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

Abstract

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...

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

“…Crystallization temperature calculated with the chlorite thermometers of Vidal et al () and Bourdelle et al () are reported in Figure a, and range between 180 and 350°C. The minimum content of X Fe 3+ in chlorite calculated with the method of Vidal et al () ranges between 0.1 and 0.55, which is realistic for low‐ T chlorite (Masci et al, ). The entire iron content was considered divalent for the thermometer of Bourdelle et al (), as recommended in that study.…”

Low-Temperature Chlorite Geothermometry and Related Recent Analytical Advances: A Review

“…The most remarkable cationic substitutions occurring in chlorites are, therefore, as follows: All these cationic substitutions can be combined to form a multitude of exchange vectors. Additional substitutions can also be examined, such as the "hydroxyl" substitution and the deprotonation process, written Fe 3+ + 2OH − = Fe 3+ + O 2− + OH − and Fe 2+ + H + = Fe 3+ , respectively [11,16] (see Section 5). Consequently, octahedral sheets can theoretically exhibit three out of three occupied cationic sites (tri-tri octahedral structure; clinochlore, daphnite), two out of three occupied cationic sites (di-dioctahedral structure; dombassite), or one or other combination of these configurations, i.e., two out of three sites occupied for M1(M2) 2 and three out of three sites occupied for (M3) 2 M4 (di-trioctahedral structure, sudoite), or the opposite (tri-dioctahedral structure) [17,18].…”

“…Cationic substitutions where Fe 3+ is implicated can also be redefined. Masci et al [16] investigated a wide range of chlorite compositions by XANES and EMP, and claimed that the DT substitution implicating Fe 3+ is an artefact. This substitution is usually considered as the result of the combination of the Al-DT substitution and AF exchange but without the intervention of Al.…”

“…Here, the structural formula is not calculated on a 14-oxygen basis but on a 15-oxygen basis, as R 2+ + H + = Fe 3+ , leading to a reduction of octahedral vacancies. Masci et al [16] campaigned for the consideration of an oxychlorite member (or "oxidized chlorite"), close to tri-trioctahedral chlorite, and for a new cation site assignment as a part of a chlorite solid-solution accounting for proton loss in M1 and M2 sites. If the deprotonation becomes a verified process, the published data on Fe 3+ -rich chlorites with a high vacancy content will have to be reappraised, and the implications for chlorite thermometry are far-reaching.…”

Automatic 3D image registration for nano-resolution chemical mapping using synchrotron spectro-tomography

“…The synchrotron's advantages in spatial coherence (Eastwood et al, 2015;Zhu et al, 2010), monochromaticity (Torikoshi et al, 2003) and, subsequently, compositional and chemical sensitivities (Kao et al, 2013; collectively open vast scientific opportunities. One of the most popular approaches is to combine X-ray tomography with X-ray absorption spectroscopy (Yang et al, 2014(Yang et al, , 2019Yu, Farmand et al, 2018;Hitchcock, 2015;Masci et al, 2019;Nelson et al, 2011;Zhang et al, 2020;Zhu et al, 2020;Wang et al, 2016), featuring a much desired experimental capability that could spatially resolve chemical heterogeneity by probing ISSN 1600-5775 # 2021 International Union of Crystallography spatially resolved X-ray spectroscopic signals. This is because the structural and chemical heterogeneities are not only ubiquitous in real-world functional materials but also are often interdependent, complicating the material system while also providing an opportunity for advanced functional materials engineering.…”