Taranakite—H7.09K1.97(Al,Fe)4.98(PO4)8.00l9.7H2O—occurs as aggregates of fine powdery crystals intercalated in clay sediments in the Onino-Iwaya limestone cave at Hiroshima Prefecture, Japan. Cell dimensions of the mineral determined by least-squares method on the basis of a hexagonal six-layer structure are a = 8.718(2) Å and c = 95.18(3) Å. Three endothermic DTA peaks occur at 120°C, 180°C, and 990°C, and an exothermic peak at 550°C. The TG curve shows a weight loss of 31.1 percent in the temperature interval from 100 to 200°C. The observed phase changes may be summarized as follows: (natural sample) 120°C, a partially dehydrated form; 180°C, a non-crystalline state; 550°C, a recrystallized state; 990°C, a transformed state. Taranakite may be formed as bat guano in clay sediments takes up potassium and aluminum ions dissolved from the clays.