Bat Guano Minerals Reveal a Surprisingly Easy Way to Lose Their Water

Deep inside caves, where colonies of bats have roosted for centuries, the ground beneath them quietly becomes a chemistry laboratory. Bat guano, rich in organic matter, decomposes and releases phosphoric acid that attacks the surrounding bedrock and sediments, triggering the precipitation of a colourful cast of phosphate and sulfate minerals. Among the most common of these is taranakite, a hydrated potassium aluminium phosphate first discovered in 1865 as an alteration product of seabird guano on the Sugar Loaf Islands of New Zealand. Now, a team of Italian researchers has worked out exactly how, and at what temperature, this mineral sheds part of its structural water and transforms into a rarer cousin called francoanellite, a finding that helps explain one of the most elusive minerals in cave environments.

The study, led by Yuri Galliano of the University of Genoa together with colleagues from Genoa, Roma Tre and Bologna, was published in the European Journal of Mineralogy. The researchers combined three high-temperature, in situ analytical techniques with equilibrium thermodynamic calculations to pin down the transformation. Their target reaction is deceptively simple: taranakite, with the formula K3Al5(PO3OH)6(PO4)2·18H2O, loses six water molecules to become francoanellite, K3Al5(PO3OH)6(PO4)2·12H2O. Both minerals share the same layered architecture of aluminophosphate sheets, but taranakite carries additional water arranged in hexagonal interlayers that strikingly resemble the structure of ordinary ice. Francoanellite lacks these interlayers entirely, with hydrogen bonds directly connecting the layers in their place.

To predict the temperature at which this reaction reaches equilibrium, the team turned to the polyhedral approach, a modelling technique that estimates the bulk thermodynamic properties of a mineral by summing the contributions of its individual cation coordination polyhedra. Using published temperature-dependent functions for the Gibbs free energy of the constituent oxide and water units, they calculated an equilibrium transformation temperature of 369.12 kelvin, equivalent to about 96 degrees Celsius. That figure might seem far removed from the cool, damp interiors of caves, and indeed it is. The real story, as the experiments revealed, lies not in equilibrium thermodynamics but in the remarkably low kinetic barrier of the reaction.

The experimental evidence came from three complementary probes. High-temperature X-ray powder diffraction, performed at the University of Padua, tracked the long-range crystal structure as the sample was heated step by step from room temperature to 438 kelvin. Taranakite reflections remained stable up to about 358 kelvin, above which they steadily weakened while the peaks of francoanellite grew, until francoanellite stood alone at 398 kelvin. The basal spacing of the layered structure contracted from roughly 15.85 angstroms to 13.80 angstroms, a direct signature of the disappearing water interlayers. Rietveld refinement showed that taranakite’s unit cell expanded slightly through ordinary thermal expansion before dehydration began, while the francoanellite cell shrank slowly until the material collapsed into an X-ray amorphous state above about 418 kelvin.

Infrared spectroscopy added a molecular-level view of the water loss. The FTIR spectra of taranakite are a complex convolution of phosphate, hydrogen phosphate, ammonium, hydroxyl and water vibrational modes, and the researchers followed how each family of bands evolved with temperature. The integrated absorbance of the water bending and stretching regions decreased gently up to about 363 kelvin, then changed slope in a way compatible with the onset of francoanellite formation, before accelerating dramatically above roughly 403 kelvin as the remaining crystallisation water was expelled and the structure amorphised. Crucially, the spectra showed that the transformation begins with the breaking of weak hydrogen bonds that tether the interlayer water molecules to the aluminophosphate layers, the weakest links in the structural chain.

Micro-Raman spectroscopy proved to be the most sensitive witness of the phase change. The symmetric stretching vibration of the phosphate units appears near 952 wavenumbers in taranakite and shifts to about 933 wavenumbers in francoanellite, positions that can even be converted into precise phosphorus-oxygen bond lengths of 1.559 and 1.565 angstroms, matching the structural models beautifully. By fitting these two peaks in spectra collected during heating, the team could watch taranakite fade and francoanellite emerge in real time. Better still, by holding the sample at fixed temperatures of 353, 358, 363 and 373 kelvin and recording spectra every few minutes, they turned their spectrometer into a kinetic instrument, measuring how fast the transformation proceeds at each temperature.

From those isothermal runs, the researchers extracted the activation energy of the dehydration using several kinetic approaches, including Avrami and Austin-Rickett plots and a model-free method known as time to a given fraction. Within the temperature window where the reaction behaves consistently, the activation energy came out at just 7.6 plus or minus 0.7 kilojoules per mole. That is astonishingly low. Comparable dehydration reactions in variscite, montmorillonite clays and zeolites all demand more than 30 kilojoules per mole. The reason, the authors explain, is that the water molecules being released are not part of cation hydration shells but sit loosely in interlayers, bound by weak hydrogen bonds with long oxygen-oxygen distances. Once those bonds break, the transformation is essentially topotactic: the aluminophosphate layers survive intact and only new hydrogen bonds between layers need to form, keeping the energy barrier minimal. The slope values from the kinetic plots, all below one, suggest the reaction is diffusion-controlled, with water molecules escaping parallel to the layers along the interlayer channels.

These numbers solve a long-standing puzzle about francoanellite’s rarity and origin. Direct measurements inside actively decomposing guano mounds, such as those in Mulu Cave in Borneo, show temperatures seldom exceeding 30 degrees Celsius, far below the calculated equilibrium temperature of 96 degrees. But because the kinetic barrier is so small, only a modest temperature rise sustained over long periods is needed to push the reaction forward. The heat released by the exothermic microbial oxidation of organic matter in guano-admixed sediments, accumulating over tens to hundreds of years, is a plausible trigger. The researchers also note that anthropogenic heat sources, such as the powerful halogen spotlights used in tourist caves like Castellana in Apulia, where francoanellite was first discovered in 1976, could contribute. Alternative pathways, including direct precipitation from percolating solutions or dissolution-recrystallisation driven by humidity swings, remain possible but are now less necessary to invoke.

The study carries practical warnings as well as scientific insights. Taranakite transforms into francoanellite within tens of minutes at temperatures as low as 358 kelvin, conditions easily reached during routine sample preparation for X-ray diffraction, where heating is sometimes used to drive off environmental moisture. Curators and researchers handling phosphate-bearing cave samples must therefore take special care to avoid inadvertently dehydrating their specimens and destroying the very mineralogical record they hope to study. Beyond Earth, the work resonates with planetary science: hydrated sulfates and iron phosphates confirmed in the Martian regolith may have undergone similar dehydration histories, and understanding the thermodynamics and kinetics of such reactions could help reconstruct the hydrologic and atmospheric evolution of the red planet. For now, the humble guano piles of Italian caves have once again proven that even the smallest, strangest mineral archives can rewrite our understanding of how minerals live, change and die.

Subject of Research: Thermodynamics and kinetics of the dehydration transformation of guano-derived taranakite into francoanellite in cave environments

Article Title: Dehydration reactions in guano-derived minerals: the taranakite-to-francoanellite transformation

Article References: Galliano, Y., Campomenosi, N., Bellatreccia, F., Belmonte, D., De Waele, J., & Carbone, C. (2026). Dehydration reactions in guano-derived minerals: the taranakite-to-francoanellite transformation. European Journal of Mineralogy, 38(3), 353-372. https://doi.org/10.5194/ejm-38-353-2026

Image Credits: AI Generated

DOI: 10.5194/ejm-38-353-2026

Keywords: taranakite, francoanellite, bat guano, cave minerals, dehydration, phosphate minerals, X-ray diffraction, Raman spectroscopy, FTIR spectroscopy, thermodynamics, activation energy, mineralogy