Insights into the Formation Conditions and Alteration Histories of Li-Rich Pegmatites from Spodumene Iron Oxidation States

Pegmatites are plutonic bodies characterized by complex mineral intergrowth textures and large (>2.5cm) crystals. Spodumene (LiAlSi2O6) is found in granitic, Li-rich pegmatites and crystallizes either from (1) highly-fractionated magma, (2) miarolitic fluid, or (3) the isochemical breakdown of petalite (LiAlSi4O10) into spodumene-quartz intergrowth (SQI). As a pyroxene mineral, spodumene can incorporate trace to minor amounts of redox-sensitive elements like Fe into its structure. Fe3+ likely substitutes for Al3+ in the M1 site; however, Fe2+ coordination in spodumene is poorly understood. Identifying differences in Fe oxidation states within single spodumene crystals - and between spodumene formed through the three processes listed above - could provide constraints on spodumene stability, as well as offer insights into pegmatite crystallization conditions, crystal growth mechanisms, and alteration history.

Spodumene crystal growth zoning was characterized using cathodoluminescence and/or X-ray fluorescence (µ-XRF). Minor and trace element concentrations were quantified using LA-ICP-MS. Fe K-edge X-ray absorption spectroscopy (XAS) and synchrotron-XRF were performed at the GeoSoilEnviro-CARS sector, Advanced Photon Source, Argonne National Laboratory. Crystallographic orientation effects on X-ray absorption were systematically studied both in single crystals of spodumene cut at known orientations and mounted in epoxy pucks and in oriented crystals mounted on a goniometer. In both methods, samples were rotated 180° with spectra collected at every 10° (goniometer) or 20° (epoxy pucks) to reveal the range of spectral variation caused by orientation. Most samples contain both Fe3+ and Fe2+, with prominent pre-edge peaks at ~7112.8 and ~7114.1 eV associated with Fe3+, and a weaker pre-edge bump at ~7111.2 eV and rising-edge shoulder at ~7120.5 eV associated with Fe2+. Fe oxidation varies within individual crystals; however, this variation is seemingly unrelated to concentration zoning. In the case of a magmatic crystal with late-stage hydrothermal alteration, the unaltered portions contain both Fe3+ and Fe2+, while altered areas contain only Fe3+. In addition to XAS methods, Fe oxidation states of the samples used in this study will be quantified using Mössbauer spectroscopy.