Thermal analysis of halotrichites

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 formula of the halotrichite minerals were determined as halotrichite (Fe0.752+,Mg0.25)SO4·Al2(SO4)3·22H2O, apjohnite (Mn0.642+,Mg0.28,Zn0.08)SO4·Al2(SO4)3·22H2O, pickeringite (Fe0.222+,Mg0.78)SO4·Al2(SO4)3·22H2O, wupatkiite as (Co0.45,Fe0.552+)SO4·Al2(SO4)3·22H2O. Three low temperature decomposition steps (a) between 0 and 44 °C, 50 and 76 °C, 72 and 88 °C were attributed to dehydration. An additional dehydration step at around 317-330 °C was confirmed by in situ mass spectrometry. The higher temperature decomposition steps between 516 and 738 °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.