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

Abstract

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 . 8 0 4 Mn 0 . 0 0 4 Mg 0 . 0 6 7 Fe 2 + 0 . 0 5 4 K 0 . 0 0 2 Na 0 . 0 0 3 ](P 5 . 9 3 4 S 0 . 0 6 6 )O 2 3 . 9 2 9 (OH) 2 . 0 7 1 } has a = 9.434(3) A, c = 6.868(3) A and V = 529.7(3) A 3 , D x = 3.142 g/cm 3 , D = 3.14(1) g/cm 3 , ω = 1.647(1) and e = 1.641(1). CO 3 bands in the infrared absorption spectrum (ν 3 = 1466 cm - 1 , ν 3 ' = 1430 cm - 1 , ν 2 = 872 cm - 1 ) show that hydroxylapatite from Meresti is carbonate-bearing. Brushite has the mean chemical-structural formula [Ca 0 . 9 8 4 Mn 0 . 0 0 2 Mg 0 . 0 0 9 Fe 2 + 0 . 0 0 4 K 0 . 0 0 1 Na 0 . 0 0 1 ](HPO 4 ) 0 . 9 7 6 (SO 4 ) 0 . 0 2 4 . 2H 2 O. Both unit cell [a = 5.812(5) A, b = 15.169(5) A, c = 6.239(4) A and β = 116.35(13)°] and physical parameters [D = 2.32(1) g/cm 3 , a = 1.540(2), β = 1.546(2), γ c a l c = 1.551, for 2V α = 86°) characterize a term close to stoichiometry. Ardealite has the mean chemical structural formula {[Ca 1 . 9 9 2 Mg 0 . 0 0 4 Fe 2 + 0 . 0 0 2 K 0 . 0 0 1 Na 0 . 0 0 1 ](HPO 4 ) 1 . 0 2 1 (SO 4 ) 0 . 9 7 9 . 4H 2 O]. Its mean unit-cell parameters are a = 5.718(3) A, b = 30.998(24) A, c = 6.248(5) A and β = 117.17(5)°, and the measured physical constants a = 1.529(2), β = 1.537(2), y (calculated for 2V α = 86°) = 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 moonmilk flows.