A bright yellowish brown deposit on the walls and bases of water races in an area of paddy fields near Hontsuka Cone, Aso-Dani, is shown to be siliceous ferrihydrite. The deposit contains close to 50% Fe, 6% Si, and 13% H₂O (110°C basis). Two broad peaks at 0.275 nm and 0.155 nm, with possibly a third (much less perceptible) at about 0.22 nm, are observed in X-ray diffraction and an infrared absorption maximum for Si-O vibrations occurs at 964 cm^-1. The deposit forms as groundwater pumped from about 100 m depth and containing close to 20 g m^-3 ferrous Fe and 35-75 g m^-3 SiO₂ is released to the atmosphere and oxidised. This occurrence is similar to one in a streambed below a spring on an andesitic volcano in New Zealand and is a further example of the weathering of volcanic rocks giving rise to ferrihydrite.