The taranakite – francoanellite dehydration reaction: new data and possible implications for minerogenetic processes in cave environments.

Taranakite (K3Al5(PO3OH)6(PO4)2·18H2O, trigonal, R3C) is a hydrated phosphate occurring in caves as a reaction product between bat guano and detrital silicates. Its presence has been documented widely throughout the world (HILL & FORTI, 1997). Previous experimental work has determined its high sensitivity to temperature change, which can lead to the loss of part of its structural water and the formation of the lower hydrate form francoanellite. The latter has also been proven to form in nature by BALENZANO et al. (1976), transforming taranakite into francoanellite (K3Al5(PO3OH)6(PO4)2·12H2O, trigonal, R3C). The name derives from Franco Anelli, former Italian karstologist and discoverer of the Castellana Caves (Apulia, Italy), where the first occurrence of the mineral was documented. With this contribution, we present a new occurrence of taranakite from the Pollera Cave (Liguria, Italy) and new experimental data regarding its thermally induced dehydration and consequent formation of francoanellite. Hence, we discuss a possible genetic mechanism for this rare and seldom investigated cave mineral both in natural (wild) caves and in show caves (such as in Castellana Cave, where the mineral was first discovered).