Sulfurous gas, sulfur or sulfates are frequently contained in volcanic ejecta. We found white patchy materials (WPM) in a road-cut profile on Miyake Island, Japan. The layer containing WPM consists of scoria deposit in 1874. The major materials we identified in the WPM were CaSO_4・2H_2O and amorphous silica according to X-ray diffraction and energy dispersive X-ray analyses. A possible process for crystallization of CaSO_4・2H_2O in the WPM is the dissolution of CaSO_4・2H_2O contained in the overlying 2000 ash and its re-precipitation on the surface of the scoriacious road-cut profile. Emission of sulfur dioxide gas, converted to sulfuric acid in water, has been so abundant since the 2000 eruption that we further examined reaction products between crushed scoria and dilute H_2SO_4 (0.1-2.5mol L^). CaSO_4・2H_2O was also identified in the reaction products as well as alunogen, iron sulfate, etc. Because alunogen is highly soluble in water, CaSO_4・2H_2O was the major crystalline product after rinsing with water
