REE-Enriched Mn-Oxide Precipitates in Water-Bearing Fractures in the Ytterby Mine, Sweden

The Ytterby mine, Sweden, is known for the discovery of eight elements, including yttrium and five of the rare earth elements (REE). The mine was in operation from 1750 to 1933 and was after closure used as a storage depot for fuel from the 1950s to 1995. A tunnel was opened in the 1950s through the bedrock into the mine to allow access to the storage depot. Recent water monitoring campaigns (20122015) in the mine revealed a black substance (denoted YBS) in some fractures opening into the tunnel. Analysis of the YBS (elemental analysis, phase analysis by XRD, SEM with energy dispersive X-ray spectrometry, IR-and EPR-spectroscopy, preferential leaching at pH 4) showed that the main mineral component of the YBS is the manganese oxide birnessite. Also minor quantities of other less well defined manganese oxides were found, as well as silicates (quartz grains, possibly feldspar grains) and calcite. Birnessite has typically the composition Mx(Mn3+, Mn(2)(4+))O(4)xAq, with M= Na, Ca and x= 0.5. The birnessite component in YBS had a Mn3+/Mn4+ ratio of 1.04/0.96 with M = 0.42 Ca + 0.03 (REE+Y), 0.03 Mg and 0.03 other metals. All of these metals were firmly associated with the structure, since no release was observed at pH 4, except for significant fractions of the total Na, Mg, Ca-contents. Thus, REE+Y correspond to 1% of the total YBS mass and up to 3% of the metal content in the birnessite phase. This corresponds to an REE enrichment factor of the order 106 (YBS-birnessite/ fracture water). Birnessite with a substantial fraction of REE in the lattice has not previously been reported. The formation of birnessite is a microbial process. Identification of the microorganisms present in the Ytterby system is in progress.