Abstract--Vernad{te (MnO2" nH~O) is a mineral with a poorly ordered structure. Its synthetic analogue is designated 6-MnO2. Birnessite and vernadite are independent mineral species and cannot be described further under the same name. They have similar hexagonal unit-cell parameters, a 0 ~ but different co parameters. Rancieite has a structure similar to that of birnessite. Calcium beanng, 14-A birnessite occurring in nature was first described by the authors. In addition to the todorokite having the parameters a,, = 9.75 A, b0 = 2.84 ,~, and c o = 9.59 ]k, other species of natural todorokite are known having aa parameters that are multiples of 4.88/~ equal to 14.6 and 24.40/~, the b0 and ca parameters being the same.

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The availability of Fe(III)‐bearing minerals for dissimilatory Fe(III) reduction was evaluated in sediments from a petroleum‐contaminated sandy aquifer near Bemidji, Minnesota (USA). First, the sediments from a contaminated area of the aquifer, in which Fe(III) reduction was the predominant terminal electron accepting process, were compared with sediments from a nearby, uncontaminated site. Data from 0.5 m HCl extraction of different size fractions of the sediments revealed that the clay size fraction contributed a significant portion of the ‘bio‐available’ Fe(III) in the background sediment and was the most depleted in ‘bio‐available’ Fe(III) in the iron‐reducing sediment. Analytical transmission electron microscopy (TEM) revealed the disappearance of thermodynamically unstable Fe(III) and Mn(IV) hydroxides (ferrihydrite and Fe vernadite), as well as a decrease in the abundance of goethite and lepidocrocite in the clay size fraction from the contaminated sediment. TEM observations and X‐ray diffraction examination did not provide strong evidence of Fe(III)‐reduction‐related changes within another potential source of ‘bio‐available’ Fe(III) in the clay size fraction – ferruginous phyllosilicates. However, further testing in the laboratory with sediments from the methanogenic portion of the aquifer that were depleted in microbially reducible Fe(III) revealed the potential for microbial reduction of Fe(III) associated with phyllosilicates. Addition of a clay size fraction from the uncontaminated sediment, as well as Fe(III)‐coated kaolin and ferruginous nontronite SWa‐1, as sources of poorly crystalline Fe(III) hydroxides and structural iron of phyllosilicates respectively, lowered steady‐state hydrogen concentrations consistent with a stimulation of Fe(III) reduction in laboratory incubations of methanogenic sediments. There was no change in hydrogen concentration when non‐ferruginous clays or no minerals were added. This demonstrated that Fe(III)‐bearing clay size minerals were essential for microbial Fe(III) reduction and suggested that both potential sources of ‘bio‐available’ Fe(III) in the clay size fraction, poorly crystalline Fe(III) hydroxides and structural Fe(III) of phyllosilicates, were important sources of electron acceptor for indigenous iron‐reducing microorganisms in this aquifer.

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