Bridgmanite, with (Mg,Fe)SiO3 as the major component, is the most abundant mineral on Earth and is stable under lower mantle conditions. When incorporating trivalent cations (Al3+ and Fe3+), bridgmanite can form various components such as the oxygen vacancy [(Mg,Fe2+)(Al,Fe3+)O2.5] and A-site vacancy [(Al,Fe3+)2/3 square 1/3)SiO3, square= vacancy] components in addition to the charge-coupled (Fe2O3, Al2O3, FeAlO3) components. These vacancy-bearing components potentially allow the storage of volatiles in the lower mantle and the modification of the viscosity and compressibility of the lower mantle. However, our understanding of these defect components has been limited due to the previous focus on relatively simple systems. This study determined the compositions of bridgmanite and its associated phases in the MgO-SiO2-Al2O3-Fe2O3-FeO system as a function of temperature (1700-2300 K) at a fixed pressure of 27 GPa, relevant for uppermost lower mantle conditions. Our results suggest the presence of oxygen vacancies in bridgmanite, implying that the Earth's lower mantle may store water and noble gases. Furthermore, our results agree with geophysical observations indicating slab stagnation in the deep mantle.
Superstructures with modulated vacancy order are a characteristic trait of the pyrrhotite solid-solution series (Fe-1-S-x, 0 < x <= 0.125), and they strongly affect the magnetocrystalline anisotropy that causes Fe-spin rotation. The effect of a stochastic vacancy order and the resulting structural heterogeneity at the atomic level on the magnetization is not resolved yet in detail. Here, we demonstrate that a thermally tailored antiferromagnetic pyrrhotite made up of fivefold and sixfold stacking modulations with five different Fe-vacancy ordering variants forms two distinct vacancy ordering schemes that govern the spin rotation. A field-dependent thermomagnetic anomaly between 220 and 170 K and a secondary hysteresis loop in the magnetization curve in a field of around 3 Tesla provide compelling evidence for two different Fe-spin rotation patterns. The simultaneous occurrence of the magnetic anomaly and two enthalpy changes link the magnetocrystalline anisotropy to structural distortions of Fe-Fe bonds in the vacancy-ordering schemes, thereby explaining the presence of the two spin rotation patterns. This finding highlights magnetic effects induced by vacancy order and underscores the relevance of its architecture in a key sulfide material extensively used in geoscience and cosmology and with potential for technical applications.
Abstract Iron (Fe), a multivalent element abundant in Earth’s crust, mantle, and core, occurs in multiple oxidation states that are closely linked to the redox conditions of Earth’s deep interior. Quantifying Fe oxidation states in deep-mantle minerals, such as garnet and bridgmanite, is therefore essential for constraining the redox evolution of Earth’s interior. Because deep-mantle minerals commonly occur at micro- to submicron scales, high spatial resolution analytical techniques are required for reliable determinations of Fe redox. Here, we optimize the workflow and establish an instrument-specific calibration curve for quantifying Fe3+/ΣFe using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). The calibration curve is based on a suite of independently characterized reference samples, including Al-bearing and Al-free bridgmanite, garnets, magnetite, and hematite, analyzed under the same STEM–EELS acquisition and data-processing conditions. The relationship between the Fe L3/L2 white-line intensity ratio, I(L3)/I(L2), and independently constrained Fe3+/ΣFe values can be described by a single calibration curve for these reference samples on our instrument. Importantly, none of the bridgmanite standards show a systematic offset relative to garnets and Fe oxide standards within the uncertainty of the present dataset. Interlaboratory comparisons further demonstrate that the previously proposed EELS “universal curve” cannot yield accurate Fe3+/ΣFe values across different instruments and analytical protocols, underscoring the need for instrument-specific calibration. We further apply the calibrated workflow to bridgmanite-bearing products synthesized using large-volume press and laser-heated diamond anvil cell techniques. These applications demonstrate that instrument-specific STEM–EELS calibration can provide quantitative, phase-specific Fe oxidation-state information from complex and fine-grained high-pressure experimental products. Our results provide a practical analytical framework for Fe redox analysis of bridgmanite and other deep-mantle minerals.
Abstract Nitrides are a small group of minerals whose formation is primarily associated with reducing conditions. Roaldite (Fe4N or γ′-Fe4N) and siderazot (Fe3N1.33 or ɛ-Fe3N) are chemically similar minerals, yet they are not commonly found together. While roaldite is typical of iron meteorites and has not been detected on Earth, siderazot is predominantly found in terrestrial rocks and has never been found in iron meteorites. We studied the Sikhote-Alin iron meteorite using optical and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), and electron backscatter diffraction (EBSD) and demonstrated the presence of siderazot. We also demonstrated that roaldite is a typical accessory mineral of the Sikhote-Alin iron, and that nitride detection can be easily accomplished using routine nitrogen mapping. In the Sikhote-Alin iron, both roaldite and siderazot contain very low amounts of nickel (< 5 wt. % Ni), which is typical of the host kamacite (α-Fe). It is noteworthy that roaldite and siderazot in the Sikhote-Alin iron differ only slightly in nitrogen content (∼6 and 8 wt. % N), which makes it difficult to identify them without the information on the crystal structure. However, roaldite and siderazot can be reliably identified based on EBSD data, given the significant differences in their unit cell parameters and crystal structures (space groups Pm-3m and P6322, respectively). We propose that siderazot formation is associated with impacts on primary roaldite and marks high-pressure transformations of meteoritic material. Searching for siderazot (and other iron nitrides like Fe2N or Fe16N2) in other iron meteorites (especially those in which roaldite has previously been detected) may help to establish the mechanism of their formation and transformation.
Abstract The study the holotype specimen of natrodufrénite from Rochefort-en-Terre (Morbihan, Brittany, France) and selected comparative samples from Poniklá (Liberec Region, Czech Republic) provided new structural, chemical, and spectroscopic data on this natural sodium containing hydrated basic iron phosphate. Its crystal structure, solved using single-crystal X-ray diffraction data obtained for the greenish variety from the Poniklá locality, was refined to R1 = 2.75% for 3879 unique reflections with Fo > 4σ(Fo) and confirmed for the holotype specimen by three-dimensional electron diffraction technique combined with Rietveld analysis of powder X-ray data. Natrodufrénite is monoclinic with space group C2/c and cell parameters a = 25.9973(4), b = 5.15580(13) Å, c = 13.8990(7) Å, β = 111.466(3)°, V = 1733.75(10) Å3 and Z = 4. The mineral is isostructural with dufrénite, having the structural framework built by two different types of PO4 tetrahedra, four different face or corner sharing FeØ6 octahedra and NaØ8 polyhedra in the channels along the b axis. The characteristic structural unit is the trimer of FeØ6 coordination octahedra. Natrodufrénite is so far the only member of the group with Fe3+-dominance at all octahedral sites. The revised ideal formula NaFe3+Fe3+5(PO4)4O(OH)5(H2O)2 (IMA 25-F) is in good agreement with that given in the original description of natrodufrénite.
