Abstract. 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).