Oxidation of biotite

Another example involves planar defects found in biotite, which were probably formed by oxidation of biotite at high temperatures (Kogure and Nespolo 2000). The specimen used is oxybiotite from the Ruiz Peak ash flow, which was described above. Two kinds of planar defects were found, although they were not abundant in the specimen. The numbers of each defect observed are similar. One of the two defects is... 

Figure 5 (right). Stability limits with respect to oxidation, of biotites (solid lines) and olivines (dashed lines) at 2070 bars (after Wones and Eugster, 1965). Contours are in mol per cent annite and fayalite, respectively. Note the oblique intersection of the contours. 

The autoradiographs of the rock and mineral thin sections (Figures 4 to 6) also confirm the importance of iron oxides although biotite.(K(Mg,Fe Si AlCLQ(0H) ) and hornblende ((Na,Ca2)(Mg,Fe )(Al,Fe )(Si AlO OH ) contain ferrous iron, sorption appears to take place solely on the small opaque (iron-oxide) inclusions. In the case of biotite, these oxides are located between the basal planes, and are randomly distributed in the hornblende. Similar distributions are observed for olivine, pyroxene, and epidote. The results for pyroxene further confirm the low sorption results obtained with gabbro, where it is one of the major minerals. 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

Eggleton RA, Banfreld JF (1985) The alteration of granitic biotite to chlorite. Am Mineral 70 902-910 Ferrow E (1987) Mossbauer and X-ray studies on the oxidation of annite and ferriannite. Phys Chem Minerals 14 270-275... 

Several studies looked at the thermal oxidation and dehydroxylation of biotite, including Rice and Williams (1969), Bagin et al. (1980), Vicente-Hemandez et al. 

Ivanitskiy VP, Kahnechenko AM, Matyash IV, Khomyak TP (1975a) Mossbauer and PMR studies of oxidation and dehydroxylation in biotite. Geochem Int l 12 145-151 Ivanitskiy VP, Matyash IV, Rakovich FI (1975b) Effects of irradiation on the Mossbauer spectra of biotites. Geochem Int l 12 151-157... 

Mathew J, Charma AL (1991) Thermal oxidation of natural biotite using Mdssbauer technique. Indian J Pure Appl Phys 29 147-149... 

Gurman SJ (1989) Stmctural information in extended X-ray absorption fine stmcture (EXAFS)./n SS Hasnain (ed) Synchrotron Radiation and Biophysics. Ellis Horwood, Chichester, UK, p 9-42 Guttler B, Niemaim W, Redfem SAT (1989) EXAFS and XANES spectroscopy study of the oxidation and deprotonation of biotite. Mineral Mag 53 591-602... 

Reduction of Fe(III) occurs on the surfaces of basalt and Fe(II) containing silicates (2, 5). Reduction of Cr(VI) by biotite has been experimentally investigated (4, 5) and the oxidation of structural Fe(II) by interlayer Cu(II) has been observed in both natural (6) and experimentally reacted biotites (7, S). The adsorption of Cr(VI) and partial reduction to Cr(III) also occurs on a-FeOOH that contains small amounts of Fe(II) (P). Recently the reduction of aqueous metal species including Fe(III), Cr... 

The loss of easily soluble potassium from the trioctahedral mica biotite will be compensated for by ion exchange with H3O ions, by oxidation of Fe ions, and by the replacement of Al in the octahedral layer by Si. On the other hand, the dioctahedral mica muscovite (Figure 2.6) undergoes similar potassium loss by degradation and associated charge deficiency (Table 2.3). In the resulting dioctahedral illites, the ideal Si/Al ratio of 3 in the tetrahedral layer of muscovite is changed to between 5 and 40 (Hower and Mowatt, 1966). In addition, water is intercalated between the layer stacks. 

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