The Structure and Thermochemistry of Three Fe-Mg Chlorites

Aja, Stephen U.; Omotoso, Oladipo; Bertoldi, Christian; Dachs, Edgar; Benisek, Artur

doi:10.1346/ccmn.2015.0630502

Cited by 6 publications

(6 citation statements)

References 49 publications

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“…Unfortunately, methods allowing detailed investigations of the cation distribution (such as single-crystal X-ray diffraction) are difficult to set up for ferric chlorite due to frequent zoning, twinning, and crystal deformation; consequently models ground on a limited set of reliable structure refinements. Most studies concur in allocating Mg and Fe 2+ to M1, M2, and M3 sites (Smyth et al 1997;Lougear et al 2000;Aja et al 2015), and filling the smaller M4 site with Al (Rule and Bailey 1987;Walker and Bish 1992;Nelson and Guggenheim 1993;Welch and Marshall 2001;Zazzi et al 2006;Aja et al 2015). Smyth et al (1997) and Inoue and Kogure (2016) suggest filling the M4 site with Fe 3+ when measured, consistent with an Al 3+ = Fe 3+ exchange.…”

Section: Introductionmentioning

confidence: 60%

“…Remaining unknowns include Al vs. Fe 3+ occupancy in the M4 and M2 sites, which is debated (Zheng and Bailey 1989;Smyth et al 1997;Aja et al 2015;Inoue and Kogure 2016). The similarly small ionic radii of Al and Fe 3+ (Shannon 1976) make them both candidates for preferential incorporation into the M4 site, as suggested by Vidal et al (2006Vidal et al ( , 2016.…”

Section: Incorporation Of Fe 3+mentioning

confidence: 99%

“…• Remaining trivalent cations are distributed on M1, then M3 if Table 4. Atom site and charge (Q) distribution of clinochlore (Nelson and Guggenheim 1993;Smyth et al 1997;Aja et al 2015) compared to two tentative magnesian di-ferri-sudoite end-members • Fe 3+ is preferentially incorporated in M4 over Al, the remaining Fe 3+ and Al are randomly mixed in M1 and M3.…”

Section: Cation Site Assignment For Chlorite Solid Solutionsmentioning

confidence: 99%

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A XANES and EPMA study of Fe3+ in chlorite: Importance of oxychlorite and implications for cation site distribution and thermobarometry

Masci

Dubacq

Verlaguet

et al. 2019

American Mineralogist

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Chlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other phyllosilicates of petrological interest, the oxidation state of iron in chlorite may differ from the usually assumed divalent state. However, the crystal chemistry of trivalent iron in chlorite remains poorly known, and the thermodynamic properties of ferric chlorite are missing from databases used for petrological modeling. As part of an attempt to fill this gap, we present results from in situ, micrometer-scale measurements of the oxidation state of iron in various chlorite-bearing samples. X-ray absorption near-edge spectroscopy (XANES) was combined with electron probe microanalysis (EPMA) on the same crystals. Results show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ~25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe 2 O 3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ~30 wt% Fe 2 O 3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe 3+ substitution. In Fe-rich samples, Fe 3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O 10 (OH) 8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI (Mg, Fe 2+) = VI o + 2 VI Fe 3+ , as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe 3+ content, up to ~ 2 H + pfu in the Fe 3+-rich chlorite samples, corresponding to a net exchange vector of the type R 2+ + H + = Fe 3+. These results do not support substitutions toward di-trioctahedral ferric endmembers, and highlight the need for considering substitution toward an "oxychlorite" (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O 12 (OH) 6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored. They are deterring if H deficiency is ignored but, given the sensitivity of most thermometers to octahedral vacancy, the assumption Fe total = Fe 2+ is still safer than using high measured Fe 3+ contents and the standard 28 charge basis, which artificially increases vacancies. In such ferric chlorites, EPMA measurement of oxygen allows a fair estimate of H content if Fe 3+ /Fe 2+ is known; it should be more systematically implemented. For the same reasons, literature data reporting Fe 3+-ri...

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Section: Introductionmentioning

confidence: 60%

Section: Incorporation Of Fe 3+mentioning

confidence: 99%

Section: Cation Site Assignment For Chlorite Solid Solutionsmentioning

confidence: 99%

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A XANES and EPMA study of Fe3+ in chlorite: Importance of oxychlorite and implications for cation site distribution and thermobarometry

Masci

Dubacq

Verlaguet

et al. 2019

American Mineralogist

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“…The chlorite mineral group is structurally and chemically more complex than smectites (and illites) and thus provide an excellent test case to evaluate these fundamental questions on the validity of modeling clay-mineral solubility with equilibrium constant formalism. The stability of Fe-Mg chlorites has recently been investigated from a number of perspectives including their interaction with aqueous solutions (Aja, 2019a;Aja et al, 2015;Aja and Dyar, 2002;Aja, 2002;Aja and Small, 1999 4 in Aja and Small, 1999).…”

Section: Chlorite -Aqueous Solution Equilibriamentioning

confidence: 99%

“…DOI: https://doi.org/10.2138/am-2022-8213. http://www.minsocam.org/ reaction, the predicted slope and the disposition of the experimental data in The Windsor chamosite and low-Fe clinochlore are monomineralic natural chlorite samples whose detailed structural chemistries have been determined (Aja et al, 2015); these monomineralic phases proved to be the solubility-controlling phases despite the presence of fractional subscripts in their structural formulae and even vacancies in their structures. These structural and chemical complexities of the chlorite group of minerals did not preclude either rational solubility patterns or the demonstration of equilibrium solubility, which observations contradict the postulate of the disequilibrium solid model.…”

Section: Chlorite -Aqueous Solution Equilibriamentioning

confidence: 99%

Resolving the conundrum of equilibrium solubility of smectites

Aja

2022

American Mineralogist

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Smectites are common clay minerals in surface and near surface terrestrial environments and have recently been shown to be ubiquitous on Mars. Because these minerals are products of water-rock interaction, the thermodynamics of their interaction with fluids constitutes a vital part of resolving the utility of smectite mineralizations as petrogenetic and paleoenvironmental indicators on both Mars and Earth's near surface environments. Smectites, and other clay minerals of comparable compositional complexity, have been purported to be disequilibrium solids whose complexity derive from steep chemical gradients in their environments of formation rather than from crystal-chemical constraints. Solubility investigates of several natural smectites wherein none exhibited the predicted inverse correlation between pH -1/3pAl and pSi(OH) 4 were adduced by May et al. (1986) as empirical proof of the disequilibrium solid concept and hence they asserted unequivocally that "it is obviously impossible to obtain valid ion activity quotients for smectite solubilities in these systems". However, the unattainability of equilibrium smectite solubility in those experimental systems was probably an artifact of the extremely high fluid-solid ratios employed therein. In subsequent experimental studies using significantly lower fluid-solid ratios, smectitefluid interactions (Kittrick and Peryea, 1989;Gaboreau et al. 2020) and chlorite-fluid interactions (Aja and Dyar, 2002) yielded solubility data amenable to laws of chemical thermodynamics and thus invalidate the disequilibrium solid model. Therefore, the notion of smectite metastability and/or instability anchored on the disequilibrium solid model is false and warrants a more constrained definition of smectite metastability. This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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Standard methods for heat capacity measurements on a Quantum Design Physical Property Measurement System

Rosen

Woodfield

2020

The Journal of Chemical Thermodynamics

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The Structure and Thermochemistry of Three Fe-Mg Chlorites

Cited by 6 publications

References 49 publications

A XANES and EPMA study of Fe3+ in chlorite: Importance of oxychlorite and implications for cation site distribution and thermobarometry

A XANES and EPMA study of Fe3+ in chlorite: Importance of oxychlorite and implications for cation site distribution and thermobarometry

Resolving the conundrum of equilibrium solubility of smectites

Standard methods for heat capacity measurements on a Quantum Design Physical Property Measurement System

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