Geology and mineralogy of the Haft Sandogh alunite deposit (NW Takestan): implication for alumina and potassium extraction

The Haft Sandogh alunite deposit is located in the northwest of Takestan (about 60 Km west of Qazvin), and is a part of large Takand alunite deposit. Mineralogical and field study reveal that there is a zonation in the alteration zones of the Haft Sandogh deposit and surrounding area including silica zone in the central part, and quartz-alunite (quartz alunite), kaolinite and propylitic formed toward outer parts. Quartz and alunite are the main minerals of the quartz alunite. Acid-sulfate alteration can be used as a tool in the exploration of epithermal-porphyry ore deposits associated with Cenozoic magmatism in Iran. Regarding to alunite, it can be considered as a potential alternative resource for potassium and alumina production and the decomposition of alunite is a principal step to extract valuable components. Thermal decomposition of alunite was characterized by using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and X-ray powder diffraction (XRD) in this research. The DTA curve shows three endothermic peaks at 546 °C, 787.9 °C and 1195.4 °C and an exothermic peak at 737.2 °C. The first peak is associated with the formation of dehydroxylated basic sulphate (KAl3O3 (SO4)2). The second exothermic peak represents the formation of a new crystalline form of sulphate (KAl(SO4)2). The third endothermic peak represents the massive desulphuration by the formation of a basic aluminium and potassium sulphate (KAlO.SO4). Final endothermic peak represents the formation of Al2O3 and leucite (KAlSi2O6).