Electronic environments of ferrous iron in rhyolitic and basaltic glasses at high pressure

Abstract

The physical properties of silicate melts within Earth's mantle affect the chemical and thermal evolution of its interior. Chemistry and coordination environments affect such properties. We have measured the hyperfine parameters of iron‐bearing rhyolitic and basaltic glasses up to ~120 GPa and ~100 GPa, respectively, in a neon pressure medium using time domain synchrotron Mössbauer spectroscopy. The spectra for rhyolitic and basaltic glasses are well explained by three high‐spin Fe2+‐like sites with distinct quadrupole splittings. Absence of detectable ferric iron was confirmed with optical absorption spectroscopy. The sites with relatively high and intermediate quadrupole splittings are likely a result of fivefold and sixfold coordination environments of ferrous iron that transition to higher coordination with increasing pressure. The ferrous site with a relatively low quadrupole splitting and isomer shift at low pressures may be related to a fourfold or a second fivefold ferrous iron site, which transitions to higher coordination in basaltic glass, but likely remains in low coordination in rhyolitic glass. These results indicate that iron experiences changes in its coordination environment with increasing pressure without undergoing a high‐spin to low‐spin transition. We compare our results to the hyperfine parameters of silicate glasses of different compositions. With the assumption that coordination environments in silicate glasses may serve as a good indicator for those in a melt, this study suggests that ferrous iron in chemically complex silicate melts likely exists in a high‐spin state throughout most of Earth's mantle.

Pressure‐Induced Coordination Changes in a Pyrolitic Silicate Melt From Ab Initio Molecular Dynamics Simulations

“…Iron was interpreted to be predominantly in sixfold coordination in albite‐diopside glasses (Keppler, ), basalt glass (Bell & Mao, ), and a variety of Fe‐Ti glasses (Nolet et al, ) with optical absorption spectroscopy. Using time‐domain Mössbauer and optical absorption spectroscopy on reduced basaltic glass, Solomatova et al () found that less than 15–20% of ferrous iron is in fourfold coordination and the rest of iron is consistent with fivefold and/or sixfold coordination. Thus, although the proportions of the site environments depend on composition, the reported coordination environments also depend strongly on the experimental technique and method of analysis and interpretation.…”

A Synchrotron Mössbauer Spectroscopy Study of a Hydrated Iron-Sulfate at High Pressures

“…Although szomolnokite exists on the surface of Earth and likely on Mars, it is unknown if it exists at depth within either planet. Although it is also known that hydrated sulfates play important roles in the metal and hydrologic exchanges [29], FeCO 3 siderite [30], FeSiO 3 ferrosilite [27], basaltic and rhyolitic glasses [24], and FeSO 4 ·H 2 O szomolnokite (this study). For szomolnokite, we include Model 2 (where site 3 undergoes a high-to low-spin transition) and Model 3 (where a fifth low-spin site gradually increases its weight fraction over a broad range of pressures).…”

“…Although there is only one crystallographic site for iron in szomolnokite at ambient conditions [13][14][15], two Mössbauer sites are required to fit the data. As discussed in previous works on iron-bearing phases Minerals 2020, 10, 146 5 of 11 characterized by a single crystallographic site for ferrous iron, such as bridgmanite and (Mg,Fe)O ferropericlase [23][24][25][26], additional Fe 2+ -like sites in the data evaluation of Mössbauer spectra may arise from differences in the next nearest-neighbor environments. Atomic-specific probes like Mössbauer spectroscopy are therefore capable of resolving such local-environment characteristics.…”

“…For example, the occurrence of spin transitions in iron-bearing materials of Earth's lower mantle has received increased attention in the last 15 years owing to their potential geophysical, geochemical, and geodynamical implications [28]. At room-temperature, (Mg,Fe)O ferropericlase and FeSiO 3 ferrosilite exhibit a similar broad spin crossover behavior with pressure [24,29], whereas the iron-bearing carbonate FeCO 3 siderite experiences a sharp spin transition [30]. In Figure 5, we schematically compare these transitions with szomolnokite, along with iron-bearing silicate glasses.…”

“…Minerals 2020, 10, x FOR PEER REVIEW 8 of 11 Figure 5. Schematic comparison of the fraction of high-spin ferrous iron (relative to low-spin ferrous iron) as a function of pressure at 300 K in various Fe 2+ -bearing phases: (Mg0.75Fe0.25)O ferropericlase, [31], (Mg0.8Fe0.2)O ferropericlase [32], (Mg0.52Fe0.48)O ferropericlase [29], FeCO3 siderite [30], FeSiO3 ferrosilite [27], basaltic and rhyolitic glasses [24], and FeSO4·H2O szomolnokite (this study). For…”