The effect of thermal annealing on charoite, K 5 Ca 8 (Si 6 O 15 ) (Si 2 O 7 ) (Si 4 O 9 ) (OH)•3H 2 O, have been studied by X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetry (TG) and thermoluminescence (TL). The chemical composition, established by electron-probe micro-analysis (EPMA), indicates significant amounts of Na, Ba, Fe and F; the environmental scanning electron microscope (ESEM) results reveal the presence of unusual minerals such as tinaksite and aegirine. The cell parameters of both natural and preheated charoite, determined by XRD, are (i) a 19.786(2), b 32.003(3), c 7.8565(9) Å, a 90, b 97.159, g 90°, with a F(30) of 43 for the non-annealed sample, and (ii) a 19.567(2), b 31.821(3), c 7.1171(7) Å, a 90, b 94.000(2), g 90°, with a F(30) of 19 for charoite heated to 710°C. The thermal effect indicates a shortening of the cell edges, Da 0.219(2), Db 0.182, Dc 0.739 Å, and the b angle tilts only by 3.16°. The DTA-TG and TL emission suggest: (i) a thermal interval of 80°-290°C, explained by the dehydration of charoite, involving different types of H 2 O, i.e., hydrogen-bonded to the framework and non-bonded H 2 O, and (ii) an interval 290°-480°C, resulting from a dehydroxylation process.
