Original paper

Hydroxylapatite, brushite and ardealite in the bat guano deposit from Pestera Mare de la Meresti, Persani Mountains, Romania

Marincea, Ştefan; Dumitraş, Delia-Georgeta; Diaconu, Gabriel; Bilal, Essaï d


The paper aims to give details on the phosphate association from the Mereşti Cave, Perşani Mountains, Romania. Ca-phosphates (i.e., hydroxylapatite, brushite and ardealite) are the main components of the guano deposit inside this cave. Associated minerals include gypsum, low quartz, 2M1 illite, interstratified kaolinite-illite, calcite and X-ray amorphous iron sesquioxides. Hydroxylapatite {[Ca9.804Mn0.004Mg0.067Fe2+0.054K0.002Na0.003](P5.934S0.066)O23.929(OH)2.071} has a = 9.434(3) Å, c = 6.868(3) Å and V = 529.7(3) Å3, Dx = 3.142 g/cm3, D = 3.14(1) g/cm3, ω = 1.647(1) and ε = 1.641(1). CO3 bands in the infrared absorption spectrum (ν3 = 1466 cm–1, ν3' = 1430 cm–1, ν2 = 872 cm–1) show that hydroxylapatite from Mereşti is carbonate-bearing. Brushite has the mean chemical-structural formula [Ca0.984Mn0.002Mg0.009Fe2+0.004K0.001Na0.001](HPO4)0.976(SO4)0.024 . 2H2O. Both unit cell [a = 5.812(5) Å, b = 15.169(5) Å, c = 6.239(4) Å and β = 116.35(13)°] and physical parameters [D = 2.32(1) g/cm3, α = 1.540(2), β = 1.546(2), γcalc = 1.551, for 2Vα = 86°) characterize a term close to stoichiometry. Ardealite has the mean chemical structural formula {[Ca1.992Mg0.004Fe2+0.002K0.001Na0.001](HPO4)1.021(SO4)0.979 · 4H2O}. Its mean unit-cell parameters are a = 5.718(3) Å, b = 30.998(24) Å, c = 6.248(5) Å and β = 117.17(5)°, and the measured physical constants α = 1.529(2), β = 1.537(2), γ (calculated for 2Vα = 86°) =1.544, and D = 2.327(3) g/cm3. The textural relationships between the three calcium phosphates, observed by scanning electron microscopy and energy-dispersive electron microprobe analysis, suggests that their sequence of crystallization was from hydroxylapatite to brushite and fi- nally ardealite. All the three species clearly resulted from the reaction between the strongly acidic solutions derived from the guano mass and the cave floor or moon-milk flows.


hydroxylapatitebrushiteardealitephysical propertiescrystal chemistryunit-cell parametersinfrared absorption datapestera mare from merestipersani mountainsromania