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Unusual Phosphate Concretions Related to Groundwater Flow in a Continental Environment

¹ Ecole des Mines de Paris, 35 rue St Honoré, 77305 Fontainebleau, France, and CNRS-UMR 7619 Sisyphe "Structure et fonctionnement des systèmes hydriques continentaux"; medard.thiry{at}ensmp.fr
² Association des Naturalistes de la Vallée du Loing, 26 rue du Monceau, 77210 Avon, France
³ Ecole des Mines de Paris, 35 rue St Honoré, 77305 Fontainebleau, France, and CNRS-UMR 7619 Sisyphe "Structure et fonctionnement des systèmes hydriques continentaux"

Occurrence of phosphate cemented-sandstone concretions in alluvial sand containing archaeological layers implies that they have formed during the last 2000 years. Morphology and petrography of the concretions indicates a relationship with groundwater cementation. The phosphate cement of the concretions consists of concentric isopachous aureoles, about 10 µm thick, that wrap the detrital grains. Aureoles are formed of collophane, the cryptocrystalline or amorphous variety of hydroxyapatite.

Geochemical modeling indicates that the apatite cement likely results from the leaching of a preexisting phosphate deposit (bones?) in an acidic soil environment and the precipitation of dissolved phosphate under pH control at the mixing zone of down-moving soil–water with the calcite-saturated groundwater.

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