Alternative sources of inorganic fertilizer are being studied mainly in relation to potassium. The reactivity of rocks and therefore the K+ availability can be changed by modifications in mineral structure. In the present study, K+ availability of phonolite rock was investigated after thermal treatment at different temperatures between 300-1200 degrees C and cooled by two different conditions: slow cooling in air and rapid cooling in iced water. All the samples were characterized by XRD, FTIR and SEM/EDS and K+ availability was investigated in 0.01 mol L-1 citric acid solution. Results showed that thermal treatment promotes K+ and Na+ availability due to changes in mineral structures. The main structural modifications observed were: 1) transformation of analcime [(Na,K)AlSi2O6.H2O] into intermediate phase at 600 degrees C; 2) formation of leucite [(Na,K)AlSi2O6] at 900 degrees C and 3) attainment of vitreous material at 1200 degrees C. Thermal treatment at 600 or 1200 degrees C promotes an increase in K+ availability and a reduction of Na+ availability for both types of cooling, slow or rapid. These results encourage studies on phonolite rock as an alternative potassium source. (C) 2015 Elsevier B.V. All rights reserved.