Rare earth element anomalies in crandallite group minerals from the Schugorsk bauxite deposit, Timan, Russia

Crandallite group minerals from the Devonian bauxite deposit of Schugorsk, Timan, Russia, were studied by electron microprobe. Special attention was g iven to the distribution of the rare earth elements (REE). Of the A-site ca tions, essential are (oxides in wt.%) Ca (0.2 - 7.9), Sr (0.4 - 5.9), Ba (0 .1 - 5.1), Pb (0.01 - 8.4), Y (to 2.4) and REE (to 21.6). Bi (A-site), Ga ( B-site) and V (X-site) occur in trace amounts. Two main types of REE distri bution in crandallite group minerals are distinguished. The first shows a c lear negative Ce anomaly and partially depleted Pr; crandallite group miner als of this type formed under strong oxidising conditions (positive Eh and neutral to slightly alkaline pH values) and lost Ce due to its oxidation to Ce4+ and accumulation in other minerals such as anatase. The second type d isplays a positive Sm anomaly and a negative Pr anomaly. Individual crandal lite group crystals belonging to this type, 1 - 40 mum in size, show a clea r compositional zoning: the core is enriched in Ca, while the rim is enrich ed in S, Sr, Pb and the REE. This crandallite formed under reducing conditi ons related to stripping of Fe from the weathering profile. The presence of Sm2+ in the crandallite lattice is proposed, and the role of organic mater ial in its reduction is discussed. A Pr anomaly was inherited from the pare nt rock. A comparison of REE distribution in crandallite group minerals fro m different weathering profiles suggests that these minerals can be used to distinguish conditions of weathering.