The naturalist on HMS Challenger, John Murray, became a wealthy man (unlike his modern counterparts) as a result of joining an oceanographic expedition, as a result of his interest in a mining venture which resulted from the discovery of phosphorite on Christmas Island in the western equatorial Pacific. What is more, the British Crown's Treasury recovered the expenses of the expedition, through the taxes derived from the phosphorite produced.

Phosphorites are cryptocrystalline apatites and vary in composition. A general formula is Ca₁₀ (P0₄, C0₃ )6 F_2?3, with increases in carbonate running parallel to increases in fluoride (or hydroxide). Modem phosphorites typically occur in areas of high productivity - off Southern and Baja California, off Peru, off South Africa. They occur as black or brown nodules from pellets up to head size, and as irregularly shaped cakes. They are interpreted as replacement products of fine?grained lithified carbonates, as mineralization of pre?existing organic matter, as precipitates from microorganisms filling cavities within carbonates, or as direct precipitates from interstitial waters.

Off California these deposits contain about 25 to 30 % P₂ 0₅ and 40 to 45 % CaO, but these values vary in other regions. The common depth of deposition is on the shelf and upper slope. Fossil phosphorites are abundant in Florida and Georgia. Miocene phosphorites are mined on a large scale; in West Africa, Eocene deposits are being exploited. Seamounts bearing Cretaceous carbonates commonly carry phosphotites.

The association of geologically young phosphorite deposits with present?day regions of upwelling suggests that the source of the phosphorus is organic matter. Apparently, the algae growing in these regions, in the surface waters, extract the phosphorus from the water. Also, crustaceans and fish concentrate it further in their bodies and excrement. During decomposition of organic debris on the sea floor, much phosphate is released to the interstitial water (and also to seawater). Thus, interstitial waters fight below the sea floor may become saturated with the phosphate mineral apatite. Precipitation of apatite, replacement of preexisting carbonate minerals, and impregnation of sediment can then proceed. Off Peru, nodules with diameters of several centimeters grow within soft sediments with a rate of some mm/1000 years, indicating diffusive upward flux of dissolved phosphorus. The resulting phosphatic concretions are resistant to transport by currents; they can be mechanically exhumed and concentrated during periods of sediment reworking. Indeed, concretions are commonly associated within the sediment with hiatuses or other discontinuity surfaces with drastically reduced net deposition rates.

Phosphorites are largely used for fertilizers, but also as a source of phosphorus in the chemical industry. Marine phosphorites are enriched in some trace elements compared with marine shales (Ag 20x, U 30x, Cd 50x). Offshore phosphorites typically contain less than 25 % P₂0₅.

Weathering processes on land concentrate P₂0₅ up to 30 to 40 %. Such residual phosphorites are mined, for example, in Florida and Morocco (Fig. 10.7) Concentrations within marine P rich deposits as reported from off Califomia, western South America, South Africa, or on the Chatham Rise, east of New Zealand, are up to 80 kg/m² phosphorites with up to 25 % P₂0₅. They occur over large areas and in water depths of less than 400 m. Exploitation will depend on the prices charged by producers on land and on the domestic demand and supply situation. In any case, the consumption of phosphorites from land increased from 3.5 million t P in 1900 to 140 million t in 1971.

Text and figures are all from Seibold and Berger (1993): The Sea Floor. An introduction to marine geology. Second Edition, Springer Verlag, pages 285-286, figures 10.7. All rights are with Fischer Verlag and the authors of this book.

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