previous work (Goulding and Talibudeen, 1980;Talibudeen and Goulding, 1983), these groups can be assigned to vermiculite (13.6 kJ/eq), true mica (10.9 kJ/eq), hydrous micas (9.4 and 8.2 kJ/eq), and montmorillonite (7.2 and 5,1 kJ/eq) on the assumption that kaolinite layers have no permanent negative charge. The amounts of these 2:1 phyllosili- cates, calculated on this basis (Table 4), suggest that, despite the XRD evidence (Table 1), all the kaolins contain small amounts of phyllosilicates.…”

“…The methods for determining changes in the differential heat of exchange, -d(AHx)/dx, with fractional K saturation, x, and for relating sites with constant differential heats to specific 2: I phyllosilicates were described by Goulding and Talibudeen (1980) and Talibudeen and Goulding (1983).…”

Layer Charge and Charge Distribution of Smectites: A Parameter Which Controls Important Physical Properties of Bentonites

“…In the case of smectites characterization of layer charge is of economic and geologic importance, because layer charge strongly affects key smectite properties such as swelling (McEwan and Wilson 1980), cation exchange capacity, and ion exchange selectivity (Maes and Cremers 1977). Smectites often are compositionally and structurally heterogeneous (Stul and Mortier 1974, Lagaly and Weiss 1975, Talibudeen and Goulding 1983, Nadeau et al 1985, Decarreau et al 1987, Iwasaki and Watanabe 1988, Lagaly 1994, Christidis and Eberl 2003. This heterogeneity contributes significantly to layer charge heterogeneity, both in terms of charge location (tetrahedral or beidellitic versus octahedral or montmorillonitic charge) and charge magnitude (i.e., individual smectite 2:1 layers may differ in charge, with the total layer charge for a sample being an average of these different charges).…”

“…Smectite total layer charge and the heterogeneity of this charge from layer to layer can been measured by a variety of methods, including: (1) micro-calorimetry (Talibudeen and Goulding 1983), in which the heat released during determination of an exchange isotherm is related to different types of exchange sites and hence to charge heterogeneity; (2) measurement of the structural formula using chemical or microbeam methods (Weaver and Pollard 1973, Newman and Brown 1987, Christidis 2006, in which the oxide content of a purified smectite sample is measured and then converted into a structural formula; (3) by XRD analysis after saturation with inorganic or organic cations (Tettenhorst and Johns 1966, Cicel and Machajdik 1981, Stul and Mortier 1974, Lagaly 1981, Olis et al 1990) and (4) by computer modeling of XRD traces of K-saturated ethylene glycol solvated smectites (Christidis and Eberl 2003). Microcalorimetry does not yield quantitative estimation of the layer charge.…”