Magnetic properties of hydrothermally synthesized greigite (Fe3S4)- II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diVraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20-30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100-120 mT at 4 K, and the remanent coercive force increases from approximately 50-80 mT at room temperature to approximately 110-180 mT at 4 K

An electrokinetic study of synthetic greigite and pyrrhotite

The isoefectric points of synthetic greigite, Fe&, and pyrrhotite, Fe, _& in 10 -3- 1.1 10 -* M NaCl solution at or, < lop4 and as < 10W4 were determined to be pH 3 and 2, respectively. Below pH 2 for pyrrhotite and below pH 3 for greigite, the surfaces are positively charged; above these pH values the surfaces are negatively charged. Upon addition of sulfidic sulfur (H$, HS-), traces of terrestrial humic acids, or a combination of sulfidic sulfur and terrestrial humic acids, the surfaces are negatively charged over the entire pH range (2-11) studied. Because humic acid and sulfidic sulfur influence the zeta potential of the iron sulfide surfaces, they must he adsorbed within the shear plane of the double layer, close to the actual surface of the minerals. The isoelectric points for greigite and pyrrhotite in the absence of either sulfidic sulfur or humic acid are very close to those for natural pyrite (Fe&) and sphalerite (ZnS), as well as synthetic ZnS, CuS, CdS, galena (PbS), chalcopyrite (CuFeSa), and elemental sulfur. It is plausible that the speciation of thiol surface groups (ES-H i, ~--II*, =S- -) dominates the surface charge on all these phases. The increase in net zeta potential upon the addition of sulfidic sulfur is, however, not well understood

Chemistry of iron sulfides in sedimentary environments

Recent advances in understanding the chemistry of iron sulfides in sedimentary environments are beginning to shed more light on the processes involved in the global sulfur cycle. Pyrite may be formed via at least three routes including the reaction of precursor sulfides with polysulfides, the progressive solid-state oxidation of precursor iron sulfides and the oxidation of iron sulfides by hydrogen sulfide. The kinetics and mechanism of the polysulfide pathway are established and those of the H2S oxidation pathway are being investigated. Preliminary considerations suggest that the relative rates of the three pathways are H2S oxidation > polysulfide pathway much greater than solid-state oxidation. The kinetics and mechanisms of iron(II) monosulfide formation suggest the involvement of iron bisulfide complexes in the pathway and iron bisulfide complexes have now been identified by voltammetry and their stabililty constants measured. The framboidal texture commonly displayed by sedimentary pyrite appears to be an extreme example of mosaicity in crystal growth. Framboidal pyrite is produced through the H2S oxidation reaction. Frontier molecular orbital calculations are beginning to provide theoretical underpinning of the reaction mechanisms. Recent progress in understanding iron sulfide chemistry is leading to questions regarding the degree of involvement of precursor iron sulfides in the formation of pyrite in sediments. Spin-offs from the work are addressing problems relating to the involvement of iron sulfides in the origin of life, the nature of metastability, the mechanism of precipitation reactions and the use of iron sulfides in advanced materials

Chemistry of iron sulfides in sedimentary environments

Recent advances in understanding the chemistry of iron sulfides in sedimentary environments are beginning to shed more light on the processes involved in the global sulfur cycle. Pyrite may be formed via at least three routes including the reaction of precursor sulfides with polysulfides, the progressive solid-state oxidation of precursor iron sulfides and the oxidation of iron sulfides by hydrogen sulfide. The kinetics and mechanism of the polysulfide pathway are established and those of the H2S oxidation pathway are being investigated. Preliminary considerations suggest that the relative rates of the three pathways are H2S oxidation > polysulfide pathway much greater than solid-state oxidation. The kinetics and mechanisms of iron(II) monosulfide formation suggest the involvement of iron bisulfide complexes in the pathway and iron bisulfide complexes have now been identified by voltammetry and their stabililty constants measured. The framboidal texture commonly displayed by sedimentary pyrite appears to be an extreme example of mosaicity in crystal growth. Framboidal pyrite is produced through the H2S oxidation reaction. Frontier molecular orbital calculations are beginning to provide theoretical underpinning of the reaction mechanisms. Recent progress in understanding iron sulfide chemistry is leading to questions regarding the degree of involvement of precursor iron sulfides in the formation of pyrite in sediments. Spin-offs from the work are addressing problems relating to the involvement of iron sulfides in the origin of life, the nature of metastability, the mechanism of precipitation reactions and the use of iron sulfides in advanced materials

Magnetic properties of hydrothermally synthesized greigite (Fe3S4)- II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diVraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20-30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100-120 mT at 4 K, and the remanent coercive force increases from approximately 50-80 mT at room temperature to approximately 110-180 mT at 4 K

Precipitation from supersaturated aluminate solutions. IV. Influence of citrate ions

The influence of citrate on the precipitation in unseeded and seeded supersaturated aluminate solutions is reported. At high Cit/Al ratios (>0. 1) extensive complex formation takes place but already at low ratios (10−4–10−2), where this effect hardly changes the supersaturation, the precipitation rate is strongly retarded. The formation of bayerite appears to be more sensitive to citrate concentration than that of pseudoboehmite and can even be prevented at rather low Cit/Al ratios (0.01). Transformation of pseudoboehmite into bayerite is also inhibited at these low Cit/Al ratios. Citrate may therefore be used to selectively precipitate pseudoboehmite. A simple adsorption model is introduced to explain the role of citrate in retarding growth. Evidence is presented to show the dual role played by citrate in the nucleation of bayerite as revealed by long time lags in relaxation experiments. More nuclei are generated with increasing citrate concentration but the growth of these particles is slowed down