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Factors affecting the reliability of results using 1,10‐phenanthroline (phen) in the quantitative assay of minerals for Fe2+ and total Fe were studied. The greatest source of variability was the photochemical reduction of ferric‐phen species during Fe2+ analyses. It is believed that oxo‐bridged dinuclear complexes of Fe2+ and phen are formed in the mineral digests, which then are susceptible to photochemical reduction by wavelengths < 400–500 nm. The rate of photoreduction of a solution obtained by digesting oxidized nontronite was sufficient to increase the predicted Fe2+ content of the clay from about 0 to 2.45% by simply leaving the sample for 1 hour in normal fluorescent light in the laboratory; and for partially reduced nontronite, from 3.25 to 4.68%. The rates of increase for the two clays were 0.0024 and 0.0014 absorbance units/min, respectively. Interferences due to this phenomenon were eliminated when samples were kept in the dark or under subdued red light. Other factors that adversely influence the accuracy and precision of the standard curve included (i) the amount of 48% HF used to digest the sample; (ii) addition of chemical reducing agents such as hydroxylamine hydrochloride and hydroquinone; (iii) the order in which phen is added relative to other reagents; (iv) pH; and (v) the length of time allowed for color development if phen is added only to the final dilution.

Chemical and physical properties of the clay mineral nontronite were studied as the oxidation state of structural iron was altered. Magnesium exchange capacities (CEC) were determined for different nontronite samples that were unaltered, dithionite‐reduced, and reduced‐reoxidized, and were compared to predicted values calculated from the molecular formula. Observed CEC increased from 125 meq/100 g as predicted with increasing Fe2+ content until the Fe2+ content reached about 53 mmoles/100 g then the CEC remained constant at 142 meq/100 g even though the Fe2+ content was increased further to 139 mmoles/100 g. A two‐step mechanism is proposed that involves first, an initial reduction of Fe3+ to a Fe2+ content of 53 mmoles/100 g with an accompanying increase in layer charge and no structural changes; and second, further reduction to a Fe2+ content of 139 mmoles/100 g during which a constant layer charge is maintained by elimination of structural OH and the coordination number of iron in the octahedral sheet is decreased. The mechanism explains changes in oxidation state and electronic environments monitored by Electron Spectroscopy for Chemical Analysis (ESCA) and Mossbauer spectroscopy. The Fe3+(2p3/2) electron binding energy for oxidized nontronite was observed at 711.8 eV, the Mossbauer isomer shift (I.S.) at +0.44 mm/sec and quadrupole splitting (Q.S.) was zero. Hydrazine‐reduced nontronite showed a binding energy at 711.8 eV for Fe3+ and 708.6 eV for Fe2+. Dithionite‐reduced nontronite shifted the binding energy for Fe3+ to 711.0 eV, and Fe2+ remained at 708.6 eV. Mossbauer I.S. moved down to +0.10 mm/sec for Fe3+ and the Q.S. increased to 1.11 mm/sec, and for Fe2+ the I.S. was centered at 1.17 mm/sec with Q.S. near 2.77 mm/sec. The proposed mechanism also accounts for shifts in infrared vibrational energies for the O‐H stretching mode, the FeO‐H deformation, and the Fe‐OH octahedral vibrational modes.

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Abstract--Structural Ire in ferruginous smectite (sample SWa-1, Source Clays Repository of the Clay Minerals Society) was reduced by a mixture of five Pseudomonas species of bacteria in a defined Fe-free medium to determine the effect of microbial reduction on clay swelling. Iron(II), total Fe, and gravimetric water content (row/too) were determined in clay gels equilibrated at applied pressures of 0.1, 0.3, and 0.5 MPa. The water content of microbially reduced SWa-1 decreased at all three applied pressures as the Fe(II) content approached about 0.8 mmol Fe(II)/g-clay. As Fe(II) increased from 0.8 mmol/g-clay, however, further change in mJmc was negligible. Concurrent with microbial reduction of structural Fe was a significant decrease in the swelling pressure (PI) of SWa-1: for example, when mw/mc = 1.2 (g/g), PI changed from 0.47 MPa at Fe(II) = 0.2, to 0.19 MPa at Fe(II) = 0.9 mmol/g-clay. Both biologically and chemically reduced smectites displayed lower values of mJmc and a concurrent decrease in II as Fe(II) content increased, but the effect of Fe(II) on mJmc was greater for the microbially reduced smectites at all applied pressures.

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