Six volcanogenus dark red soils were examined and compared with a paleoclimatical red soil and a burnt soil (reddened under a forest fire). Volcanogenus dark red soils contained iron oxides both inside and on the surface of their mineral particles, whereas in red and burnt soils, the mineral particles had iron oxides only on their surface as weathering products. Volcanogenus dark red soils contained better-crystalline hematite than those of red and burnt soils, and their XRD peaks were so sharp that they could be recognized without any concentrating treatment. Grinding test showed that the grain size of hematite was a possible factor in the darkness of red color. The observation of parent materials in thin sections showed that the dark-reddening process includes deuteric and thermal alterations. The features of volcanogenus dark red soils mentioned above should be helpful for classifying red- and dark red colored soils.
