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

摘要

The paper aims to give details on the phosphate association from the Meresti Cave, Persani 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 {[Ca_(9.804)Mn_(0.004)Mg_(0.067)Fe~(2+)_(0.054)K_(0.002)Na_(0.003)](P_(5.934)S_(0.066_)O_(23.929)(OH)_(2.071)) has a = 9.434(3) A, c = 6.868(3) A and V = 529.7(3) A~3, D_x = 3.142g/cm~3, D = 3.14(1) g/cm~3, omega = 1.647(1) and epsilon = 1.641(1). CO_3 bands in the infrared absorption spectrum (V_3 = 1466cm~(-1), V_3' = 1430cm~(-1), v_2 = 872cm~(-1)) show that hydroxylapatite from Meregti is carbonate-bearing. Brushite has the mean chemical-structural formula [Ca_(0.984)Mn_(0.002)Mg_(0.009)Fe~(2+)_(0.054)K_(0.001)Na_(0.001)](HPO_4)_(0.976)(SO_4)_(0.024) centre dot 2H_2O. Both unit cell [a = 5.812(5) A, b = 15.169(5) A, c = 6.239(4) A and beta = 116.35(13) deg] and physical parameters [D = 2.32(1) g/cm~3, alpha = 1.540(2), beta = 1.546(2), gamma_(calc) = 1.551, for 2V_(alpha) = 86 deg] characterize a term close to stoichiometry. Ardealite has the mean chemical structural formula {[Ca_(1.992)Mg_(0.004)Fe~(2+)_(0.002)K_(0.001)Na_(0.001)](HPO_4)_(1.021)(SO_4)_(0.979) centre dot 4H_2O}. Its mean unit-cell parameters are a = 5.718(3) A, b = 30.998(24) A, c = 6.248(5) A and (3 = 117.17(5) deg, and the measured physical constants a = 1.529(2), (3 = 1.537(2), y (calculated for 2V_alpha = 86 deg) = 1.544, and D = 2.327(3) g/cm~3. 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 finally 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.