An interesting suite of iron phosphate minerals formed by biogeochemical processes occurs at Bomi Hill and Bambuta in western Liberia. The phosphates at Bomi Hill form bouldery aprons on the slopes below two prominent cliffs and occur in place in caves and open fissures in cliffs of high-grade iron ore. They were formed by the interaction of bat dung, or substances derived from it, with exposed iron oxide. The minerals thus formed include strengite and its dimorph, phosphosiderite; a member of the rockbridgeite group (formerly included with dufrenite); and, most noteworthy, leucophosphite, hitherto known only from Western Australia. From chemical, optical, and x-ray data, together with a recent synthetic study, leucophosphite is established as a valid and well-defined mineral species and, so far, the only known alkali-iron phosphate mineral of sedimentary origin. Chemical studies by soils chemists have been correlated with natural occurrences, giving a better understanding of the geochemistry of phosphatic fertilizers and of phosphate minerals.
The phosphates from Bambuta differ from those at Bomi Hill; the differences, however, may reflect incomplete knowledge of the two deposits rather than any significant difference of origin. In both, the source of the phosphorus is believed to be the dung of bats.