Four halotrichites from different origins were analysed by thermogravimetric and differential thermogravimetric analysis. The halotrichites were analysed by powder X-ray diffraction and were phase pure. The chemical composition was analysed using EDX techniques. The for mula of the halotrichite minerals were determined as halotrichite (Fe0.752+Mg0.25)SO4.Al-2(SO4)(3)(.)22H(2)O, apjohnite (Mn0.642+Mg0.28,Zno(0.08))SO4. Al-2(SO4)(3).22H(2)O, pickeringite (Fe-0.22(2+) Mg-0.78)(SO4Al2)-Al-.(SO4)(3)(.)22H(2)O, wupatkiite as (CO0.45,Fe-0.55(2+))(SO4Al2)-Al-.(SO4)(3)(.)22H(2)O. Three low temperature decomposition steps (a) between 0 and 44 degrees C, 50 and 76 degrees C, 72 and 88 degrees C were attributed to dehydration. An additional dehydration step at around 317-330 degrees C was confirmed by in situ mass spectrometry. The higher temperature decomposition steps between 516 and 738 degrees C are attributed to the decomposition of sulphate to sulphur dioxide and oxygen as confirmed by mass spectrometry. A comparison of the thermal decomposition of jarosites is made. (c) 2007 Elsevier B.V. All rights reserved.