Iron Oxidation and Reduction Effects on Structural Hydroxyl and Layer Charge in Aqueous Suspensions of Micaceous Vermiculites

Abstract

Abstract--Four Na2S20,-reduced Na-vermiculites, each with some trioctahedral mica interstratified, were oxidized with H20 2 at pH 6.5 and again reduced with Na2S20 * in suspensions at pH 7.5-8.0. The layer charge (CEC + K +), measured at pH 6.50, did not change significantly when octahedral Fe was oxidized (7-92 mmole I00 g-1) or reduced (6-71 mmole 100 g-1). Electroneutrality was maintained within the octahedral sheet when Fe was oxidized or reduced. When Fe(II) was oxidized, electroneutrality was maintained by deprotonation of octahedral OH-groups,and by ejection of (dissolution of structural) octabedral metallic cations, The weathering increment was small since, of the total amount of Fe + Mg, less than 1"3 per cent was ejected from any of the four vermiculitic materials. When biotite was K-depleted, about 20 m-equiv of layer charge per 100g (300~ basis) was lost, while 51 mmole of Fe(II) per 100g was oxidized in the presence of Na2SzO 4 and 82 mmoles in its absence in the aqueous suspensions. Since sequential reduction-oxidation-reduction treatments of K-depleted biotite and mica-containing vermiculites did not cause signifcant changes in layer charge (r 2 = 0"04), the layer charge changes were concluded to be entirely independent of the oxidation or reduction of Fe in these minerals.

Mechanisms and Crystal Chemistry of Oxidation in Annite: Resolving the Hydrogen-Loss and Vacancy Reactions

“…It is also believed to occur in the laboratory oxidation or re-equilibration of other Fee+-bearing and OH-bearing minerals (e.g. Borggaard et al, 1982;Clowe et al, 1988;Lear and Stucki, 1985;Veith and Jackson, 1974) and to represent a main mechanism of H incorporation in nominally anhydrous minerals such as pyroxenes (Skogby and Rossman, 1989;Skogby, 1994). On the other hand, in several studies of natural and laboratory weathering of biotite, the vacancy mechanism is seen to dominate (e.g.…”

Mössbauer Effect Studies of Iron in Kaolin. I. Structural Iron

“…It has been suggested (e.g., Veith and Jackson, 1974;Jefferson et al, 1975) that such oxide coatings arise when structural Fe 2+ oxidizes to Fe 3+ and migrates to the clay surface during weathering, forming a noncrystalline gel coating. Such gel coatings are usually only observed on clays having little or no structural Fe 2+.…”

Oxidation and Reduction of Structural Iron in Chlorite at 480°C

“…The two processes may be separated by thermal oxidation of the iron succeeded by acid dissolution of the hydroxide interlayer (Ross and Kodama, 1974;Goodman and Bain, 1979). During dry thermal oxidation, the layer charge remains constant by the release of one hydrogen atom per oxidized Fe(II), as suggested in the oxidation of biotite (Vedder and Wilkins, 1969;Farmer et al, 1971;Veith and Jackson, 1974;Hogg and Meads, 1975;Tripathi et al, 1978;Bagin et al, 1980), chlorite (Makumbi and Herbillon, 1972;Goodman and Bain, 1979), and smectite Heller-Kallai, 1976a, 1976b). For chlorite, the mechanism of this hydrogen release is obscure, as is the reversibility of the oxidation step.…”