Iron in Silicate Glasses and Melts

Abstract

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Atomic structure and physical properties of peridotite glasses at 1 bar

“…Indeed, as previously presented, a variety of spectroscopic methods attest to the prevailing role of Fe 2+ as a network modifier cation in silicate melts, similar to Mg 2+ or to Ca 2+ , whereas Fe 3+ behaves, at least partially, as a network former cation like Al 3+ (see for a review of this Le Losq et al, 2021b,;Mysen and Richet, 2019). In general, this similarity arises from their similar ionic radii and charges, where Z/r for IV Al 3+ = 7.7 and for IV Fe 3+ = 6.1, while Z/r for VI Mg 2+ = 2.78 and VI Fe 2+ = 2.6 (Shannon et al, 1976).…”

“…Similarly, the NBO/T of Q 1 units is 3, that of Q 2 is 2, and that of Q 3 is 1. Regarding Fe 2+ and Fe 3+ , existing data generally indicate that the first acts as a network modifier cation, whereas the second acts as a network former cation (e.g., see the review in Le Losq et al, 2021b). Assuming such roles for Fe 2+ and Fe 3+ , the present glasses have an estimated NBO/T (calculated from composition as [2O-4T]/T with T the tetrahedral cations Si 4+ , Al 3+ and Fe 3+ ) that ranges from ~2.10 (Fe 3+ /Fe TOT = 0.44) to ~2.44 (Fe 3+ / Fe TOT = 0.01).…”

“…This structural picture is broadly corroborated by variations in the viscosity of silicate melts upon changing the redox state of iron. That is, at a given temperature, melt viscosity decreases as Fe 3+ is reduced to Fe 2+ Dingwell, 1989;Liebske et al, 2003;Chevrel et al, 2013;Le Losq et al, 2021b). However, this general picture still is subject to debate because the coordination numbers (CNs) of Fe 3+ and Fe 2+ may vary with melt composition, which can, in turn, induce variations in melt properties.…”

“…This is important as the Fe 3+ /Fe 2+ ratio in turn modifies melt viscosity, because ferric and ferrous iron occupy different structural roles in silicate melts 1989;Dingwell, 1991;Liebske et al, 2003;Chevrel et al, 2013). Indeed, in silicate melts, it is widely accepted that Fe 3+ acts as a network former, participating more readily in the construction of the aluminosilicate polyhedral disordered network due to its preference for tetrahedral coordination, whereas Fe 2+ acts as a network modifier that breaks T-O-T bonds (T = Si, Al), tending to occupy octahedral sites (see Le Losq et al, 2021b for a review). However, the coordination number (CN) of Fe 2+ and Fe 3+ varies as a function of composition, temperature, and pressure (e.g., Jackson et al, 2005;Zhang et al, 2017), implying that the relationship between melt structure, and, hence, viscosity with iron oxidation state, may not be straightforward.…”

Iron coordination in liquid FeAl 2 O 4

“…The ability of iron to exist in different valence states can play a crucial role in altering the physical properties, such as viscosity, density and melting point, of the geological melts that control geochemical processes in the Earth's deep interior [1][2][3]. Due to its important geological role, extensive experimental and simulation studies have been conducted to understand how iron is incorporated in the structure of oxide liquids and glasses (see reviews [4][5][6][7][8] and references therein). However, despite this extensive body of research, the local coordination of Fe by oxygen in these melts remains uncertain.…”