Natural and Experimental Insights into the Formation, Preservation and Composition of Rhabdophane (REEPO4 · xH2O, REE = La - Lu, x = 0 - 1) in Rare Earth Element Deposits

Toby G Bamforth

In transitioning away from coal, oil and natural gas as primary energy sources, the world will require a significant volume of raw materials that are essential for the development and manufacture of renewable technologies. Many of these materials are designated globally as ‘critical metals’ – a title bestowed upon those that are increasingly high in demand, but whose supply chains are subject to geopolitical disruption. Among the most critical metals are the seventeen rare earth elements (REEs, La – Lu, Y and Sc), which are vital for the manufacture of permanent magnets and the production of wind turbines and electric vehicles. However, natural REE deposits are both: 1) a relatively recent focus of study, given the more modern nature of the REE’s increasing end-uses, and; 2) mineralogically and geochemically complex, given the requirement to track and understand the behaviours of seventeen different metals in ore systems than are often subject to many hydrothermal, magmatic and supergene processes. Most REE extraction has therefore focussed on the exploitation of only two minerals – monazite (REEPO4) and bastnaesite (REECO3F) – though others may also represent important economic sources of the REEs. Rhabdophane (REEPO4 · xH2O, x = 0 – 1) is one such mineral. While rhabdophane appears analogous to monazite, it is: 1) known to precipitate exclusively as a low temperature hydrothermal mineral, which provides information as to the nature of ore formation, and; 2) mineralogically distinct, such that the REEs and trace elements (i.e., U) behave differently between its precipitation and the precipitation of monazite. Despite these variations, studies of natural rhabdophane are sparse and the two minerals are rarely discerned. Following an introductory Chapter 1, this thesis characterises the primary geochemical and mineralogical features of rhabdophane in natural REE deposits so as to better understand its potential as a source of critical metals. Chapter 2 initiates this by describing a rhabdophanerich saprolite from the Kapunda Cu mine, South Australia. It finds that the rhabdophane formed via the interaction of acidic, REE-rich weathering fluids and apatite (Ca5[PO4]3[OH, F, Cl]). Rhabdophane precipitation (and REE accumulation, up to 12.37 wt.%) is subsequently linked to these highly-acidic weathering fluids that: 1) mobilised REEs from primary minerals, and; 2) expediated the liberation of phosphate into the weathering fluid. These results show how acidic weathering fluids might cause REE mobilisation or accumulation in supergene systems. Chapter 3 then provides a concise insight into the composition of the Kapunda rhabdophane, with specific regard to its high concentrations of uranium (U, up to 5100 ppm) – a radioactive contaminant that can taper the economic and environmental value of a REE ore. Synchrotron XANES analysis of the Kapunda rhabdophane demonstrates that it likely hosts pentavalent U(V). This contradicts the prevailing theory of how U substitutes into rhabdophane, which insists that it must be tetravalent, U(IV), to undergo the same cheralite ([Ca,{Th,U}][PO4]2) and huttonite ([Th,U]SiO4) type coupled substitutions found in monazite. Hydrothermal REEphosphates are thus described for the first time to accommodate ‘mobile’ U(VI) uptake, via a mechanism that is: 1) most likely prominent under oxidising conditions, where Ce (III) is also oxidised to cerianite (CeO2) and results in the reduced uptake of Ce (III) (and heavier uptake of La, Nd and U) into rhabdophane, and; 2) facilitated by breaking of the uranyl ion (UO2 2+) during uptake, which causes the U(VI) to bond in octahedral co-ordination prior to its reduction to U(V) by adjacent Ce(III) ions as is indicated by adjacent geometry optimisation modelling. Chapter 4 expands outwards to determine how deposit-scale rhabdophane-rich horizons can be targeted in deeply weathered terrains (i.e., supergene REE deposits) using whole-rock geochemistry. Unsupervised multivariate analyses (i.e., K-means clustering and Principal Component Analysis, PCA) were applied to over 3000 whole-rock assays from the Splinter Rock REE Prospect, Western Australia, and were successful in delineating the rhabdophaneenriched saprolite from its mineralogically indistinguishable overlying sediments. PCA was then used to develop tailored geochemical indices with which to assign new assays to their relevant regolith horizons. Results highlight the efficacy of multivariate analysis for REE exploration in regolith while demonstrating how economic REE enrichment may be facilitated by: 1) cerium oxidation, and; 2) greater metal accumulation at the saprolite-saprock boundary. Lastly, Chapter 5 details the experimental replacement of natural apatite by rhabdophane and of rhabdophane by monazite, under controlled conditions that mimic those identified in prior chapters. In reacting with a Ce-rich fluid at 30°C, apatite was replaced by rhabdophane where voids of varying sizes developed between both phases due to anisotropic apatite dissolution. Rhabdophane was then replaced by monazite at 180°C, in REE-barren fluids of increasing phosphate concentrations which systematically increased the rate of replacement. Monazite precipitation was thus identified as the rate-limiting step, though rhabdophane aggregates were pseudomorphically preserved at the micro-scale where individual nano-scale needles were not. In both experiments, trace U partitioned into the reaction solution. Results demonstrate how: 1) rhabdophane is a suitable precursor to monazite in supergene systems; 2) both minerals should be differentiated crystallographically; 3) mineral replacement influences U mobilisation in REE ores, and; 4) the rate of this replacement is dependent on the composition of the fluid.

Natural and Experimental Insights into the Formation, Preservation and Composition of Rhabdophane (REEPO4 · xH2O, REE = La - Lu, x = 0 - 1) in Rare Earth Element Deposits

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