The article discusses the formation of pyrite (FeS₂) in marine sediments, focusing on the chemical and biological processes involved. Experimental studies show that pyrite can be synthesized at neutral pH in concentrated sulfide solutions and in natural sediments through the reaction of precipitated FeS with elemental sulfur at 65°C. This reaction is likely to occur at sedimentary temperatures but may take several years to complete. Synthetic pyrite formed by this reaction is framboidal. The major steps in sedimentary pyrite formation include bacterial sulfate reduction, reaction of H₂S with iron minerals to form iron monosulfides, and the reaction of iron monosulfides with elemental sulfur to form pyrite. Key factors limiting pyrite formation in marine sediments include the availability of organic matter for sulfate-reducing bacteria, sulfate diffusion into sediments, the total concentration and reactivity of iron minerals, and the production of elemental sulfur. In Connecticut coastal sediments, the main limiting factor for pyrite formation is the availability of metabolizable organic matter. Reactive iron and dissolved sulfate are present in excess, and essentially all FeS is transformed to pyrite by elemental sulfur. In sediments with lower iron content, especially carbonates, the principal limiting factor is the concentration of reactive iron. In most sediments, total iron content and sulfate diffusion are not limiting. The formation of pyrite involves the transformation of iron monosulfide (FeS) to pyrite (FeS₂). Experimental data support the hypothesis that the overall transformation reaction is FeS + S° → FeS₂. The rate and mechanisms of this process are not well documented, but experimental data show that pyrite can form in a few weeks at neutral pH and 65°C. The study also examines the factors limiting pyrite formation in marine sediments, including organic matter, iron, and sulfate. Organic matter is a primary limiting factor, as it controls the concentration of H₂S, which in turn affects the degree of pyritization. Iron is not a limiting factor in Connecticut coastal sediments, as shown by the relatively low values of P (degree of pyritization). Sulfate diffusion is not a limiting factor in these sediments, as evidenced by the presence of appreciable concentrations of dissolved sulfate in sediment pore waters. Elemental sulfur is a potential limiting factor in pyrite formation, as it is the only oxidizing agent capable of transforming FeS to FeS₂. In sediments overlain by aerobic waters, elemental sulfur is readily formed through the oxidation of H₂S and FeS by dissolved oxygen. In sediments overlain by anaerobic waters, the origin of elemental sulfur is more problematical, as it may be formed by anaerobic processes or deposited from above. The rate of pyrite formation in Connecticut coastal sediments is estimated to be about 0.2% S per year, based on the concentration of pyrite sulfur inThe article discusses the formation of pyrite (FeS₂) in marine sediments, focusing on the chemical and biological processes involved. Experimental studies show that pyrite can be synthesized at neutral pH in concentrated sulfide solutions and in natural sediments through the reaction of precipitated FeS with elemental sulfur at 65°C. This reaction is likely to occur at sedimentary temperatures but may take several years to complete. Synthetic pyrite formed by this reaction is framboidal. The major steps in sedimentary pyrite formation include bacterial sulfate reduction, reaction of H₂S with iron minerals to form iron monosulfides, and the reaction of iron monosulfides with elemental sulfur to form pyrite. Key factors limiting pyrite formation in marine sediments include the availability of organic matter for sulfate-reducing bacteria, sulfate diffusion into sediments, the total concentration and reactivity of iron minerals, and the production of elemental sulfur. In Connecticut coastal sediments, the main limiting factor for pyrite formation is the availability of metabolizable organic matter. Reactive iron and dissolved sulfate are present in excess, and essentially all FeS is transformed to pyrite by elemental sulfur. In sediments with lower iron content, especially carbonates, the principal limiting factor is the concentration of reactive iron. In most sediments, total iron content and sulfate diffusion are not limiting. The formation of pyrite involves the transformation of iron monosulfide (FeS) to pyrite (FeS₂). Experimental data support the hypothesis that the overall transformation reaction is FeS + S° → FeS₂. The rate and mechanisms of this process are not well documented, but experimental data show that pyrite can form in a few weeks at neutral pH and 65°C. The study also examines the factors limiting pyrite formation in marine sediments, including organic matter, iron, and sulfate. Organic matter is a primary limiting factor, as it controls the concentration of H₂S, which in turn affects the degree of pyritization. Iron is not a limiting factor in Connecticut coastal sediments, as shown by the relatively low values of P (degree of pyritization). Sulfate diffusion is not a limiting factor in these sediments, as evidenced by the presence of appreciable concentrations of dissolved sulfate in sediment pore waters. Elemental sulfur is a potential limiting factor in pyrite formation, as it is the only oxidizing agent capable of transforming FeS to FeS₂. In sediments overlain by aerobic waters, elemental sulfur is readily formed through the oxidation of H₂S and FeS by dissolved oxygen. In sediments overlain by anaerobic waters, the origin of elemental sulfur is more problematical, as it may be formed by anaerobic processes or deposited from above. The rate of pyrite formation in Connecticut coastal sediments is estimated to be about 0.2% S per year, based on the concentration of pyrite sulfur in