Abstract
Thermochemical data for many basic iron phosphates have been estimated by a method which regards these compounds as components on joins between the simple phosphates and hydroxides. The data so derived have been used to develop models depicting the phase compatibilities and the general diagenetic behavior of iron phosphates in subaqueous freshwater environments. The models are combined with measurements on the interstitial waters of Lake Erie to define the chemical requirements for the formation of vivianite and other iron phosphates in the Great Lakes sediments. In aerobic lake sediments, ferrosoferric (and manganousferric) hydroxyphosphates are the stable minerals which may be derived by phosphating ferromanganese oxides or by the oxidation of ferrous phosphates. Strengite and simple ferric phosphates are unlikely to be important diagenetic constituents of freshwater sediments. The stable and most probable phosphate minerals in reducing environments are vivianite, reddingite, and anapaite. The precipitation and dissolution of these iron phosphates (particularly vivianite and ludlamite, the dominant phases) in the Great Lakes sediments are considered an important buffer mechanism which regulates both the levels of phosphorus in the interstitial waters and the release of phosphorus to the overlying lake waters.
