APATITE-(CaOH) IN THE FOSSIL BAT GUANO DEPOSIT FROM THE 'DRY' CIOCLOVINA CAVE, S.UREANU MOUNTAINS, ROMANIA

摘要

Apatite-(CaOH) is the most abundant phosphate in the deposit of fossil bat guano in the "dry" Cioclovina Cave, Sureanu Mountains, South Carpathians, Romania. Initial deposits, of both biogenic and authigenic origin, were chemically equilibrated during diagenesis. Individual crystals are tabular, roughly hexagonal, platy on (0001) and usually between 3 and 15 mu m across and up to 1 mu m thick. The mean indices of refraction measured for 10 representative samples are epsilon 1.645(2) and omega 1.653(1). The mean measured density [D_m = 3.17(2) g/cm~3] is in good agreement with the individual calculated values. The unit-cell parameters calculated as an average of 20 sets of values are a 9.436(13), c 6.868(8) A. The mineral is Ca-deficient, carbonate-and sulfate-bearing. Less than 2.26 percent of the phosphate groups are protonated, and less than 4.66 percent are replaced by sulfate. The cumulative incorporation of other cations in the Ca sites accounts for only 0.71 to 4.07 percent (mean 2.07 percent). Both unit-cell parameters and thermal behavior are characteristic for a hydrous A-type carbonated apatite-(CaOH), with molecular H_2O and carbonate substituting for hydroxyl in the structural channels. The multiplicity of the bands in the infrared absorption spectrum (3 upsilon_3 + 1 upsilon_1I + 3 upsilon_4 + 2 upsilon_2) is consistent with a C_6 point symmetry of the phosphate anion. Whitlockite obtained by thermal breakdown at 1000 deg C is sulfate-bearing. The authigenesis of the Cioclovina apatite-(CaOH) involved a reaction between calcium carbonate from the moonmilk flows or the cave floor and phosphoric solutions derived from guano, with brushite or an X-ray amorphous phase as a precursor.