The conventional process of lithium extraction from α-spodumene (LiAlSi2O6) is energy-intensive and associated with high byproduct management cost. Here, we investigate an alternative process route that uses potassium sulfate (K2SO4) to extract lithium while producing leucite (KAlSi2O6), a slow release fertilizer. Presenting the first-ever in situ record of the reaction of α-spodumene with potassium sulfate, we use synchrotron X-ray diffraction (XRD) and differential scanning calorimetry (DSC) to document the reaction sequence during prograde heating. From 780 °C, we observe a broad endothermic DSC peak, abnormal expansion of the α-spodumene structure, and an increase in α-(Li, K)-spodumene peak intensity during heating with potassium sulfate, indicative of the exchange between lithium and potassium in the spodumene structure. When 11 ± 1% K occupancy in the M2 site of α-(Li, K)-spodumene is reached, the mechanism changes from ion exchange to a reconstructive transformation of α-(Li, K)-spodumene into leucite, evidenced by a decrease in α-spodumene and potassium sulfate abundance concurring with formation of leucite over a narrow temperature range between 850 and 890 °C. The increasing background intensity in synchrotron XRD above 870 °C suggests that a lithium sulfate-bearing melt starts to form once >90% of α-spodumene has been converted during the reaction. This fundamental understanding of the reaction between α-spodumene and potassium sulfate will enable future development of lithium extraction routes using additives to significantly decrease energy intensity and to produce marketable byproducts from α-spodumene.