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Large Miocene phosphorite deposits occur in parts of four states and extend along the Atlantic Coastal Plain from North Carolina into the center of the Florida peninsula. The reserves and resources of this large deposit are estimated at billions of metric tons of phosphate rock. The bulk of this sediment consists of phosphate particles (pellets and other grain types), quartz and clay minerals. There are locally important and variable amounts of calcite and dolomite with minor amounts of silicates, opal, iron-aluminum phosphates (in weathering zones) and heavy minerals.

The observed variations in mineralogy over the region are the result of primary compositional differences as well as postdepositional alteration. The latter includes diagenetic changes that resulted in the formation of authigenic minerals as well as weathering of the deposits to alter the compositions and phases to those that are seen today.

Carbonate fluorapatite (francolite) and clay minerals are the two groups of phases that may be the most indicative of geochemical changes. They are also the two most important constituents from an economic and environmental point of view. Francolite is the only economically important phosphate mineral present; the clays represent a significant source of impurities in the ore and present by-product disposal problems. Thus, this brief summary will concentrate on these two groups of minerals.

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