Trace element geochemistry of spodumene megacrystals: A combined portable-XRF and micro-XRF study

Abstract

Formation mechanisms of spodumene (LiAlSi2O6) megacrystals, such as those historically mined in the Black Hills of South Dakota or recently discovered in the Plumbago North deposit, Newry, Maine, remain puzzling, despite decades of observations and speculations. In-situ trace element geochemistry and zoning patterns in spodumene could provide insights into the primary growth mechanisms and optimize the extraction of lithium, but the size of these giant crystals greatly exceeds the chamber size of conventional compositional-mapping techniques. Here we propose a novel two-step approach for documenting the distribution of trace elements in sliced crystals, several decimeters in size, using 2D compositional mapping via 1) portable X-ray Fluorescence (pXRF) collected across a square grid with a step size of 0.5 to 1 cm and 2) benchtop micro-X-ray Fluorescence (μXRF) at step-size ≥ 25 μm using crystals or portions of crystals that were pre-screened via pXRF. Mineral-specific analytical optimization included specific filters and/or detector settings in order to avoid the Bragg peaks interfering with the Ka lines of Mn, Fe, Ge, Ga, ± Sn in spodumene when using a Bruker TRACER 5i pXRF unit, an M4 TORNADO μXRF instrument, or an M4 TORNADOPlus instrument.Two subhedral spodumene samples analyzed along crystallographically-oriented sections have a relatively homogenous, several decimeter-wide core (Zone 1) postdated by a strongly zoned, several centimeter-thin rim (Zone 2). With the increase in spatial resolution by three orders of magnitude, extraordinary oscillatory ± sector growth zoning, 0.1 mm to >1 cm wide, was revealed using μXRF in selected areas of Zone 2, that were pre-screened by pXRF. Typically, both Zones 1 and 2 are characterized by coupling of Fe with Ga ± Ti and decoupling from Mn, Sn, and Ge. An exception is Sn that decouples from Mn and couples with Fe,Ti, and Ga in Zone 2. This consistent zoning in crystals from distinct localities suggests two distinct growth episodes, marked by a sudden change in the crystallization mechanism, possibly from an undersaturated magma to fluid-saturated system. This integrated pXRF-μXRF method may assist with compositional and alteration characterization of lithium ore to optimize the processing and extraction methods. Additionally, the integrated techniques open a new venue of investigation of chemical heterogeneities and zoning patterns within coarse to megacrystic minerals and may serve as an excellent sample pre-screening prior to other microanalytical techniques.