Marcasite Formation Research Articles

Biogeochemistry and element speciation in sapropel from freshwater Lake Dukhovoe (East Baikal region, Russia)

This paper presents results of biogeochemical investigation of the Holocene organic-rich sapropelic sediments of Lake Dukhovoe situated not far from the Baikal. The main source of organic matter in Lake Dukhovoe is phytoplankton, which serves as a biogeochemical barrier where elements are accumulated from water and then buried in the bottom sediments. Microorganisms (e.g., bacteria, micromycete and actinomycetes) play significant role in decomposition of organic matter and formation of organic-mineral complexes in four distinguished types of sapropels: organic, organic-mineral, mineral-organic and mineral. Diagenetic transformations of the Lake Dukhovoe sapropels under influence of mechanical, biochemical, microbiological and physicochemical processes lead to transformation of element speciation. A boundary between oxidizing and reducing diagenesis conditions is at the depth of 167 cm in the sapropel core. Change of the conditions forms geochemical barrier in bottom sediments and control the formation of secondary phases (organic-mineral complexes, diatomite, framboidal pyrite, vivianite, carbonates, etc.) as well as processes of accumulation and leaching of chemical elements. Additionally, high contents of Fe, Mn, Cu, Zn, Pb, Cd, and Mo in the upper 10 cm of the sapropel might have been caused by forest fires and anthropogenic factors. The fact of marcasite formation in the stomatocysts of Chrysophyte algae (golden algae) is new and was established for the first time. Based on the results and geochemical equilibrium modelling, the stomatocysts of Chrysophyte algae can be considered as a microreactor for iron sulfide formation.

Rapid Marcasite to Pyrite Transformation in Acidic Low-Temperature Hydrothermal Fluids and Saturation Index Control on FeS2 Precipitation Dynamics and Phase Selection

Pyrite is the most abundant metal sulfide in the Earth’s crust and is also found on Mars. It can form by direct hydrothermal precipitation or by polymorphic phase transformation from marcasite. However, the control on the dynamics of hydrothermal pyrite precipitation is poorly understood, and the kinetics of the hydrothermal transformation from marcasite to pyrite is unknown. To address these issues, we quantified pyrite and marcasite formation in hydrothermal fluids at pH 1, using pyrrhotite as a precursor mineral to produce supersaturated solutions. In situ powder X-ray diffraction experiments at 190 °C showed that hydrothermal fluids rich in ΣS(-II) (0.2 molal) favored the precipitation of nanocrystal pyrite (∼20 nm) due to a high saturation index (>105), while ΣS(-II)-free fluids produced a mixture of marcasite and pyrite nanocrystals (21–46 nm) due to a low saturation index (<104). The fluid/rock ratio (70 and 120 g/g at 210 °C) can also affect the saturation indices of these fluids, influencing nucleation and crystal growth dynamics of pyrite and marcasite and resulting in complex evolution of crystallite size, phase abundance, and the pyrite/marcasite ratio. Ex situ hydrothermal experiments at 210 °C showed rapid transformation from marcasite to pyrite, with around 95% marcasite being transformed to pyrite in 20 weeks, compared to more than 6.3 million years at 210 °C under dry conditions based on extrapolation from previous kinetic studies. These results suggest that saturation index influences hydrothermal precipitation dynamics and controls phase selection between pyrite and marcasite and that marcasite may not survive over geological time in low-temperature (<200 °C), water-saturated environments.

Effects of Charge Compensation on the Thermoelectric Properties of (La1-zCez)0.8Fe4-xCoxSb12 Skutterudites

La/Ce-partially double-filled and Co-charge-compensated (La1−zCez)0.8Fe4−xCoxSb12 skutterudites were synthesized, and their thermoelectric properties were studied by varying the filling ratio and charge compensation. X-ray diffraction analysis revealed that the matrix phase was skutterudite and a secondary phase was determined to the marcasite FeSb2. However, the formation of marcasite could be inhibited by increasing the Co content. Rare-earth antimonides, including LaSb2 and CeSb2, which were formed in fully filled La1−zCezFe4−xCoxSb12, were not found after La/Ce partial filling. La/Ce filling and Co substitution were confirmed by the decrease in lattice constants, from 0.9137 to 0.9099 nm, with increasing Ce and Co contents. Electrical conductivity showed negative temperature dependence, indicating metallic or degenerate semiconductor characteristics. Intrinsic conduction resulted in the maximum Seebeck coefficient at temperatures between 723 and 823 K. As the Co-substitution and Ce-filling contents increased, the Seebeck coefficient increased, while electrical and thermal conductivities decreased. This was considered to be due to difference in the valences of La&lt;sup&gt;3+&lt;/sup&gt; and Ce&lt;sup&gt;3+/4+&lt;/sup&gt; and the increase in carrier concentration caused by Co charge compensation. However, because they had similar atomic masses and ionic radii, the effects of the La/Ce filling ratio were not significant. Instead, Co charge compensation had the dominant effect on thermoelectric properties. The maximum Seebeck coefficient of 165.4 µVK&lt;sup&gt;-1&lt;/sup&gt; was obtained for (La0.25Ce0.75)0.8Fe3CoSb12 at 823 K, and the highest electrical conductivity of 2.27 × 10&lt;sup&gt;5&lt;/sup&gt; S m&lt;sup&gt;-1&lt;/sup&gt; was achieved for (La0.75Ce0.25)0.8Fe4Sb12. (La0.25Ce0.75)0.8Fe3CoSb12 exhibited the lowest thermal conductivity of 2.15 W m&lt;sup&gt;-1&lt;/sup&gt;K&lt;sup&gt;-1&lt;/sup&gt; at 523 K and (La0.75Ce0.25)0.8Fe3.5Co0.5Sb12 showed the highest power factor of 2.53 mW m&lt;sup&gt;-1&lt;/sup&gt; K&lt;sup&gt;-2&lt;/sup&gt; at 723 K. The maximum dimensionless figure of merit, ZTmax = 0.71, was achieved at 723 K for (La0.75Ce0.25)0.8Fe3CoSb12.

On the origin of a phosphate enriched interval in the Chattanooga Shale (Upper Devonian) of Tennessee—A combined sedimentologic, petrographic, and geochemical study

The Devonian Chattanooga Shale contains an uppermost black shale interval with dispersed phosphate nodules. This interval extends from Tennessee to correlative strata in Kentucky, Indiana, and Ohio and represents a significant period of marine phosphate fixation during the Late Devonian of North America. It overlies black shales that lack phosphate nodules but otherwise look very similar in outcrop. The purpose of this study is to examine what sets these two shales apart and what this difference tells us about the sedimentary history of the uppermost Chattanooga Shale.In thin section, the lower black shales (PBS) show pyrite enriched laminae and compositional banding. The overlying phosphatic black shales (PhBS) are characterized by phosbioclasts, have a general banded to homogenized texture with reworked layers, and show well defined horizons of phosphate nodules that are reworked and transported. In the PhBS, up to 8000 particles of P-debris per cm2 occur in reworked beds, whereas the background black shale shows between 37–88 particles per cm2. In the PBS, the shale matrix contains between 8–16 phosphatic particles per cm2. The shale matrix in the PhBS contains 5.6% inertinite, whereas just 1% inertinite occurs in the PBS. The shale matrix in both units is characterized by flat REE patterns (shale-normalized), whereas Phosbioclast-rich layers in the PhBS show high concentrations of REEs and enrichment of MREEs. Negative Ce-anomalies are common to all samples, but are best developed in association with Phosbioclasts. Redox-sensitive elements (Co, U, Mo) are more strongly enriched in the PBS when compared to the PhBS. Trace elements associated with organic matter (Cu, Zn, Cd, Ni) show an inverse trend of enrichment.Deposited atop a sequence boundary that separates the two shale units, the PhBS unit represents a transgressive systems tract and probably was deposited in shallower water than the underlying PBS interval. The higher phosphate content in the PhBS is interpreted as the result of a combination of lower sedimentation rates with reworking/winnowing episodes. Three types of phosphatic beds that reflect different degrees of reworking intensity are observed. Strong negative Ce anomalies and abundant secondary marcasite formation in the PhBS suggests improved aeration of the water column, and improved downward diffusion of oxygen into the sediment. The associated oxidation of previously formed pyrite resulted in a lowering of pore water pH and forced dissolution of biogenic phosphate. Phosphate dissolution was followed by formation of secondary marcasite and phosphate. Repeated, episodic reworking caused repetitive cycles of phosphatic dissolution and reprecipitation, enriching MREEs in reprecipitated apatite. A generally “deeper” seated redox boundary favored P-remineralization within the sediment matrix, and multiple repeats of this process in combination with wave and current reworking at the seabed led to the formation of larger phosphatic aggregates and concentration of phosphate nodules in discrete horizons.

Mineralogical and sulphur isotopic evidence for the influence of sulphatereducing and -disproportionating bacteria on pyrite and marcasite formation in the Georgia kaolins

AbstractThe Georgia, USA, kaolins contain three major occurrences of Fe-sulphide minerals, namely (1) in burrow structures, (2) as nodules associated with lignite and (3) as finely disseminated material. We have used sulphur isotope analysis and X-ray powder diffraction to understand these three occurrences in order to gain insight into the diagenetic conditions and microbial influences affecting the kaolin mineralogy. Burrows and nodules are dominated by corroded marcasite with thin pyrite overgrowths. The δ34S composition of each sulphide sample has internal fractionations (δ34Shigh – δ34Slow) ranging from 22‰ to 67‰. The secondary pyrite phase tends to be the most 34S depleted, with a low δ34S value of –47‰, whereas marcasite displays a δ34S range from 30‰ to +33‰. Early acidic environments from organic acids could have facilitated the initial marcasite crystallization, with a pH increase during later diagenesis producing pyrite overgrowths. Sulphide formation would have continued until burial restricted the diffusion of sea-water sulphate. At this point, the metabolism of sulphate-reducing bacteria would have been retarded, producing δ34S values ranging from –30‰ (open sulphate supply) to +40‰ (highly restricted sulphate supply). Under the nutrient-starved conditions of later-diagenesis, disproportionate sulphur-reducing bacteria would likely have become more dominant, yielding additional δ34Ssulphide fractionation and producing the observed fractionation (δ34Ssulphate – δ34Ssulphide) values of +67‰.

Marcasite in Black Shales--a Mineral Proxy for Oxygenated Bottom Waters and Intermittent Oxidation of Carbonaceous Muds

Abstract Black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions. Although bottom-water anoxia favors deposition of black shales, certain biological indicators, such as benthic fossils and bioturbation, suggest that bottom-water conditions may not have been as persistently anoxic as commonly presumed. Examination of twelve marine carbonaceous shale-bearing units, ranging in age from Middle Proterozoic to Cretaceous, suggests that in addition to paleontological clues, early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen-bearing bottom waters. Marcasite requires low pH values to form, and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition. In the studied examples, degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments, and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low pH values required for marcasite formation. Thus, observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts. Pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed. The latter consideration may, for example, prove important for those instances where a black shale is associated with an oceanic anoxic event (OAE), because these are commonly presumed to have been deposited under fully anoxic conditions.

An experimental study of the mechanism of the replacement of magnetite by pyrite up to 300 °C

We present the results of an experimental study into the sulfidation of magnetite to form pyrite/marcasite under hydrothermal conditions (90–300 °C, vapor saturated pressures), a process associated with gold deposition in a number of ore deposits. The formation of pyrite/marcasite was studied as a function of reaction time, temperature, pH, sulfide concentration, solid-weight-to-fluid-volume ratio, and geometric surface area of magnetite in polytetrafluoroethylene-lined autoclaves (PTFE) and a titanium and stainless steel flow-through cell. Marcasite was formed only at pH 21°C <4 and was the dominant Fe disulfide at pH 21°C 1.11, while pyrite predominated at pH 21°C >2 and formed even under basic conditions (up to pH 21°C 12–13). Marcasite formation was favored at higher temperatures. Fine-grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite (grain size <125 μm). This pyrrhotite eventually gave way to pyrite. The transformation rate of magnetite to Fe disulfide increased with decreasing pH (at 120 °C; pH 120°C 0.96–4.42), and that rate of the transformation increased from 120 to 190 °C. Scanning electron microscope (SEM) imaging revealed that micro-pores (0.1–5 μm scale) existed at the reaction front between the parent magnetite and the product pyrite, and that the pyrite and/or marcasite were euhedral at pH 21°C <4 and anhedral at higher pH. The newly formed pyrite was micro-porous (0.1–5 μm); this micro-porosity facilitates fluid transport to the reaction interface between magnetite and pyrite, thus promoting the replacement reaction. The pyrite precipitated onto the parent magnetite was polycrystalline and did not preserve the crystallographic orientation of the magnetite. The pyrite precipitation was also observed on the PTFE liner, which is consistent with pyrite crystallizing from solution. The mechanism of the reaction is that of a dissolution–reprecipitation reaction with the precipitation of pyrite being the rate-limiting step relative to magnetite dissolution under mildly acidic conditions (e.g., pH 155°C 4.42). The experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized Banded Iron Formations: pyrite, marcasite and pyrrhotite have been found to exist or co-exist in different sulfidized Banded Iron Formations, and the microtextures show no evidence of sub-μm-scale pseudomorphism of magnetite by pyrite.

Oxidation of detrital pyrite as a cause for Marcasite Formation in marine lag deposits from the Devonian of the eastern US

Late Devonian black shale successions in the eastern US contain numerous discontinuities that are characterized by lags containing variable amounts of quartz sand mixed with reworked pyritic and phosphatic debris. These lags can be partially cemented by iron sulfides with a radial fibrous to massive morphology. Examination of iron sulfide cements via electron backscatter diffraction (EBSD) shows that these cements can consist entirely or partially of marcasite. It is proposed here that oxidation and dissolution of reworked pyrite debris led to favorable conditions for intermittent rapid marcasite formation by forcing a substantial pH drop in the surface sediment and by raising dissolved iron concentrations in the pore waters.

Epitaxial overgrowths of marcasite on pyrite from the Tunnel and Reservoir Project, Chicago, Illinois, USA: Implications for marcasite growth

An unusual occurrence of marcasite epitaxially overgrown on pyrite was found along fractures in the Racine Dolomite, in the main tunnel of the Des Plaines unit of the Chicago Tunnel and Reservoir Project. The marcasite occurs as thin tabular crystals, up to 300 μm on edge, dominated by {010}. These grow around the [001] zone of pyrite octahedra such that {100} [001] pyrite | {101} [010] marcasite. Development of overgrowth varies from initial marcasite formation to complete coverage of the pyrite. The marcasite apparently grows around the pyrite octahedra by a layer growth mechanism as evidenced by macrosteps terminating partially developed layers on the marcasite {010}. Other forms present on the marcasite are {110}, {011}, {101}, {001} , and {100}. Thermodynmically, pyrite is the stable form of FeS 2. However, it is well documented that FeS 2 crystallization from solutions with pH less than 5 results in marcasite. These samples show that marcasite can directly precipitate in an epitaxial relationship onto a pyrite substrate. It is suggested that the epitaxy observed in this study might be initiated through a reconstruction of the surface most layers of the {100} face of pyrite to a configuration closely resembling the marcasite structure, while maintaining the common feature, FeSSFe … periodic bond chains. This proposed reconstruction of the pyrite surface into a marcasite-like arrangement, rather than simply continued growth of the thermodynamically stable phase (pyrite), may he due to interactions of a protonated disulfide species with the pyrite during growth.

Mechanisms of pyrite and marcasite formation from solution: III. Hydrothermal processes

The formation of pyrite and marcasite from solutions between 100 and 300°C has been examined experimentally, and the solubility product for the initial precipitate upon mixing of Fe2+ and H2S solutions has been determined.Below 300°C, pyrite and marcasite form via an FeS precursor. The precursor phase is crystalline, nearly stoichiometric FeS with a solubility product of 102.9±0.2 (K = aFe2+aH2S(aH+)2). It reacts progressively to mackinawite, hexagonal pyrrhotite, and/orgreigite before forming pyrite or marcasite. In the presence of elemental sulfur, thiosulfate, or polysulfides, the rate of reaction is extremely fast (minutes at 150°C). Without these sulfur species and only hydrogen sulfide or bisulfide present, the conversion rate drops significantly ( 1 h at 300°C). In the absence of any aqueous sulfur source, the conversion proceeds at an indetectably slow rate.Because the rate of direct FeS2 nucleation is insignificant in (slightly) acidic solutions below 300°C, most pyrite and marcasite in hydrothermal ores form via the conversion of an FeS precursor.

Reactions forming pyrite and marcasite from solution: II. Via FeS precursors below 100°C

The formation of pyrite and marcasite from a FeS precursor has been examined experimentally. In aging experiments at 65°C, the conversion of precursor amorphous FeS depends on these geologically relevant variables: concentration of sulfur-contributing species, acidity, redox state, time, Fe(II)/S(−II) ratio in solution, and addition of an organic ligand (citrate). The results indicate that pyrite and marcasite formation proceed at a significant rate only if intermediate sulfur species (i.e., polysulfides, polythionates, or thiosulfate) are present in solution. In the absence of any sulfur contributor or with only hydrogen sulfide or bisulfide present, no FeS 2 formed within 16 days. Sulfidation of the precursor proceeds through progressively more sulfur-rich, Fe-S phases: am FeS (Fe 1.11S-Fe 1.09S) → mackinawite (FeS 0.93-FeS 0.96) → greigite (Fe 3S 4) → pyrite/marcasite FeS 2. Greigite is absent under very reduced environments. The conversion sequence found in this study is in good agreement with iron-sulfide distribution patterns found in modern marine sediments.

Kinetic paths for low temperature pyrite and marcasite formation from solution

Kinetic paths for low temperature pyrite and marcasite formation from solution

Electrochemical Formation of Sulfur at Stainless Steel Surfaces

The electrochemical formation of elemental sulfur at stainless steel electrodes has been studied in aqueous solutions at temperatures between 298° and 373°K and a total pressure (with ) of 1.4 MPa. A deposit of sulfur was found only in solutions containing a low concentration of soluble iron (≲ 10−6 mole dm−3); at higher iron concentrations, pyrite and marcasite formation occurred. All three deposits formed from a common polysulfide intermediate, the nature of the deposit depending on the relative concentrations of soluble iron and polysulfide at the surface of the stainless steel.

Marcasite in Roll-Type Uranium Deposits: ABSTRACT

Iron disulfide minerals occur in anomalously high concentrations in uranium-bearing zones of roll-type uranium deposits and are thus important components of the geochemical system that governs formation of these uranium orebodies. These high concentrations have been interpreted by previous workers as resulting from an ore-stage generation of sulfide superimposed upon a pre-ore generation. We have examined petrographically over 500 polished sections from five roll-type deposits, three from Texas and two in Wyoming, and have recognized both pre-ore and ore-stage generations of iron disulfides in each deposit. The two generations may be distinguished by textural relations. In each deposit, marcasite is present as an ore-stage sulfide; in End_Page 514------------------------------ two Texas deposits (one in Webb County, the other in Live Oak County), it is the dominant ore-stage sulfide. This ore-stage marcasite occurs as intergrowths within and overgrowths on uranium-bearing phases and in close association with ferroselite near the redox boundary. Ore-stage marcasite occurs commonly as overgrowths on pre-ore sulfides that are dominantly pyrite both in ore and in a more extensive halo around the altered tongue. In two of the Texas deposits, ore-stage marcasite is present as much as 200 m (Webb County) and 400 m (Live Oak County) downdip from the roll front. Because of the close association of marcasite and uranium mineralization, understanding the conditions that lead to marcasite precipitation enables clearer determination of the geochemical environment of ore deposition. Kinetic factors are shown to favor marcasite over pyrite and we suggest that undersaturation with monosulfide-type phases such as mackinaurite and greigite are a prerequisite for marcasite formation. End_of_Article - Last_Page 515------------

Effect of temperature and acidity in the formation of marcasite (FeS 2 ) and wurtzite (ZnS); a contribution to the genesis of unstable forms

Effect of temperature and acidity in the formation of marcasite (FeS 2 ) and wurtzite (ZnS); a contribution to the genesis of unstable forms