Rare earth nanominerals in bentonite deposits of the Eocene Texas coastal plains

Mineral phases and modes of occurrence of rare earth elements and yttrium (REE) in weathering environments are poorly understood but constitute a potential economic source. In Texas, deposits of bentonite that formed by alteration of rhyolitic ash in a coastal and marine influenced environment show REE enrichment comparable to some North American coal and deposits that are under consideration for extraction. We investigated the post depositional mineralization and modes of REE occurrence in a bentonite deposit from Gonzales, Texas, in which total REE concentration exceeds 450 ppm. Scanning electron microscopy (SEM) revealed the presence of sub-micron to nano-size REE phosphate particles dispersed in the montmorillonite matrix, aggregated at the edge of incompletely altered phenocrysts, or in the void space separating montmorillonite stacks. Chemical composition distinguishes a prevalent group of phosphate with Ce > La ~ Nd > Y > Pr > Sm, also bearing Ca and trace amounts of Th and U, and a less common Y-phosphate with trace amounts of other REEs. Transmission electron microscopy (TEM) analysis indicated that the most common particles exhibit a platy morphology, with a diameter smaller than 200 nm and a thickness of less than 100 nm. Electron diffraction, 3D electron diffraction, and high-resolution TEM clearly showed that the mineral phase was monazite. We interpreted the observed REE geochemistry and mineralogy as dissolution of volcanic glass deposited in an alkaline aqueous environment that released REEs into pore water that became oversaturated due to precipitation of montmorillonite, the availability of P in the solution prompted precipitation of monazite nanoparticles. The reduction in permeability following formation of montmorillonite and precipitation of opal in void spaces facilitated accumulation.