Occurs in nature as the minerals: beraunite, cacoxenite, dufrenite, koninckite, phosphosiderite, strengite. Prepn from Fe(H2PO4)3: Remy, Boulle, Compt. Rend. 253, 2699 (1961); from Fe(CO)5 and H3PO4: Cate et al., Soil Sci. 88(3), 130 (1959); from phosphate rock: Vickery, US 2914380 (1959 to Horizons Inc.); from mill scale and H3PO4: Alexander, Mathes, US 3070423 (1962 to Chemetron).