Precipitates formed due to Fe(II)-oxidation catalysed by Acidithiobacillus ferrooxidans between pH 1.6 and 3.2 have been studied by powder X-ray diffraction, infrared spectroscopy and Mössbauer spectroscopy. The precipitates consist of well crystalline, ammonium-containing jarosite at pH 1.6, and mixtures of jarosite and poorly crystalline schwertmannite at pH 2.5 and 3.2. At low temperatures (10 and 20 K) the components of the two phases overlap strongly. Jarosites ordered (c. 45 K – defect antiferromagnetic) at temperatures well below the ordering temperature for schwertmannite (c. 80 K at pH 3.2 and c. 70 K at pH 2.5). Thus thermoscans measured between 100 and 10 K facilitates characterization of these two minerals when occurring in mixtures.