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The world's largest insular phosphate deposit is found on Nauru, an uplifted coral atoll in the western equatorial Pacific. This deposit, which is draped over a dolomitized karstic surface, has an earthy pelletal texture; it is oolitic at its base and structureless in its upper part. The only phosphate mineral found is a carbonate fluorapatite with the stoichiometry Ca10(PO4)5.6(CO3)0.6F1.6(OH)1-x. This mineral is slightly depleted in F- and CO32-, relative to PO43-, to be considered a true francolite.
Abundant specimens of corals and micromollusks within the dolomite are representative of two contrasting atoll environments: a coral reef and a deep-water lagoon. The biostratigraphy has not been determined. Radiometric dates give a minimum age of 200,000 yr.
The source of the phosphorus is bird guano. The ^dgr18O and ^dgr13C values of the apatite and dolomite suggest that phosphatization occurred in meteoric water, possibly within and above a Ghyben-Herzberg lens; whereas dolomitization occurred in hypersaline water that refluxed from the lagoon when it became isolated from the open ocean during uplift. This interpretation is supported by the occurrence of as much as 15% gypsum in lagoonal sediments.
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