phosphates

phosphates Phosphate is present in many sedimentary rocks as grains of apatite minerals (calcium phosphate). Rocks that are largely composed of phosphate (phosphorites) are less common but of economic importance as a resource for fertilizers. These phosphorites are formed either directly from a marine source or indirectly through biological uptake.

Marine deposits such as the Upper Cretaceous rocks of North Africa and the Middle East (a leading world resource) form in specific environments. Phosphate in the shallow photic zones is used up by marine organisms, but it will accumulate below this zone if oxygen is sufficiently depleted. This phosphate will form deposits if there is a low influx of sediments, particularly where there are upwelling currents of cold phosphate-rich waters. The phosphate replaces the carbonate in skeletons and lime mud; and phosphate in decaying organic matter can be liberated by bacterial decay. Together this phosphate accumulates as small nodules or crusts and can be seen forming on the ocean shelves today.

Biogenic phosphorites may form where phosphate-rich matter, such as bones, teeth, or excrement, is concentrated and cemented together or dissolved and later precipitated. Guano is made from seabird and bat droppings; it may be up to 30 m thick and is an economic resource in the eastern Pacific. These biogenic deposits may be cemented together into rocks or may be dissolved and re-precipitated within limestones as cements or as nodules.

N. Mann

Bibliography

Shelley, R. C. (1988) Applied sedimentology. Academic Press, London.
Tucker, M. E. (1981) Sedimentary petrology: an introduction. Blackwell Scientific Publications, Oxford