Taranakite is a hydrated alkali iron-aluminium phosphate mineral with chemical formula (K,Na)₃(Al,Fe³⁺)₅(PO₄)₂(HPO₄)₆·18H₂O. It forms from the reaction of clay minerals or aluminous rocks with solutions enriched in phosphate derived from bat or bird guano or, less commonly, from bones or other organic matter. Taranakite is most commonly found in humid, bat inhabited caves near the boundary of guano layers with the cave surface. It is also found in perennially wet coastal locations that have been occupied by bird colonies.