Bat guano deposits in caves host diverse assemblages of authigenic phosphates and sulfates. Previous field observations have proposed that some of them (e.g. brushite, gypsum, taranakite) can be subject to dehydration reactions as a response to varying environmental conditions. To evaluate the thermodynamic and kinetic constrains that regulate the dehydration reaction leading guano-derived taranakite (K3Al5(PO3OH)6(PO4)2·18H2O) to transform into francoanellite (K3Al5(PO3OH)6(PO4)2·12H2O), we conducted a multi-analytical in situ in- vestigation consisting of temperature-resolved X-ray powder diffraction, Fourier transform infrared, and microRaman spectroscopy on a sample of taranakite. Thermodynamic calculations were also performed by means of the polyhedral approach to predict the phase transformation temperature at equilibrium, estimated to be 369.12 K (95.97 °C). Laboratory experiments conducted under increasing temperature conditions confirmed that the breaking of weak hydrogen bonds between interlayer water molecules and the aluminophosphate layers of taranakite is responsible for the onset of the dehydration reaction. Monitoring the evolution of the phosphate-stretching Raman peaks over time under isothermal conditions enabled us to estimate the activation energy of the process to be 7.6 ±0.7 kJ mol−1 by means of the “time to a given fraction” method. The release of heat concomitant to the oxidation of organic matter in guano-admixed cave sediments is proposed as a viable trigger for the transformation, given the low kinetic barrier associated with it.
Galliano, Y., Campomenosi, N., Bellatreccia, F., Belmonte, D., De Waele, J.o., Carbone, C. (2026). Dehydration reactions in guano-derived minerals: the taranakite-to-francoanellite transformation. EUROPEAN JOURNAL OF MINERALOGY, 38(3), 353-372 [10.5194/ejm-38-353-2026].
Dehydration reactions in guano-derived minerals: the taranakite-to-francoanellite transformation
De Waele, Jo;
2026
Abstract
Bat guano deposits in caves host diverse assemblages of authigenic phosphates and sulfates. Previous field observations have proposed that some of them (e.g. brushite, gypsum, taranakite) can be subject to dehydration reactions as a response to varying environmental conditions. To evaluate the thermodynamic and kinetic constrains that regulate the dehydration reaction leading guano-derived taranakite (K3Al5(PO3OH)6(PO4)2·18H2O) to transform into francoanellite (K3Al5(PO3OH)6(PO4)2·12H2O), we conducted a multi-analytical in situ in- vestigation consisting of temperature-resolved X-ray powder diffraction, Fourier transform infrared, and microRaman spectroscopy on a sample of taranakite. Thermodynamic calculations were also performed by means of the polyhedral approach to predict the phase transformation temperature at equilibrium, estimated to be 369.12 K (95.97 °C). Laboratory experiments conducted under increasing temperature conditions confirmed that the breaking of weak hydrogen bonds between interlayer water molecules and the aluminophosphate layers of taranakite is responsible for the onset of the dehydration reaction. Monitoring the evolution of the phosphate-stretching Raman peaks over time under isothermal conditions enabled us to estimate the activation energy of the process to be 7.6 ±0.7 kJ mol−1 by means of the “time to a given fraction” method. The release of heat concomitant to the oxidation of organic matter in guano-admixed cave sediments is proposed as a viable trigger for the transformation, given the low kinetic barrier associated with it.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
