Sequential dehydration of the phosphate–sulfate association from Gura Dobrogei Cave, Dobrogea, Romania

A rich association of primary guano minerals, including taranakite, hydroxylapatite, brushite and gypsum with relicts of illite, kaolinite, alpha (low) quartz and calcite, was identified in the fossil bat guano deposit from Gura Dobrogei Cave, Dobrogea County, Romania. Gypsum and Ca phosphates developed preferentially on the carbonate bedrock or on fallen carbonate blocks in the guano mass, whereas taranakite was identified in the clay-rich, detritic sequences. The mineral species from the cave were characterized by optical methods, scanning electron microscopy, X-ray powder diffraction, Fourier-transform infrared and inductively coupled plasma–atomic emission spectrometry analysis. Chemically induced local dehydration of primary minerals, characterized by low temperatures (up to 100 ∘C or even lower) and critically depending on exothermal reactions in the guano mass, prompted the formation of a secondary association, consisting of francoanellite, bassanite and monetite. Topotactic substitutions were observed in the cases of francoanellite on taranakite, bassanite on gypsum and monetite on brushite. In its turn, ardealite was partially replaced by monetite and bassanite. The sequential dehydration process seems driven by the degradation of organic matter by microbial action and also, presumably, by other exothermic reactions at local scale (e.g., oxidation of ammonia, allogenic pyrite or other organic compounds).