Secondary lanthanide phosphate mineralization in weathering profiles of I-, S-, and A-type granites
Rare earth elements (REEs, "lanthanides") constitute a vital commodity for technological applications. Although these elements occur at trace levels in many minerals, they can comprise major constituents of low abundance phosphate, carbonate, silicate and oxide minerals, some of which form during granite weathering. REE-phosphate phases can be a source of phosphorus for essential biomolecules and certain REEs are required in the active site of some bacterial enzymes involved in the oxidation of
... in the oxidation of methanol, an important compound in the global biogeochemical carbon cycle. The mechanisms that promote the dissolution of lanthanide phosphate minerals are largely unknown, but likely vary with the lanthanide phosphate mineralogy of weathered rock and soil. Here, we studied weathering of five I-type, three S-type and one A-type granites to determine the extent of weathering of primary REE and/or P-bearing minerals apatite, allanite and monazite, and the formation of secondary REE/P-minerals. We found evidence for greater mobilization of REE and P in weathered I-type and A-type granites than in S-types, reflecting the higher solubility of apatite and allanite relative to monazite. Although monazite persisted in highly weathered Stype granites, some alteration was detected. Secondary REE/P-minerals were not detected in two S-type profiles, while spherical shaped secondary REE/P-mineral aggregates were abundant throughout the third S-type profile. Secondary euhedral REE/P-crystals were abundant even in the slightly weathered I-type and A-type granite material, yet they were not detected in the highly weathered material, indicating that these minerals had dissolved. Our findings indicate that mineralogy substantially constrains, but does not completely control, lanthanide availability as a function of degree of weathering. These results have implications for predicting REE and phosphate bioavailability in soils derived from granitic rock types and suggest that highly weathered I-type granites may provide inocula for bioleaching experiments.