A black substance exuding from fractures was observed in 2012 Ytterby mine, Sweden, and identified 2017 as birnessite with the composition Mx[Mn(III,IV)]2O4∙(H2O)n. M is usually calcium sodium, x around 0.5. The unique, being calcium, magnesium, also rare earth elements (REEs) constituting up to 2% of total metal content. biogenic origin established 2018. Analysis microbial processes leading fo...