Phosphate minerals formed by reaction of bird guano with basalt at Cooks Head Rock and Green Island, Otago, New Zealand

The seldom-described potassium-bearing phosphate minerals taranakite and leucophosphite occurs as veins, crusts, and alteration pools in jointed basalt of a sea stack and in basaltic breccia of Green Island off the Otago coast. All occurrences are microcrystalline aggregates. Electron microprobe analyses of taranakite reveal compositions very similar to the ideal formula. In contrast, leucophosphite analyses are lower in potassium and richer in titanium and aluminium than previously studied occurrences. Both localities host seabird populations and it is inferred that bird guano supplied both the phosphorus and potassium essential to these minerals. Aluminium, iron, and titanium, on the other hand, are probably derived from the basalt host rock.