Abstract

Traditionally, researchers have considered that only Al 3 § substitutes in the SiO4 tetrahedral layer of 2:1 phyllosilicates, particularly, in smectites. The tradi-tional approach of assigning all Si present in a chemical analysis to the tetrahedral layer and then making up the atomic unit cell deficit with Al 3+ was well estab-lished by Ross and Hendricks (1945). They were unable to distinguish tetrahedral Fe 3+ (lVFe3+), however, and therefore the possible tetrahedral substitution of Fe 3§ was never taken into consideration. If a calculation of the chemical formula of a particular nontronite showed so much Fe as to suggest WFe3+, they appear to have assumed that the additional Fe was present as an im-purity. Osthaus (1954) used acid dissolution rates to demonstrate that this simplistic approach was not war-ranted, and that Fe 3 § was indeed present in Garfield nontronite inboth the tetrahedral nd octahedral sheets. Several nontronite analyses were presented by Good-man et al. (1976) and Cardile and Johnston (1985) which indicated on a chemical formulae basis such a low AP + content and such a high Fe 3+ content hat WFe3+ substitution must exist. If ~VFe3+ is indeed present in a given smectite, its presence should be accounted for when presenting structural formulae. This is clearly not being done. Furthermore, disagreement still exists in the literature concerning the fitting ofIVFe 3 + resonances tothe M6ss-bauer spectra of nontronites. This is exemplified by Lear and Stucki (1987) who disputed the JVFe3+ as-signment of Goodman et al. (1976) for standard SWa- 1 nontronite from Grant County, Washington, as sup-