Original paper

Authigenic burbankite in the Cioclovina Cave sediments (Romania)

Onac, Bogdan P.; Bernhardt, Heinz-Jürgen; Effenberger, Herta

Abstract

The complex rare-earth-bearing anhydrous carbonate burbankite, A3B3(CO3)5, occurs as microcrystalline yellow greyish aggregates in the lower part of a lacustrine-like sediment sequence in the Cioclovina Cave, Romania. From this occurrence, foggite, churchite-(Y) and colourless or milky-white needle-like brushite and gypsum were also documented. The empirical formula (calculated from the electron-microprobe results on the basis of five carbonate groups pfu) is (Na2.46Ca0.98Sr1.71Ba0.32Y0.05Ce0.17La0.08Nd0.08Pr0.02Th0.09)Σ = 5.96(CO3)5. Single-crystal X-ray investigations gave a = 10.514(3) and c = 6.477(2) Å, space group P63mc, Z = 2. The structural refinement converged at R1 = 0.030 for 827 F0 > 4σ(F0). The crystal structure refinement was performed to verify ordering at the two sites, A and B. The A site is [6 + 2] coordinated with an average A—O bond length of 2.491 Å; the B site is [10] coordinated, the average B—O bond length is 2.678 Å. As expected the Na and Ca atoms are concentrated in the smaller AO8 polyhedron whereas the larger cations occupy the B site. The three crystallographically different carbonate groups are planar within the accuracy of structure refinement, C—O bonds vary from 1.268(4) to 1.294(3) Å. The δ13C and δ18O values for Cioclovina burbankite compare to other low-temperature cave carbonates, and thus clearly distinguish it from the more common burbankite occurring in igneous alkaline rocks. Precipitation of burbankite in cave settings is attributed to the reaction between percolating REEs, Sr- and Na-rich solutions and carbon dioxide, in an alkali-balanced environment, under dry and poor or no drainage conditions.