Abstract Hydrothermal synthesis of kalsilite was facilitated by the inclusion of potassium carbonate (K2CO3) as the source of the potassium ion. After a 24 h reaction at 220 °C, kaolin clay treated with 1.25 M K2CO3 showed the most significant peaks at 2θ of 28.5° and 34.7° by X-ray diffraction corresponding to the hexagonal kalsilite. In addition, field emission scanning electron microscopy images also revealed hexagonal particles proving the formation of the desired mineral. The orthorhombic boehmite and monoclinic bayerite were formed as the dominant phases at lower K2CO3 molarity (<1.0 M), whereas kalsilite was recognized as the minor crystalline phase. The crystallinity of hexagonal kalsilite increased at higher K2CO3 concentration (>1.0 M) while the reaction temperature remained at 220 °C. Furthermore, the energy dispersive spectroscopy pattern of kalsilite showed a significant atomic percentage of potassium in the aluminosilicate material, indicating its formation.