Abstract
Detailed mineralogical analysis of soils from the UK’s historical uranium mine, South Terras, was performed to elucidate the mechanisms of uranium degradation and migration in the 86 years since abandonment. Soils were sampled from the surface (0–2 cm) and near-surface (25 cm) in two distinct areas of ore processing activities. Bulk soil analysis revealed the presence of high concentrations of uranium (<1690 p.p.m.), arsenic (1830 p.p.m.) and beryllium (~250 p.p.m.), suggesting pedogenic weathering of the country rock and ore extraction processes to be the mechanisms of uranium ore degradation. Micro-focus XRF analysis indicated the association of uranium with arsenic, phosphate and copper; µ-XRD data confirmed the presence of the uranyl-arsenate minerals metazeunerite (Cu(UO2)2(AsO4)2·8H2O) and metatorbernite (Cu(UO2)2(PO4)2·8H2O) to be ubiquitous. Our data are consistent with the solid solution of these two uranyl-mica minerals, not previously observed at uranium-contaminated sites. Crystallites of uranyl-mica minerals were observed to coat particles of jarosite and muscovite, suggesting that the mobility of uranium from degraded ores is attenuated by co-precipitation with arsenic and phosphate, which was not previously considered at this site.
Highlights
The UK’s most important uranium-producing mine, South Terras, is located in the St
The occurrence of metazeunerite at South Terras has been previously inferred,[18] to our knowledge, this study provides conclusive and verifiable evidence for this phase at the site, and for any studied uranium-contaminated vadose sediments.[14,15,16,17]
Our data indicate that metazeunerite and metatorbernite were found to occur in solid solution, which has not been previously observed at other uranium-contaminated sites where uranyl-micas are present.[14,15,16,17]
Summary
The UK’s most important uranium-producing mine, South Terras, is located in the St. Austell District of Cornwall (SW England, Supplementary Fig. 1), which operated between 1873 and 1930.The primary ore, pitchblende (primarily UO2 and U3O8), is associated with late stage metamorphism related to the St. The tailings were reworked to extract radium through BaSO4 recovery methods.[1]
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