The hydrothermal reaction of Ca(OH)(2) with reagent quartz at temperatures between 175 and 340 degrees C has been studied to obtain fundamental information on coal demineralization with Ca(OH)(2). X-ray diffraction analysis was used to measure the conversion of quartz and to identify reaction products. The reaction sequence strongly depended on Ca/Si atomic ratio and on autoclaving conditions. At a Ca/Si atomic ratio of about 1, quartz was first converted to Ca-2-Si7O4 . H2O, which then reacted with residual quartz or residual Ca(OH)2, forming a silica-rich phase or a calcium-rich phase, and finally all these intermediate compounds were transformed to xonotlite upon a prolonged high-temperature treatment. At higher Ca/Si ratios, a fraction of quartz remained unreacted even after severe treatments, and the final products were jaffeite (Ca/Si = 3) and tobermorite-like compounds (Ca/Si = similar to 1) instead of xonotlite. On the basis of the above investigations on model reaction, demineralization reactions to remove quartz from coal through Ca(OH)(2)/HCl leaching were interpreted and a suitable acid-washing method was proposed.