Thermal analysis of beaverite in comparison with plumbojarosite

The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. The mineral beaverite Pb(Fe,Cu)(3)(SO4)(2)(OH)(6) decomposes in three stages attributed to dehydroxylation, loss of sulphate and loss of oxygen, which take place at 376 and 420, 539 and 844 degrees C. In comparison three thermal decomposition steps are observed for plumbojarosite PbFe6(SO4)(4)(OH)(12) at 376, 420 and 502 degrees C attributed to dehydroxylation; loss of sulphate occurs at 599 degrees C; and loss of oxygen and formation of lead occurs at 844 and 953 degrees C. The temperatures of the thermal decomposition of the natural plumbojarosite were found to be less than that for the synthetic jarosite. A comparison of the thermal decomposition of plumbojarosite with argentojarosite is made. The understanding of the chemistry of the thermal decomposition of minerals such as beaverite, argentojarosite and plumbojarosite and related minerals is of vital importance in the study known as 'archeochemistry'.