Taranakite‑to‑Francoanellite Dehydration Revealed

A new European Journal of Mineralogy paper documents the dehydration transformation from taranakite to francoanellite within bat guano deposits found in caves, offering a detailed thermodynamic and kinetic framework for this process.¹

The research was conducted by Yuri Galliano, Nicola Campomenosi, Fabio Bellatreccia, Donato Belmonte, Jo De Waele, and Cristina Carbone and appears in volume 38, pages 353–372 of Eur. J. Mineral., published in 2026 under DOI ²]

Bat guano—bird droppings that accumulate over centuries—forms complex secondary mineral assemblages inside cave systems. These environments host minerals that can undergo partial dehydration when environmental conditions shift, such as temperature or humidity changes.¹

In their study, the authors focused on the specific pathway whereby taranakite, a hydrated calcium‑phosphate mineral, releases part of its structural water molecules and reorganizes into the anhydrous phase francoanellite. They measured the relevant enthalpy and entropy changes through calorimetric experiments and monitored the kinetics using time‑resolved X‑ray diffraction.¹

Thermodynamic modeling indicated that the dehydration is exothermic and becomes favorable above approximately 120 °C, while kinetic data showed that the reaction proceeds over several weeks at typical cave temperatures of 15–20 °C, implying long‑term stability for francoanellite once formed.¹

The team also identified key morphological markers: francianellite crystals tend to appear as small tabular fragments clustered around residual taranakite grains, often forming radiated groups that reflect the original crystal habit before dehydration.¹

These findings provide evidence‑based hypotheses explaining why francoanellite appears in natural settings where bat guano has been present for extended periods but rarely occurs in freshly deposited guano. The research suggests that both thermal history and microenvironmental factors control whether taranakite will fully dehydrate to produce francoanellite.¹