Cacoxenite in Miocene Sediments of the Maryland Coastal Plain

Paul P. Hearn
1988 Clays and clay minerals  
Cacoxenite having the composition (A14.0Fe22.507.~(OH)14.3(PO4)17(H20)23.7)" 50.3H20 was identified in a bed of mature quartz sand in the Miocene Calvert Formation near Popes Creek, Maryland. This is the first reported occurrence of this mineral in Atlantic Coastal Plain sediments north of Florida. The cacoxenite occurs as silt-size to sand-size grains, both as irregularly shaped aggregates and as radiating arrays of delicate acicular crystals. The presence of discrete cores and overgrowths in
more » ... and overgrowths in some grains indicates at least two generations of crystal growth. Electron microprobe analyses reveal excess Si and A1 (relative to the ideal composition), which is believed to reflect ultra-fine clay particles within the cacoxenite grains. Admixed clays probably served as a substrate for the formation of ferric oxyhydroxides, which were subsequently converted to cacoxenite through the addition of dissolved phosphorus.
doi:10.1346/ccmn.1988.0360506 fatcat:mm4nxeadanevlekf4zxsj7zuii