Iron minerals in sedimentary phosphorites of the southeastern United States

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Virginia Polytechnic Institute and State University

The central Florida, North Carolina, and central Tennessee phosphorite deposits account for 90% of the marketable phosphate produced in the United States. Scanning electrons and reflected light microscopy reveals that the iron-bearing minerals associated with these sedimentary phosphorites are related to the geologic setting and degree of weathering of the deposit.

Pyrite and marcasite (FeS₂) are the principal iron mineral inclusions in unweathered phosphorites of Florida, North Carolina, and Tennessee, and occur as discrete, randomly disseminated microcrysts, irregularly shaped microcryst aggregates, cubes, dendrites, framboids, or bioturbation and interstitial pore space fillings. Pyrite and marcasite also fill the void spaces in skeletal phosphorites such as nerve canals in shark teeth of Florida and North Carolina and in bryozoan zooecium of Tennessee. The oxidizing conditions accompanying weathering in the terrigenous sand and clay environments of central Florida alter these inclusions to goethite (FeOOH), which occurs as pseudomorphs after the iron sulfides. However, hematite (Fe₂O₃) optically appears to be the iron sulfide pseudomorph in the phosphorites of the weathered carbonate environments in central Tennessee.

The first reported occurrences of chalcopyrite (CuFeS₂), chromite (FeCr₂O₄), and an iron-potassium-silicate sheath surrounding some iron sulfide and iron oxides as inclusions in phosphorites, supergene ferrian-millisite [Ca₃(Al,Fe³⁺)₁₂(PO4)₈(OH)₁₈ • 6H₂O)] as coatings on phosphorites in the leached zone under a central Florida gossan, and secondary ferromanganese oxide spherulites as accessory minerals in Tennessee deposits are made in this study