A Modern, Guano-Related Occurrence of Foggite, CaAl(PO\u3csub\u3e4\u3c/sub\u3e)(OH)\u3csub\u3e2\u3c/sub\u3e·H\u3csub\u3e2\u3c/sub\u3eO and Churchite-(Y), YPO\u3csub\u3e4\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO in Cioclovina Cave, Romania

This study reports foggite and churchite-(Y) from two spatially separate locations in the guano-related phosphate deposit from the Cioclovina Cave, Romania. Optical microscope observations, powder X-ray diffraction, electron microprobe analyses, and FTIR were used in the analysis of the two minerals. The chemical composition of foggite was determined to be Ca0.925(Al0.91Fe2+ 0.016)Σ0.926(P0.991Si0.043)Σ1.034O3.74(OH)2.26 · H2O and churchite-(Y) [(Y0.830Dy0.043Er0.033Gd0.029Yb0.022)Σ0.957Ca0.009]P1.023O4.00 · 2H2O. Chemical analyses of Cioclovina churchite-(Y) clearly revealed enrichment in lanthanides of even atomic number. The refined unit-cell parameters are for foggite (orthorhombic) a = 9.264(1) Å, b = 21.334(8) Å, c = 5.197(7) Å, and V = 1027.13(8) Å3 (Z = 8); for churchite-(Y) (monoclinic): a = 5.578(8) Å, b = 15.013(6) Å, c = 6.277(8) Å, β = 117.94(4)°, and V = 464.38(5) Å3 (Z = 4). FTIR spectrum of churchite-(Y) exhibits all the bands assigned to the vibrations of PO4, OH, and water groups. Unlike other documented occurrences of foggite and churchite-(Y), in Cioclovina Cave, the occurrence of these minerals are related to a process that phosphatized subjacent limestone and various cave sediments (sand, clay, and limy mud) to form a complex phosphate assemblage. The minerals are presumably derived from phosphate-rich solutions that reacted with clay earth while moving downward through the sediments. Foggite was formed at the expense of the originally precipitated crandallite. Locally concentrated yttrium, REE, and dissolved phosphate are probably responsible for the precipitation of churchite-(Y)