Oxidation of Ferrous Iron in Biotite

Gilkes, R. J.; Young, Ralph C.; Quirk, J. P.

doi:10.1038/physci236089b0

Cited by 8 publications

(33 citation statements)

References 8 publications

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“…The gradual pre-transition decrease in c (Figure 8c) can be tentatively interpreted as progressive OH axis bending away from the c axis, as more and more of the electrostatic interlayer compressional force is countered by fewer and fewer OH groups (whose protons oppose the K* interlayer cations). Such bending is consistent with the fact that, as the annite is oxidized, a larger and larger fraction of the remaining OH groups see t6]Fe s+-[6]FeZ+-t6JFe 3+ or [61Fe2+-[61FeS+-[61Fe 3+ nearest-neighbor configurations, rather than the dominant [61Fe z+-~61Fe2+-f6)Fe2+ nearest-neighbor configuration of unoxidized annite (Gilkes et al, 1972a(Gilkes et al, , 1972b.…”

Section: Pillaring Collapse Transition At Point Of Total Oh Losssupporting

confidence: 61%

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

Rancourt

2001

Clays and clay miner.

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Abstract--A synthetic octahedral-site-vacancy-free annite sample and its progressive oxidation, induced by heating in air, were studied by powder X-ray diffraction (pXRD), M6ssbauer spectroscopy, nuclear reaction analysis (NRA), Raman spectroscopy, X-ray fluorescence (XRF) spectroscopy, gas cba-omatography (GC), thennogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and size-fraction separation methods. For a set heating time and as temperature is increased, the sample first evolves along an annite--oxyannite join, until all H is lost via the oxybiotite reaction (Fe 2+ + OH-~ Fe 3+ + 0 2-+ Hd'). It then evolves along an oxyannite-feiTioxyanuite join, where ideal ferrioxyannite, KFe3+s/3[~mA1Si3012, is defined as the product resulting from complete oxidation of ideal oxyannite, KFe3+2FeZ+A1Si30~2, via the vacancy mechanism (3 Fe z+ ~ 2 Fe 3+ + I61[~ + Fe1"). A pillaring collapse transition is observed as a collapse of c near the point where FeZ+/Fe = 1B and all OH groups are predicted and observed to be lost. Quantitative analyses of H, using NRA, GC, and Raman spectroscopy, corroborate this interpretation and, in combination with accurate ferric/ferrous ratios from M6ssbauer spectroscopy and lattice parameter determinations, allow a clear distinction to be made between vacancy-free and vacancybearing annite. The amount of Fe in ancillary Fe oxide phases produced by the vacancy mechanism is measured by M6ssbauer spectroscopy to be 1 1.3(5)% of total Fe, in agreement with both the theoretical prediction of 1/9 = 11.1% and the observed TGA weight gain. The initiation of Fe oxide formation near the point of completion of the oxybiotite reaction (FeZ+/Fe = 1B) is con-oborated by pXRD, TGA, Raman spectroscopy, and appearance of an Fe oxide hyperfine field sextet in the M6ssbaner spectra. The region of Fe oxide folTnation is shown to coincide with a region of octahedral site vacancy formation, using a new MOssbauer spectral signature of vacancies that consists of a component at 2.2 mm/s in the I6~Fe3+ quadrupole splitting distribution (QSD). The crystal chemical behaviors of annite--oxyannite and of oxyannite fen-ioxyannite are best contrasted and compared to the behaviors of other layer-silicate series in terms of b vs. [D] (average octahedral cation to O bond length). This also leads to a diagnostic test for the presence of octahedral site vacancies in hydrothermally synthesized annite, based on a graph of b vs. Fe2+/Fe. The implications of the observed sequence of thermal oxidation reactions for the thermodynamic relevance of the oxybiotite and vacancy reactions in hydrothermal syntheses are examined and it is concluded that the oxybiotite reaction is the relevant reaction in the single-phase stability field of annite, at high hydrogen fugacity and using ideal starting cation stoichiometry. The vacancy reaction is only relevant in a multi-phase field, at lower hydrogen fugacity, that includes an Fe oxide equilibrium phase (magnetite) that can effectively compete ...

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Section: Pillaring Collapse Transition At Point Of Total Oh Losssupporting

confidence: 61%

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

Rancourt

2001

Clays and clay miner.

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“…Farmer et al (1971) presented "... evidence for loss of protons and octahedral iron from chemically oxidized biotities and vermiculites". Their results were confirmed by Gilkes et al (1972). The oxidation treatments in both papers were carried out in acid solutions, with either saturated bromine water and/or H202.…”

Section: Introductionsupporting

confidence: 52%

“…The oxidation of Fe(II) is accompanied by deprotonation of octahedral OH -groups, under the neutral conditions (Table 3) used in the present experiments; earlier work showed corresponding dehydroxylation under more acid conditions (Farmer et al, 1971;Gilkes et al, 1972). A deficit in structural OH-+ F-(<4 per unit cell) and therefore an excess of oxygen (> 20 per unit cell) occurs in many trioctahedral micas (Eugster and Wones, 1962;Foster, 1964;Rimsaite, 1967Rimsaite, , 1970.…”

Section: Discussionmentioning

confidence: 75%

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

Veith

1974

Clays and clay miner.

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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.

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“…and vermiculites" . Their results were confirmed by Gilkes et al (1972). The oxidation treatments in both papers were carried out with either saturated bromine water and/or H202 in acid solutions.…”

Section: Introductionsupporting

confidence: 54%

Iron oxidation and reduction effects on structural hydroxyl and layer charge in aqueous suspensions of biotitic vermiculites

Veith¹,

Jackson²

1972

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Oxidation of Ferrous Iron in Biotite

Cited by 8 publications

References 8 publications

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

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

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

Iron oxidation and reduction effects on structural hydroxyl and layer charge in aqueous suspensions of biotitic vermiculites

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