Material from the type locality, the “dry” Cioclovina Cave, Şureanu Mountains, Romania, was re-examined in order to update the descriptive mineralogy of ardealite, a rare hydrated calcium acid phosphate sulfate. Ardealite from Cioclovina has a S/P ratio ranging between 1/0.87 and 1/0.98. Although S remains the main tetrahedral cation in the structure, P is consistently present at concentrations between 19.10 and 20.45 wt% P2O5 (0.928–0.992 P atom per formula unit). Concerning the other cations, the mineral shows a very restricted range of composition, without Fe and with very low Mn, Mg, Na and K contents. The indices of refraction are α = 1.530(2), β = 1.537(2) and γ (calculated for 2Vγ = 86°) = 1.543. The measured density [Dm = 2.335(3)–2.342(5) g/cm3] agrees well with the calculated values [Dx = 2.317–2.350 g/cm3]. The average unit-cell parameters refined from 31 sets of X-ray powder diffraction data are a = 5.719(5), b = 31.012(28), c = 6.249(7) Å and β = 117.21(6)°. Thermally assisted X-ray diffraction analyses confirm that water is lost in three steps; the loss of molecular water is a two-step process and is complete below 250 °C. The first thermal breakdown products are brushite and bassanite. The band multiplicity on the IR-absorption spectrum (3ν3 + 1ν1 + 3ν4 + 2ν2) suggests that the protonated phosphate and sulfate groups have Cs point symmetry. The mineral derives from the reaction between calcium carbonate from the moonmilk flows or the cave floor and phosphoric solutions derived from bat guano, with or without hydroxylapatite as a precursor, at pH values up to 5.5.

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