Acidic waters from pyrite weathering and atmospheric deposition represent a global environmental problem that often involves a major alteration of soil chemistry. Sulfate is usually the dominant anion in these waters and its interaction with polymeric forms of Fe(III) can lead to the formation of a variety of nanocrystalline oxides and hydroxysulfates within the soil environment. The mineral phases produced play a central role in determining the solubility, mobility, and toxicity of trace elements released by acid weathering. The purpose of this work was to study the phase boundary between schwertmannite [ideally Fe₈O₈(OH)₆(SO₄)] and jarosite-type minerals using ammoniojarosite [(NH₄, H₃O)Fe₃(OH)₆(SO₄)₂] as a model basic ferric sulfate. Mineral formation was achieved by oxidation of ferrous sulfate solutions at pH 2.0 – 3.0 over a range of NH₄ concentrations (5 to 805 mM) and temperatures (22 to 65 C) in the presence of iron-oxidizing acidophiles. Ammoniojarosites were also produced by chemical (abiotic) procedures in parallel thermal experiments. At temperatures ranging from 22 to 36 C, schwertmannite was the only solid phase produced at concentrations of NH₄ < 10 mM. Between 10 and 85 mM NH₄, a mixed product of ammoniojarosite and schwertmannite was formed. In excess of 165 mM NH₄, only ammoniojarosite was precipitated. An increase in the incubation temperature using thermoacidophiles at 45 and 65 C accelerated the formation of ammoniojarosite in culture solutions containing 165 mM NH₄. The biogenic schwertmannites had characteristic x-ray diffraction profiles consisting of eight broad, overlapping peaks, but colors were redder (6.5 YR vs 10 YR) than previously observed for either synthetic or natural specimens. Both the biogenic and chemical ammoniojarosites were yellow (2Y to 4Y in Munsell hue), low surface area (< 1 m²/g) materials with average (n = 12) c_o and a_o unit cell parameters of 17.467 ± 0.048 Å and 7.330 ± 0.006 Å, respectively. The average cell lengths compared well with results from standard reference materials. There was a strong, positive correlation between concentration of NH₄ in the growth media and N content of the ammoniojarosites. All samples were N deficient compared to stoichiometric ammoniojarosite indicating the formation of limited solid solutions with hydronium jarosite. Strong positive correlations were also observed between synthesis temperature and the unit cell axial ratios (c_o/a_o) and volumes. Crystal morphologies of the biogenic jarosites ranged from pseudo-cubic to disc shaped, whereas the chemical specimens prepared at 36 to 95 C had irregular habits.