(1992), schwertmannite is the most common Fe mineral to precipitate directly from acid sulfate waters. These minerals are grouped together in a separate acidity pool (i.e.
Bigham JM, Nordstrom DK (2000) Iron and aluminum hydroxy sulfates from acid sulfate waters. Reviews in Mineralogy and Geochemistry 40, 351-403.
Our study did not attempt to isolate such a mineral; however, in East Trinity waters, the measured field data strongly support van Breemen's original 1973 model of a notional ion assemblage AlOHS[O.sub.4] regulating aluminium activity in acid sulfate waters. Acid sulfate waters are complex with several aluminium and iron hydroxy sulfate phases potentially present.
Van Breemen (1994) and Nordstrom (Bigham and Nordstrom 2000) no longer support the existence of a mineral with the stoichiometry Al:OH:S[O.sub.4] as a regulator of AI activity in acid sulfate waters. However, our field data closely match the line of constant IAP for AlOHS[O.sub.4] determined by van Breemen (1973).
(22.) Bigham JM, Schwertmann U, Traina SJ, Winland RL and Wolf M: Schwertmannite and the chemical modeling of iron in acid sulfate waters
. Geochim Cosmo Acat 60: 2111-2121, 1996.