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Andrew P. Roberts - One of the best experts on this subject based on the ideXlab platform.
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Micromagnetic simulations of first-order reversal curve (FORC) diagrams of framboidal Greigite
Geophysical Journal International, 2020Co-Authors: Miguel A. Valdez-grijalva, Andrew P. Roberts, Lesleis Nagy, Adrian R. Muxworthy, Wyn Williams, David HeslopAbstract:SUMMARY Greigite is a sensitive environmental indicator and occurs commonly in nature as magnetostatically interacting framboids. Until now only the magnetic response of isolated non-interacting Greigite particles have been modelled micromagnetically. We present here hysteresis and first-order reversal curve (FORC) simulations for framboidal Greigite (Fe3S4), and compare results to those for isolated particles of a similar size. We demonstrate that these magnetostatic interactions alter significantly the framboid FORC response compared to isolated particles, which makes the magnetic response similar to that of much larger (multidomain) grains. We also demonstrate that framboidal signals plot in different regions of a FORC diagram, which facilitates differentiation between framboidal and isolated grain signals. Given that large Greigite crystals are rarely observed in microscopy studies of natural samples, we suggest that identification of multidomain-like FORC signals in samples known to contain abundant Greigite could be interpreted as evidence for framboidal Greigite.
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Identification and environmental interpretation of diagenetic and biogenic Greigite in sediments: A lesson from the Messinian Black Sea
Geochemistry Geophysics Geosystems, 2014Co-Authors: Liao Chang, Andrew P. Roberts, Wout Krijgsman, Mark J. Dekkers, Iuliana Vasiliev, Christiaan G.c. Van Baak, John D. Fitz Gerald, Annelies Van Hoesel, Michael WinklhoferAbstract:Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of Greigite magnetofossils in the geological record is only poorly constrained. Here we investigate Mio-Pliocene sediments of the former Black Sea to test whether we can detect Greigite magnetofossils and to unravel potential environmental controls on Greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial Greigite, and suggest a potentially widespread preservation of Greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by Greigite. TEM-based chemical and structural analyses also indicate the common presence of nickel-substituted diagenetic iron sulfide crystals with a ferrimagnetic Greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic Greigite enhancements appear to be climatically controlled, with Greigite mainly occurring in warm/wet periods. Diagenetic Greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of Greigite for studying long-term climate variability in anoxic environments. Key Points: We provide evidence for the presence of biogenic Greigite in ancient sediments Diagenetic Greigite enhancements are climatically controlled Greigite is a paleoenvironmental indicator in anoxic environments
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critical single domain grain sizes in chains of interacting Greigite particles implications for magnetosome crystals
Geochemistry Geophysics Geosystems, 2013Co-Authors: Adrian R. Muxworthy, Andrew P. Roberts, Liao Chang, Michael Winklhofer, Wyn Williams, Mihaly PosfaiAbstract:Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosomes), which aid navigation (magnetotaxis). To improve the efficiency of magnetotaxis, magnetosome crystals (which can consist of magnetite or Greigite) should be magnetically stable single domain (SD) particles. Larger particles subdivide into nonuniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic (SP) behavior. In this study, we determined the stable SD range as a function of grain elongation and interparticle separation for chains of identical Greigite grains using fundamental parameters recently determined for Greigite. Interactions significantly increase the stable SD range. For example, for cube-shaped Greigite grains the upper stable SD threshold size is increased from 107 nm for isolated grains to 204 nm for touching grains arranged in chains. The larger critical SD grain size for Greigite means that, compared to magnetite magnetosomes, Greigite magnetosomes can produce larger magnetic signals without the need for intergrain interactions.
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ENIGMATIC X-RAY MAGNETIC CIRCULAR DICHROISM IN Greigite (Fe3S4)
The Canadian Mineralogist, 2012Co-Authors: Liao Chang, Richard A. D. Pattrick, Gerrit Van Der Laan, Victoria S. Coker, Andrew P. RobertsAbstract:Greigite (Fe3S4), a widely occurring iron thiospinel, was investigated using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XAS and XMCD spectra were recorded at the Fe L2,3 edges for pure synthetic and natural Greigite samples. At the Fe L3 edge, the XAS spectra reveal two main absorption peaks at 707.2 and 708.6 eV, which are interpreted to originate from Greigite and an oxidized surface layer on Greigite crystals. The XMCD spectra, which are dominated by a Greigite signal, contain three peaks at 705.3, 706.2, and 707.7 eV, all with the same sign. The expectation is that the spectrum would have two negative peaks representing Fe2+ and Fe3+ in octahedral coordination, and a positive peak representing Fe3+ in tetrahedral coordination, as found in stoichiometric magnetite (Fe3O4). A reasonable fit of the XMCD data can be achieved without the tetrahedral Fe component, which contradicts magnetic structural information provided by neutron diffraction analysis, and uses unreasonable parameters. The conundrum between theory and experimental data may be caused by the strong covalent effect in sulfides, which causes broadening of the hybridized XMCD peaks and also indicates that multiplet calculations cannot fully predict the properties of Greigite. Our results indicate covalent 3d states in Greigite, which can destroy the half-metallicity that is present in magnetite. Our measurements represent the best available XAS and XMCD spectra for Greigite, but further experimental and modeling information are needed to explain the observed XMCD spectra and to understand what it represents in terms of electronic and magnetic structure. This is important because Greigite widely contributes to the magnetic properties of sedimentary rocks.
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Magnetic properties of sedimentary Greigite (Fe3S4): An update
Reviews of Geophysics, 2011Co-Authors: Andrew P. Roberts, Liao Chang, Chorng-shern Horng, Christopher J. Rowan, Fabio FlorindoAbstract: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.
Liao Chang - One of the best experts on this subject based on the ideXlab platform.
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Identification and environmental interpretation of diagenetic and biogenic Greigite in sediments: A lesson from the Messinian Black Sea
Geochemistry Geophysics Geosystems, 2014Co-Authors: Liao Chang, Andrew P. Roberts, Wout Krijgsman, Mark J. Dekkers, Iuliana Vasiliev, Christiaan G.c. Van Baak, John D. Fitz Gerald, Annelies Van Hoesel, Michael WinklhoferAbstract:Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of Greigite magnetofossils in the geological record is only poorly constrained. Here we investigate Mio-Pliocene sediments of the former Black Sea to test whether we can detect Greigite magnetofossils and to unravel potential environmental controls on Greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial Greigite, and suggest a potentially widespread preservation of Greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by Greigite. TEM-based chemical and structural analyses also indicate the common presence of nickel-substituted diagenetic iron sulfide crystals with a ferrimagnetic Greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic Greigite enhancements appear to be climatically controlled, with Greigite mainly occurring in warm/wet periods. Diagenetic Greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of Greigite for studying long-term climate variability in anoxic environments. Key Points: We provide evidence for the presence of biogenic Greigite in ancient sediments Diagenetic Greigite enhancements are climatically controlled Greigite is a paleoenvironmental indicator in anoxic environments
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On the magnetocrystalline anisotropy of Greigite (Fe3S4)
Geochemistry Geophysics Geosystems, 2014Co-Authors: Michael Winklhofer, Liao Chang, Stephan H. K. EderAbstract:The ferrimagnetic mineral Greigite (cubic Fe3S4) is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments. Due to the lack of suitable single-crystal specimens, the magnetic anisotropy parameters of Greigite have remained poorly constrained, to the point where not even the easy axis of magnetization is known. Here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance (FMR) powder spectroscopy on hydrothermally synthesized, chemically pure Greigite microcrystals dispersed in a nonmagnetic matrix. In terms of easy axis orientations, the FMR data are consistent with or , or less likely, a more general type. With a g factor of 2.09, the anisotropy field is about 90 mT and in some samples may reach 125 mT, compared to 30 mT for cubic magnetite. This confirms the dominating role of cubic anisotropy on the magnetic properties of Greigite, which we show to be responsible for large SIRM/k values. K1 is in the range −15 … −23 J/m3 ( ) or +10 … +15 kJ/m3 ( ), yielding upper limits of 44 or 34 nm for the superparamagnetic grain size, respectively.
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critical single domain grain sizes in chains of interacting Greigite particles implications for magnetosome crystals
Geochemistry Geophysics Geosystems, 2013Co-Authors: Adrian R. Muxworthy, Andrew P. Roberts, Liao Chang, Michael Winklhofer, Wyn Williams, Mihaly PosfaiAbstract:Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosomes), which aid navigation (magnetotaxis). To improve the efficiency of magnetotaxis, magnetosome crystals (which can consist of magnetite or Greigite) should be magnetically stable single domain (SD) particles. Larger particles subdivide into nonuniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic (SP) behavior. In this study, we determined the stable SD range as a function of grain elongation and interparticle separation for chains of identical Greigite grains using fundamental parameters recently determined for Greigite. Interactions significantly increase the stable SD range. For example, for cube-shaped Greigite grains the upper stable SD threshold size is increased from 107 nm for isolated grains to 204 nm for touching grains arranged in chains. The larger critical SD grain size for Greigite means that, compared to magnetite magnetosomes, Greigite magnetosomes can produce larger magnetic signals without the need for intergrain interactions.
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ENIGMATIC X-RAY MAGNETIC CIRCULAR DICHROISM IN Greigite (Fe3S4)
The Canadian Mineralogist, 2012Co-Authors: Liao Chang, Richard A. D. Pattrick, Gerrit Van Der Laan, Victoria S. Coker, Andrew P. RobertsAbstract:Greigite (Fe3S4), a widely occurring iron thiospinel, was investigated using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XAS and XMCD spectra were recorded at the Fe L2,3 edges for pure synthetic and natural Greigite samples. At the Fe L3 edge, the XAS spectra reveal two main absorption peaks at 707.2 and 708.6 eV, which are interpreted to originate from Greigite and an oxidized surface layer on Greigite crystals. The XMCD spectra, which are dominated by a Greigite signal, contain three peaks at 705.3, 706.2, and 707.7 eV, all with the same sign. The expectation is that the spectrum would have two negative peaks representing Fe2+ and Fe3+ in octahedral coordination, and a positive peak representing Fe3+ in tetrahedral coordination, as found in stoichiometric magnetite (Fe3O4). A reasonable fit of the XMCD data can be achieved without the tetrahedral Fe component, which contradicts magnetic structural information provided by neutron diffraction analysis, and uses unreasonable parameters. The conundrum between theory and experimental data may be caused by the strong covalent effect in sulfides, which causes broadening of the hybridized XMCD peaks and also indicates that multiplet calculations cannot fully predict the properties of Greigite. Our results indicate covalent 3d states in Greigite, which can destroy the half-metallicity that is present in magnetite. Our measurements represent the best available XAS and XMCD spectra for Greigite, but further experimental and modeling information are needed to explain the observed XMCD spectra and to understand what it represents in terms of electronic and magnetic structure. This is important because Greigite widely contributes to the magnetic properties of sedimentary rocks.
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Magnetic properties of sedimentary Greigite (Fe3S4): An update
Reviews of Geophysics, 2011Co-Authors: Andrew P. Roberts, Liao Chang, Chorng-shern Horng, Christopher J. Rowan, Fabio FlorindoAbstract: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.
Chorng-shern Horng - One of the best experts on this subject based on the ideXlab platform.
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Magnetic properties of sedimentary Greigite (Fe3S4): An update
Reviews of Geophysics, 2011Co-Authors: Andrew P. Roberts, Liao Chang, Chorng-shern Horng, Christopher J. Rowan, Fabio FlorindoAbstract: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.
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Low‐temperature magnetic properties of Greigite (Fe3S4)
Geochemistry Geophysics Geosystems, 2009Co-Authors: Liao Chang, Andrew P. Roberts, Yan Tang, Qianwang Chen, Christopher J. Rowan, Petr Pruner, Chorng-shern HorngAbstract:[1] We provide comprehensive low-temperature magnetic results for Greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that Greigite has no low-temperature magnetic transitions, but we also document that it has strong domain-state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single-domain Greigite undergoes little change in magnetic properties below room temperature. For pseudo-single-domain (PSD)/MD Greigite, hysteresis properties and first-order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low-temperature demagnetization is grain size dependent for PSD/MD Greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural Greigite samples, which are probably associated with surficial oxidation. Low-temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low-temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of Greigite-bearing sediments.
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low temperature magnetic properties of Greigite fe3s4
Geochemistry Geophysics Geosystems, 2009Co-Authors: Liao Chang, Andrew P. Roberts, Yan Tang, Qianwang Chen, Christopher J. Rowan, Petr Pruner, Chorng-shern HorngAbstract:[1] We provide comprehensive low-temperature magnetic results for Greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that Greigite has no low-temperature magnetic transitions, but we also document that it has strong domain-state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single-domain Greigite undergoes little change in magnetic properties below room temperature. For pseudo-single-domain (PSD)/MD Greigite, hysteresis properties and first-order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low-temperature demagnetization is grain size dependent for PSD/MD Greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural Greigite samples, which are probably associated with surficial oxidation. Low-temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low-temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of Greigite-bearing sediments.
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carbon sulfur iron relationships in sedimentary rocks from southwestern taiwan influence of geochemical environment on Greigite and pyrrhotite formation
Chemical Geology, 2004Co-Authors: Shuhji Kao, Andrew P. Roberts, Chorng-shern Horng, Konkee LiuAbstract:The importance of the magnetic iron sulfide minerals, Greigite (Fe3S4) and pyrrhotite (Fe7S8), is often underappreciated in geochemical studies because they are metastable with respect to pyrite (FeS2). Based on magnetic properties and X-ray diffraction analysis, previous studies have reported widespread occurrences of these magnetic minerals along with magnetite (Fe3O4) in two thick Plio-Pleistocene marine sedimentary sequences from southwestern Taiwan. Different stratigraphic zones were classified according to the dominant magnetic mineral assemblages (Greigite-, pyrrhotite-, and magnetite-dominated zones). Greigite and pyrrhotite are intimately associated with fine-grained sediments, whereas magnetite is more abundant in coarse-grained sediments. We measured total organic carbon (TOC), total sulfur (TS), total iron (FeT), 1N HCl extractable iron (FeA), and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals. The studied sediments have low TS/FeA weight ratios (0.03-0.2), far below that of pyrite (1.15), which indicates that an excess of reactive iron was available for pyritization. Observed low TS (0.05-0.27%) is attributed to the low organic carbon contents (TOC=0.25-0.55%), which resulted from dilution by rapid terrigenous sedimentation. The fine-grained sediments also have the highest FeT and FeA values. We suggest that under conditions of low organic carbon provision, the high iron activity in the fine-grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded, which, in turn, favored preservation of the intermediate magnetic iron sulfides. The relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous FeS into Greigite or into pyrrhotite. Compared to pyrrhotite-dominated sediments, Greigite-dominated sediments are finer-grained and have higher FeA but lower TS. We suggest that diagenetic environments with higher supply of reactive iron, lower supply of labile organic matter, and, consequently, lower sulfide concentration result in relatively high Eh conditions, which favor formation of Greigite relative to pyrrhotite
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Contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic Greigite
Earth and Planetary Science Letters, 2001Co-Authors: Wei Teh Jiang, Andrew P. Roberts, Chorng-shern Horng, Donald R. PeacorAbstract:Abstract In several recent published studies, paleomagnetic results from Greigite-bearing sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observations have important implications for the reliability of paleomagnetic data from Greigite-bearing sediments. We have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of Greigite in mudstones from the Lower Gutingkeng Formation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng Greigite has three modes of occurrence, including nodular, framboidal and matrix Greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolved edges around detrital and early diagenetic phases, and neoformation of Greigite and Fe-rich clays around detrital phyllosilicates, indicate that all three types of Greigite have a diagenetic origin that post-dates early diagenetic pyrite. In addition, paleomagnetic data yield contradictory polarities even for Greigite-bearing sister samples from the same stratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng Greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of Greigite formation can apparently vary enough to give contradictory polarities for different Greigite components even within a single stratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from Greigite-bearing sediments.
Christopher J. Rowan - One of the best experts on this subject based on the ideXlab platform.
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Magnetic properties of sedimentary Greigite (Fe3S4): An update
Reviews of Geophysics, 2011Co-Authors: Andrew P. Roberts, Liao Chang, Chorng-shern Horng, Christopher J. Rowan, Fabio FlorindoAbstract: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.
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Low‐temperature magnetic properties of Greigite (Fe3S4)
Geochemistry Geophysics Geosystems, 2009Co-Authors: Liao Chang, Andrew P. Roberts, Yan Tang, Qianwang Chen, Christopher J. Rowan, Petr Pruner, Chorng-shern HorngAbstract:[1] We provide comprehensive low-temperature magnetic results for Greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that Greigite has no low-temperature magnetic transitions, but we also document that it has strong domain-state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single-domain Greigite undergoes little change in magnetic properties below room temperature. For pseudo-single-domain (PSD)/MD Greigite, hysteresis properties and first-order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low-temperature demagnetization is grain size dependent for PSD/MD Greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural Greigite samples, which are probably associated with surficial oxidation. Low-temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low-temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of Greigite-bearing sediments.
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reductive diagenesis magnetite dissolution Greigite growth and paleomagnetic smoothing in marine sediments a new view
Earth and Planetary Science Letters, 2009Co-Authors: Christopher J. Rowan, Andrew P. Roberts, Thomas BroadbentAbstract:Abstract In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4′N, 60°08.0′E) and northern California (38°24.8′N, 123°58.2′W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide Greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP Greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by Greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of Greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of Greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis.
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low temperature magnetic properties of Greigite fe3s4
Geochemistry Geophysics Geosystems, 2009Co-Authors: Liao Chang, Andrew P. Roberts, Yan Tang, Qianwang Chen, Christopher J. Rowan, Petr Pruner, Chorng-shern HorngAbstract:[1] We provide comprehensive low-temperature magnetic results for Greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that Greigite has no low-temperature magnetic transitions, but we also document that it has strong domain-state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single-domain Greigite undergoes little change in magnetic properties below room temperature. For pseudo-single-domain (PSD)/MD Greigite, hysteresis properties and first-order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low-temperature demagnetization is grain size dependent for PSD/MD Greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural Greigite samples, which are probably associated with surficial oxidation. Low-temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low-temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of Greigite-bearing sediments.
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Magnetic characteristics of synthetic pseudo-single-domain and multi-domain Greigite (Fe3S4)
Geophysical Research Letters, 2007Co-Authors: Liao Chang, Andrew P. Roberts, Qingsong Liu, Adrian R. Muxworthy, Yan Tang, Qianwang Chen, Christopher J. Rowan, Petr PrunerAbstract:[1] We report the magnetic properties of pure synthetic pseudo-single-domain (PSD) and multi-domain (MD) Greigite and the grain size dependence of the magnetic properties of Greigite for the first time. The dominantly PSD-like and MD-like behavior are demonstrated by hysteresis, first-order reversal curve diagrams, low-temperature cycling (LTC) of room temperature saturation isothermal remanent magnetization (SIRM) and low-temperature SIRM warming curves. Variations in a range of magnetic properties clearly correlate with grain size. Characteristic PSD/MD behavior is preserved at low temperatures, which, coupled with the small decrease in remanence during warming, rule out the presence of substantial superparamagnetic behavior in the studied samples. LTC-SIRM measurements indicate a continuous demagnetization of remanence during cooling. Knowledge of this expanded range of magnetic properties of Greigite should be widely useful in environmental magnetic and paleomagnetic studies.
Mark J. Dekkers - One of the best experts on this subject based on the ideXlab platform.
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Early diagenetic Greigite as an indicator of paleosalinity changes in the middle Miocene Paratethys Sea of central Europe
Geochemistry Geophysics Geosystems, 2017Co-Authors: Suzhen Liu, Wout Krijgsman, Mark J. Dekkers, D. V. PalcuAbstract:The Miocene epicontinental Paratethys Sea of central Eurasia has experienced multiple restriction and reconnection events to the open ocean. Magnetostratigraphy is an important dating tool to better understand the temporal and spatial paleoenvironmental variations associated with these changes. Magnetostratigraphy in the Paratethys domain, however, is complicated by the presence of Greigite (Fe3S4). Here we report rock magnetic and X-ray fluorescence data of the Tisa section (Romania) which was previously magnetostratigraphically dated at the middle Miocene (base at 12.8 Ma and top at 12.2 Ma). This section comprises the Badenian Sarmatian Extinction Event (BSEE), which is marked by a major salinity change from marine to brackish environments, related to the opening of the connection between the Central and the Eastern Paratethys basins. In the marine Badenian sediments below the BSEE, the pyritization process is shown to be complete because of abundant sulfate supply. In the brackish Sarmatian deposits, four intervals with early diagenetic Greigite are observed, and linked to insufficient sulfate in the water column. These four Greigite intervals appear to correspond to maxima in the ∼100 kyr eccentricity cycle. We propose that increased fresh water from the Eastern Paratethys basin during eccentricity maxima restricted the sulfate availability in the Tisa area, leading to a reduced HS− production and enhanced Greigite preservation. The early diagenetic formation of Greigite enables a quasi syn-depositional recording of the paleomagnetic field, which allows reliable paleomagnetic dating in this section. Our results further suggest Greigite as a potential indicator for salinity changes during marine/brackish transitions.
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Identification and environmental interpretation of diagenetic and biogenic Greigite in sediments: A lesson from the Messinian Black Sea
Geochemistry Geophysics Geosystems, 2014Co-Authors: Liao Chang, Andrew P. Roberts, Wout Krijgsman, Mark J. Dekkers, Iuliana Vasiliev, Christiaan G.c. Van Baak, John D. Fitz Gerald, Annelies Van Hoesel, Michael WinklhoferAbstract:Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of Greigite magnetofossils in the geological record is only poorly constrained. Here we investigate Mio-Pliocene sediments of the former Black Sea to test whether we can detect Greigite magnetofossils and to unravel potential environmental controls on Greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial Greigite, and suggest a potentially widespread preservation of Greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by Greigite. TEM-based chemical and structural analyses also indicate the common presence of nickel-substituted diagenetic iron sulfide crystals with a ferrimagnetic Greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic Greigite enhancements appear to be climatically controlled, with Greigite mainly occurring in warm/wet periods. Diagenetic Greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of Greigite for studying long-term climate variability in anoxic environments. Key Points: We provide evidence for the presence of biogenic Greigite in ancient sediments Diagenetic Greigite enhancements are climatically controlled Greigite is a paleoenvironmental indicator in anoxic environments
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The Tortonian reference section at Monte dei Corvi (Italy): evidence for early remanence acquisition in Greigite-bearing sediments
Geophysical Journal International, 2009Co-Authors: S. K. Hüsing, Mark J. Dekkers, Christine Franke, Wout KrijgsmanAbstract:SUMMARY The reliability of primary natural remanent magnetization (NRM) signals in Greigite-bearing sediments has been frequently questioned. Here, we show that the stable NRM in the deep marine Middle to Late Miocene sediments at Monte dei Corvi, northern Italy, is mainly carried by Greigite. Combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two Greigite populations. One fine-grained and relatively well-dispersed Greigite population (grain size between 60 and 200 nm) is most likely of magnetotactic origin. The second Greigite population with larger grain sizes (typically 700 nm to 1 μm) is most likely of authigenic (bacterially mediated) origin, and is related to post-depositional sulphidization processes. Greigite is the main magnetic remanence carrier in the older part of the section (12.8 to 8.7 Ma), whereas Greigite and fine-grained (presumably magnetotactic) magnetite are present in the younger part of the section (8.7 to 6.9 Ma). Similar remanent magnetization directions of the magnetite and Greigite components, and the likelihood of a magnetotactic origin, suggests that the NRM is of syn-depositional age. We suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine Greigite. This corroborates the reliability of the previously established magnetostratigraphy at Monte dei Corvi.
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Putative Greigite magnetofossils from the Pliocene epoch
Nature Geoscience, 2008Co-Authors: Iuliana Vasiliev, Mark J. Dekkers, Christine Franke, Johannes D. Meeldijk, Cor G. Langereis, Wout KrijgsmanAbstract:Magnetotactic bacteria produce chains of magnetite^ 1 , 2 and/or Greigite^ 3 , 4 , 5 crystals within their cell bodies called magnetosomes that are permanently magnetized^ 6 . They use these magnets to navigate along geomagnetic field lines to reach their preferred habitat^ 7 . Greigite magnetosomes have been well documented in modern sedimentary environments, but their identification in the fossil record remains controversial. Here we use transmission electron microscopy, electron diffraction patterns and rockmagnetic analyses to assess the origins of nanometre-scale Greigite crystals found in Pliocene claystones from the Carpathian foredeep of Romania. We find that, like modern magnetosomal Greigite grains, the crystals are single domain^ 8 , with few crystallographic defects and an overall shape consistent with an intracellular origin. We suggest these crystals are magnetosomal in origin, which would place them among the oldest Greigite magnetofossils identified so far. The crystals also carry a primary magnetic signal, which has remained stable since its acquisition 5.3–2.6 million years ago. We suggest that Greigite magnetofossils could therefore provide reliable records of ancient geomagnetic field variations, and that they could also be used as a proxy to assess palaeoenvironmental conditions in low-oxygen sedimentary environments. Greigite crystals of bacterial origin are widespread in modern sedimentary environments, but their occurrence in the fossil record remains controversial. Grains from Romanian Pliocene-aged sediments have now been identified as bacterial in origin, tentatively placing them among the oldest known Greigite magnetofossils.
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Putative Greigite magnetofossils from the Pliocene epoch
Nature Geoscience, 2008Co-Authors: Iuliana Vasiliev, Mark J. Dekkers, Christine Franke, Johannes D. Meeldijk, Cor G. Langereis, Wout KrijgsmanAbstract:Greigite crystals of bacterial origin are widespread in modern sedimentary environments, but their occurrence in the fossil record remains controversial. Grains from Romanian Pliocene-aged sediments have now been identified as bacterial in origin, tentatively placing them among the oldest known Greigite magnetofossils.
