Evidence for loss of protons and octahedral iron from oxidized biotites and vermiculites

Abstract

SUMMARY. Infra-red examination of a weathered biotite and of biotites that have been converted to vermiculites and subsequenOy oxidized, indicates that oxidation of octahedral ferrous ions to ferric ions is associated with a reversible conversion of hydroxyl ions to oxide ions. Subsequently, in highiron biotites, there is an irreversible loss of ferric ions from the octahedral layer, resulting in an increased number of dioctahedral sites. Electron microscopy and X-ray diffraction indicate that ejected ferric ions form either amorphous interlayer oxides or, when bromine is used as an oxidant, a crystalline external phase of ~-FeOOH. The high refractive index of some oxidized vermiculites is shown to be due largely to submicroscopic iron oxides.

Effect of Oxidation and Reduction on Potassium Exchange of Biotite

“…These increases after Na2S204 reduction are similar to those reported by Farmer et al (1971) and may be significant. There was also a significant decrease in layer charge of the phlogopite (23 m-equiv/ 100 g) during K-depletion.…”

“…This was followed by a loss of Fe a + ions from the octahedral sheet resulting in an increased number of octahedral vacancies. Both these mechanisms reduce the screening effect of hydroxyl H on interlayer K, which results in a stronger bonding of K. Unlike the irreversible loss of octahedral Fe, the loss of protons'may be reversible upon reduction (Farmer et al, 1971).…”

“…This has been attributed to an increased inclination of OH dipoles away from the normal to the basal plane which results in a more electronegative environment of K and consequently in a stronger bonding of K (Barshad and Kishk, 1968;Robert and Pedro, 1968;Juo and White, 1969;Farmer et al, 1971;Gilkes et al, 1973). Farmer et al (t971) showed that chemical oxidation of Fe-rich vermiculites and vermiculitized biotites was associated with a conversion of OH to O ions.…”

The Alteration Products of Potassium Depleted Oxybiotite

“…Rectorite (Brown and Weir, 1963) also has a regularly alternating structure of paragonite--like sheets containing no structural iron. Thus oxidation of biotite must induce some structural change such as altered hydroxyl orientation, distorted or contracted tetrahedral sheets (Gilkes et al, 1972) or loss of hydroxyl (Farmer et al, 1971) which then leads to differences in exchangeability of K from between adjacent sheets. …”

Iron-Rich Saponite (Ferrous and Ferric Forms)

“…Several concepts have been proposed regarding the oxidation of ferrous iron in hydrous silicate minerals, as follows: (i) reaction between ferrous iron, hydroxyl ion and additional oxygen, as shown in ferrous chamosite by Bfindley and Youell (1953) and in oxidized amphiboles by Addison et al (1962a, b); (ii) reaction between ferrous iron and hydroxyl ion without additional oxygen, as shown in biotite by Rimsaite (1967Rimsaite ( , 1970; (iii) reaction taking place by loss of interlayer cations; and (iv) reaction associated with a reversible conversion of hydroxyl to oxygen ions and subsequent irreversible loss from octahedral sites of ferric ions as iron oxide, as shown in oxidized biotite and vermiculite by Farmer et al (1971).…”