Raine Island is a small vegetated coral cay that is capped by recent insular phosphate rock. On the northeastern shore, phosphate rock also extends into the intertidal zone, where it is juxtaposed against calcareous beachrock. A petrological and isotopic study was carried out on the phosphate rock in order to clarify some of the uncertainties concerning the genesis of insular phosphate rocks in general and on Raine Island in particular. The main phosphate mineral is dahllite (carbonate hydroxyapatite), which occurs as microlaminated grain coatings and as a bioclast replacement. Minor pore-filling whitlockite (beta tricalcium phosphate) was recorded in one sample. Organic carbon is intimately associated with phosphate cement and accounts for color variations between microlaminae. Phosphate delta 18 O p (SMOW) values range from 14.4 to 16.6 per mil, indicating that phosphatic porewaters were entirely fresh and had not undergone evaporative 18 O enrichment. Phosphate cement records the passage of both downward-percolating near-surface porewater and seaward-moving groundwater at or close to the freshwater-saltwater interface. Intertidal phosphatic rock on the northeastern shore is a rare example of a lithified, intertidal tropical beach sand deposit with a definite fresh-water origin.

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