Iron in biotites

Raeburn SP, llton ES, Veblen DR (1997a) (Quantitative determination of the oxidation state of iron in biotite using X-ray photoelectron spectroscopy 1. Calibration. Geochim Cosmochim Acta 61 4519-4530... 

In all cases, water and carbonic acid, the latter of which is the source of protons, are the main reactants. The net result of the reaction is the release of cations (Ca " ), Mg ", K", Na" ) and the production of alkalinity via HCO. When ferrous iron is present in the lattice, as in biotite, oxygen consumption may become an important factor affecting the weathering mechanism and the rate of dissolution. 

The first extensive study of iron-silicate minerals published was by deCoster and co-workers (7, 24). Their results are summarized in Table VI. Augite is a pyroxene containing considerable Na and Ca. No iron percentage was given for the olivine studies. The other minerals are included for comparison. The values given here for biotite are averages for their three untreated samples, which have two sites described as distorted octahedra. Heat treatment at 700°C. for one hour resulted in the emergence of a new Fe peak in biotite. 

A few years ago it was pointed out to me by Sterling Hendricks that ordinary black mica, biotite, which has an intensely black color, owes this color to the presence of iron in both the ferrous and ferric oxidation state. Black tourmalines also usually contain both ferrous and ferric iron. Another intensely black mineral, with black streak, is ilvaite, with composition Ca(Fe )aFe+++(Si04)20H. 

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. 

Most kaolinites contain appreciable amounts of MgO (range 0.01—1.0% modal value between 0.2—0.3%). Bundy et al. (1965) found that MgO as well as total iron and soluble iron were directly related to the C.E.C. and suggested that the Mg and Fe were present in montmorillonite which they believe is commonly present, in amounts less than 5%, in kaolinites. This may be true in part, but electron probe studies (Weaver,1968) indicate that some of the MgO is related to the Ti02-Fe203 material and some is present in biotite. 

Faye, G. H. (1968b) The optical absorption spectra of iron in six-coordinate sites in chlorite, biotite, phlogopite and vivianite. Some aspects of pleochroism in the sheet silicates. Canad. Mineral., 9,403-25. 

Table 2 lists diffusion coefficients for many mineral-isotopic systems currently used for thermochronometry. Diffusivities of some important isotopes depend on major-element composition e.g., " °Ar diffusion in phlogopite is —88% faster than in iron-rich biotite (Giletti, 1974 Grove and Harrison, 1996). In general, however, such variations are small in comparison with mineral-to-mineral differences in the diffusivity of a specific isotope. These differences for the basis for quantitative thermochronology. 

For some applications it is essential to control the concentration of iron which may vary over a broad range, regardless of mineral type. There are muscovite types known to contain up to 5% of Fc203 (although it would be expected to contain none), whereas biotite may contain as little as 2% of Fe203 (it typically contains iron in its chemical formula). 

Iron oxidation in biotites impedes release. Since Fe oxidation decreases overall layer charge, one might think that this would facilitate layer expansion and continued mica weathering. In fact, the Fe - Fe transformation in the octahedral sheet may ultimately confer dioctahedral character to the mica (see rule 1) and render release more difficult. 

Annersten H (1975b) Mossbauer study of iron in natural and synthetic biotites. Fortschr Mineral 52 583-590... 

Guseinov AA (1999) Relationship between ion conductivity and the heat-induced processes of oxidation and dehydroxylation in ferrous-magnesian micas. Geochem Int l 37 87-90 Haggstrom L, Wappling R, Annersten H (1969) Mossbauer study of iron-rich biotites. Chem Phys Lett 4 107-108... 

Malmstrom M, Banwart S (1997) Biotite dissolution at 25°C the pH dependence of dissolution rate and stoiehiometty. Geoehim Cosmochim Acta 61 2779-2799 Malathi N, Puri SP, Saraswat IP (1969) Mossbauer studies of iron in illite and montmorillonite. J Phys Soe Japan 26 680-683... 

Mineeva RM (1978) Relationship between Mdssbauer spectra and defect structure in biotites from electric field gradient calculations. Phys Chem Minerals 2 267-277 Mizutani T, Fukushima Y, Kobayashi T (1991) Synthesis of 1 1 and 2 1 iron phyllosihcates and characterization of their iron state by Mdssbauer spectroscopy. Clays Clay Minerals 39 381-386 Moon N, Coffin CT, Steinke DC, Sands RH, Dnnham WR (1996) A high-sensitivity Mdssbaner spectrometer facilitates the study of iron proteins at natural abundance. Nncl Instr Meth Phys Res B 119 555-564... 

The Mdssbauer effect on the Fe nuclei in biotite.] Kristallografiya 17 328-331 PopperP (1951) Transmission of natural and synthetic mica in the ultra-violet. Nature 168 1119-1120 Raclavasky K, Sitek J, Lipka J (1975) Mdssbauer spectroscopy of iron in clay minerals. 5th IntT Conf Mdssbauer Spectroscopy, Proc Part II, p 368-371... 

Sanz J, Meyers J, Vielvoye L, Stone WEE (1978) The location and content of iron in natural biotites and phlogopites a comparison of several methods. Clay Minerals 13 45-52 Sanz J, Serratosa JM, Stone WEE (1984) Local ordering in trioctahedral micas study by NMR, IR and Mossbauer spectroscopies. 27-y mezhdunarodnyy geologicheskiy kongress, 27(Vol. IX, Part 2) 178-179... 

Stucki JW, Roth CB, Baitinger WE (1976) Analysis of iron-bearing clay minerals by electron spectroscopy from chemical analysis (ESCA). Clays Clay Minerals 24 289-292 Swope J (1997) Single crystal X-ray and neutron diffraction studies of the crystal chemical effects of OH=0 substitution in mantle ratile and of Cl-OH substitution in biotite, and the crystal chemistry of IM ferromagnesian trioctahedral micas. PhD dissertation, Dept Geological Sciences, University of Colorado, Botrlder, 85 p... 

Guidotti CV, Cheney JT, Guggenheim S (1977) Distribution of titanium between coexisting muscovite and biotite in metapelites from northwestern Maine. Am Mineral 62 438-448 Guidotti CV, Dyar MD (1991) Ferric iron in metamorphic biotite and its petrologic and crystallochemical implications. Am Mineral 76 161-175... 

Iron (II) silicates are widely distributed in nature. Most of the iron in the primary magmatic rocks is in the form of Fe(II) silicates (olivines, pyroxenes, amphiboles, and biotites). When exposed on the Earth s surface, they undergo oxidative hydrolytic weathering to give iron(III) hydroxyoxides such as goethite, the overall stoichiometry being described by equation (1). This form of iron is very insoluble, but subsequent microbial reduction to iron (II) makes it available to the biosphere. 

Treasure Lake Group, overlain by felsic ignimbrite sheets of the Faber Group (Goad et al. 2000). Ore minerals mainly consists of Fe-, As-, Co- and Cu-sulphides, native Au and Bi. Intense, pervasive, polyphase iron-oxide (magnetite-dominant)-hornblende-biotite-tourmaline-K-feldspar-carbonate replacive alteration occurs in the upper metasedimentary sequence below the volcanic-sedimentary unconformity... 

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