A method for the processing of potassium containing materials comprises: (i) Separation of a potassium containing mineral from gangue minerals; (ii) Acid leaching whereby substantially all potassium, iron, aluminum and magnesium is solubilized and mixed potassium/iron double salt formed; (iii) Selectively crystallizing the mixed potassium/iron double salt formed in the leach step (ii); (iv) Second separation to separate the mixed potassium/iron double salt formed in step (iii); (v) Thermal decomposition to produce an iron oxide, a potassium salt and one or more phosphates; (vi) Leaching the product of the thermal decomposition; (vii) Third separation to separate the iron oxide and phosphate from the potassium salt; (viii) Recovering the potassium salt by crystallization; (ix) Separating the iron oxide and phosphate of step (vii) by leaching and subsequent solid liquid separation; and (x) Precipitating phosphate from liquor produced in step (ix) through the addition of a base.