Sassolite has been recently found for the first time in. Japan at Iwo-dake Volcano, Iwo-zima. Near the explosio crater of Iwo-dake there are many fumaroles, from which large amount of volcanicgas is issued. Sassolite was found as one of the sublimate minerals atsome fumaroles, where the temperature was about 200°C. Sassolite formspseudohexaaonal plates, less than 0.5mm. in diameter, and has the following properties; sp, gr.(28°C)=1.478, α=1, 331, β=1.448, γ=1.453, 2Vx=about 5°. Chemical composition is B₂O₃ 56.39 and H₂O 43.61. The strongest1ines of X-ray powder pattern of the mineral are 3.16À (100), 5.98À (15), 5.86À (10), and 1.58À (5).