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Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low‐field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low‐temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo‐single‐domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe₃S₄ spinel (greigite), and its high-pressuremonoclinic phase, we show that greigite is a stable phase in the Fe–S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe₃S₄ in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.

How and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols. Plain Language Summary Remanent magnetizations preserved in sedimentary rocks serve as a continuous record of Earth’s magnetic field history and play a fundamental role in understanding the Earth system. It is commonly assumed that magnetic minerals align with the magnetic field as a particle settles through the water column, known as a depositional remanent magnetization (DRM). However, diagenesis can lead to chemical growth of magnetic minerals, known as a chemical remanent magnetization (CRM). CRM lacks stratigraphic continuity and can obscure or completely overprint the original magnetization any time after sediment deposition, leading to a magnetic record that is uncorrelated with the age of the rock. Yet, CRMs go largely unrecognized. Theory and experiments in our paper document that CRMs record the magnetic field six times more efficiently than DRMs. Our work provides a way to distinguish the two through routine laboratory protocols. Key Points Recording efficiency of chemical remanent magnetization (CRM) is six times higher than depositional remanent magnetization (DRM) Undetected chemical remanences lead to overestimated relative paleointensity estimates Comparison of natural and laboratory magnetization and demagnetization behavior help identify chemical remanent magnetizations in sediments

Magnetic greigite may be a valuable indicator for methane emissions in the geological past, if its formation pathway and diagenetic environment can be unambiguously defined. Here, we investigate sulfur isotopic compositions of iron sulfides and ferrous iron concentrations of thick greigite‐bearing sediments (TGBSs) in the South Yellow Sea, a shallow marginal sea with strong methane emissions. For the first time, isotopically heavy iron sulfides (up to 28.7‰ in δ34S and 0.19‰ in Δ33S) and enrichments of ferrous iron in the TGBSs are observed. We interpret the data as indicating synchronic occurrences of anaerobic oxidation of methane coupled to sulfate and iron reductions, which occur in coastal methanic zones with limited sulfate availability and therefore probably imply leakages of methane. Consequently, we suggest that greigite is a promising geological indicator for tracing methane liberated from coastal sediments, accounting for ∼60% of the currently rising global marine methane budget. Plain Language Summary Greigite (Fe3S4) is an easily detected ferromagnetic mineral and extensively observed in shallow coastal sediments, where strong methane emissions to the atmosphere prevail. It probably forms in a relatively sulfur‐poor environment associated with the consumption of methane by sulfate reduction. Here, we found a unique sulfur isotope fingerprinting in iron sulfides and enrichment of ferrous iron in these greigite‐bearing sediments, likely defining a methanic condition associated with potential coastal methane leakages. Our study highlights the intrinsic linkage between methane and greigite formation, supports that greigite may be a potential indicator for quantifying the global natural methane budget, and therefore has paleoenvironmental implications for geological global warming events. Key Points Isotopically heavy sulfides (both δ34S and Δ33S) and enrichment of ferrous iron are synchronically observed in greigite‐bearing sediments Pyrite with elevated Δ33S in thick greigite‐bearing sediments likely formed under a methanic condition Greigite is a potentially useful tracer for methane leakages from coastal sediments

Greigite magnetic nanoparticles (Fe 3 S 4 -MNPs) were prepared and reveal a peroxidase-like activity. Kinetic studies revealed a pseudo-enzymatic activity that is much higher than that of other magnetic nanomaterial-based enzyme mimetics. This finding was exploited to design a photometric enzymatic glucose assay based on the formation of H 2 O 2 during enzymatic oxidation of glucose by glucose oxidase, and the formation of a blue product from an enzyme substrate that is catalytically oxidized by H 2 O 2 in the presence of Fe 3 S 4 -MNPs. Glucose can be detected in the 2 to 100 μM concentration range, and the low detection limit is 0.16 μM. The method was applied to quantify glucose in human serum. In our perception, this enzyme mimetic has a large potential in that it may be used in other oxidase based assays, but also in ELISAs. Graphic Abstract Fe 3 S 4 magnetic nanoparticles (MNPs) are shown to act as peroxidase mimetics and this was used to design a glucose oxidase (GOx) based glucose assay where the H 2 O 2 formed during oxidation of glucose oxidizes tetramethylbenzidine (TMB) to give a blue product which can be quantified by photometry.

Multipolarity remanence in greigite‐bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite‐bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single‐polarity” to “multi‐polarity” samples. The inward alteration of sulfide grains is topotactic and is size‐dependent with higher alteration in smaller grains. We propose a multi‐phase self‐reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self‐reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications. Plain Language Summary Some magnetic minerals in nature can be magnetized opposite to the external geomagnetic and planetary magnetic fields—a peculiar phenomenon called “self‐reversal.” A self‐reversal magnetization process is typically observed to occur in igneous rocks during cooling in an external field. Here, using magnetic and microscopic analyses we demonstrate that sediments containing authigenic ferrimagnetic iron sulfide mineral—greigite—can acquire a self‐reversed magnetization during progressive surface alteration of greigite nanoparticles. Surface alteration produces new “magnetic shells” that are magnetized opposite to the magnetization of the parent greigite core through magnetic interactions due to the close contact between the core and shell. Post‐depositional sedimentary processes, for example, percolation of fluids or oxygenation could trigger surface alteration that leads to “self‐reversal” and complicate the primary magnetization records. This self‐reversal mechanism can explain the commonly reported anomalous magnetization records of authigenic greigite; it is very useful for correct interpretations of tectonic and paleoenvironmental processes, and geological age frames of iron sulfide bearing sediment sequences. Key Points Surface alteration of diagenetic greigite to magnetite and pyrrhotite causes magnetic softening and multipolarity remanence The original microtextures are preserved during iron sulfide alteration and the alteration extent is size‐dependent A new multi‐phase self‐reversal model of diagenetic greigite is proposed that has broad geochronological and geophysical implications

Greigite (Fe3S4) is a ferrimagnetic iron‐sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo‐ and rock‐magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well‐defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In‐situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid‐state FeS to greigite transformation with an activation energy of 78–90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 and 100 K, respectively. The mixing trend coincides with empirical data reported for natural greigite‐bearing sediments, suggesting a common SP endmember size of 5–10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20–50 nm determined by X‐ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments. Plain Language Summary Sediments provide continuous records of Earth's ancient magnetic field, which lend insights into the workings of the geodynamo and help to establish the geologic time scale through global magnetostratigraphic correlation. Greigite is a magnetic iron sulfide mineral that commonly forms after deposition, thereby remagnetizing the sediment and complicating interpretation of the magnetic record. Understanding greigite formation and detecting its presence is fundamental for obtaining reliable records of the paleomagnetic field, yet knowledge of how greigite grows and how its magnetic properties evolve during growth remains limited. This article outlines a novel approach to form greigite in sediments and to monitor its growth kinetics, grain size and magnetic remanence acquisition. The magnetic properties of the synthetic sediments resemble those of natural greigite‐bearing sediments and match well with theoretical calculations, which can help quantify grain sizes in sedimentary greigite. The reported method and our results contribute to a better understanding of greigite formation and chemical magnetic remanence acquisition in sediments. Key Points We present a new method to grow greigite in aqueous sediments and create a chemical remanent magnetization under controlled conditions Greigite grain sizes of 20–50 nm span the superparamagnetic to single domain threshold, consistent with theoretical predictions Our experimental hysteresis data coincide with calculated mixing curves allowing better quantification of greigite particle sizes in nature

Rivers play a crucial role in landscape evolution and human development, especially in arid zones, where hydrological resources are scarce and in high demand. The Atacama Desert is one of the world’s oldest and driest non-polar deserts, and aquatic systems therein have been historically subjected to anthropogenic pressure mainly associated with natural resource exploitation, such as water consumption for industrial mining activities. The mining industry has experienced a systematic development since the early 20th century, making Chile one of the main worldwide copper producers. This study analyzed sediments from two Atacama Desert rivers, the Loa and Salado Rivers (Antofagasta Region, Northern Chile). Sedimentary short-cores were obtained from sampled locations at varying distances from the confluence of the rivers. The characterization of chemical components, grain size, mineralogy, and magnetic properties of the rivers’ sediments was assessed in surface and subsurface samples to determine their respective signatures in the Inka-Coya Lake near the rivers’ confluence. The magnetic mineralogy present in the sediments of both rivers is composed of detrital magnetite and maghemite interspersed with those of authigenic origin. However, the downstream Loa River concentrated more authigenic minerals than the Salado and increased the abundance of silt-sized particles. The grain size of the Loa’s channel bed suggests low stream competency and high formation of depositional habitats. The magnetic signal and mineralogical composition of sediments from the lake are dominated by detrital pyrite, magnetite, and authigenic greigite. In contrast, the river’s sediments were dominated by magnetite and maghemite of detrital origin intercalated with those of authigenic origin. The granulometry, mineralogy, and rock magnetic properties of Inka-Coya Lake sediments indicate recent detrital input alternating with authigenic mineral-rich layers, mainly reflecting shifts in hydrological regimes. The highest concentrations of copper were observed in the upper, more recent sediment layers. Future scenarios of risky climatic conditions associated with increasing global metal demands could modify the availability of potentially toxic elements and transport capability in fluvial sediments, increasing the threats to water resource conservation in the world’s most arid desert.

We report on a systematic study on the physicochemical attributes of synthetic Fe-Cu-sulfide nanoparticles (NPs) precipitated under conditions similar to the anoxic, low-temperature aqueous, sedimentary, soil, and subsurface environments where these NPs have been repeatedly identified. Characterizing the basic attributes of these NPs is the first step in understanding their behaviors in various processes including in the bio-availability of essential and toxic metals, environmental remediation, and resource recovery. Abiotic experiments are compared to biotic experiments in the presence of the sulfate-reducer Desulfovibrio vulgaris to elucidate biological controls on NP formation. First, the single-metal end-member NPs are determined by precipitation in a solution containing either aqueous Fe(II) or Cu(II). Limited differences are observed between biogenic and abiogenic precipitates aged for up to one month; the Fe-only experiments resulted in 4-10 nm mackinawite (FeS) NPs that aggregate to form nanosheets up to ∼1000 nm in size, while the Cu-only experiments resulted in mixtures of covellite (CuS) NPs comprised of <10 nm fine nanocrystals, 20-40 × 6-9 nm nanorods, and ∼30 nm nanoplates. The crystal sizes of biogenic mackinawite and covellite are, respectively, larger and smaller than their abiogenic counterparts, indicating a mineral-specific response to biological presence. Structural defects are observable in the fine nanocrystals and nanorods of covellite in both biogenic and abiogenic experiments, indicative of intrinsic NP instability and formation mechanism via particle attachment. In contrast, covellite nanoplates are defect free, indicating high stability and potentially rapid recrystallization following particle attachment. Next, mixed-metal sulfide NPs are precipitated at variable initial aqueous Fe-to-Cu ratios (2:1, 1:1, and 1:5). With an increasing ratio of Fe-to-Cu, Fe-rich covellite, nukundamite (Cu5.5FeS6.5), chalcopyrite (CuFeS2), and Cu-rich mackinawite are formed. The Fe-rich covellite NPs are larger (100-200 nm) than covellite precipitated in the absence of Fe, indicating a role for Fe in promoting crystal growth. Chalcopyrite and nukundamite are formed through the incorporation of Fe into precursor covellite NPs while retaining the original crystal morphology, as confirmed by doping a covellite suspension with aqueous Fe(II), resulting in the formation of chalcopyrite and nukundamite within days. Additionally, in the biological systems, we observe the recrystallization of mackinawite to greigite (Fe3S4) after six months of incubation in the absence of Cu and the selective formation of chalcopyrite and nukundamite at lower initial Fe-to-Cu ratios compared to abiotic systems. These observations are consistent with NP precipitation that are influenced by the distinct (sub)micro-environments around bacterial cells compared to the bulk solution. Comparative TEM analyses indicate that the synthetic NPs are morphologically similar to NPs identified in natural environments, opening ways to studying behaviors of natural NPs using experimental approaches.

Long and continuous lacustrine sediments in Southwest China provide exceptional records of the Indian summer monsoon (ISM) evolution. Rock magnetic and environmental magnetic methods have significant roles in these lacustrine studies. However, lacustrine sedimentary environments are complex and magnetic mineral signatures can be altered by post-depositional processes. This study applies isothermal remanent magnetization (IRM) component unmixing methods to lacustrine sediments from the Heqing core, to identify and quantify magnetic mineral components. We analyzed 104 samples based on lithological variations and magnetic susceptibility (χ) to examine the composition of magnetic minerals and their relative contributions. Three distinct magnetic components were identified in IRM component unmixing results: a low-coercivity detrital component, a medium-coercivity authigenic component, and a hard magnetic component. Based on rock magnetic results, the medium-coercivity component was attributed to greigite. These components exhibit stratigraphic trends that reflect changes in paleoenvironmental conditions. The medium-coercivity component shows an upwards decrease, indicating a significant change in ISM science at about 1.8 Ma. The study highlights the importance of considering post-depositional processes when interpreting magnetic mineral signatures in lacustrine sediments. The CLG model, combined with conventional rock magnetic analyses, provides a rapid approach for characterizing magnetic assemblages in weakly magnetic sediments.