EVIDENCE OF TRACE ELEMENT SPECIATION IN SPODUMENE FROM X-RAY ABSORPTION FINE STRUCTURE (XAFS) SPECTROSCOPY

Spodumene (LiAlSi₂O₆) is a lithium ore mineral found in granitic pegmatites and readily incorporates trace elements into its pyroxene crystal structure. Understanding the incorporation of trace elements into spodumene can improve the processing of Li ores. Identifying the crystal-site distribution and speciation of redox-sensitive trace elements such as Fe and Mn could offer insight into the redox state of the pegmatite at the time of crystallization, the pegmatite crystal growth mechanisms, and the alteration history. Our study uses a combination of cathodoluminescence (CL), X-ray fluorescence (XRF), LA-ICP-MS, and X-ray absorption fine structure (XAFS) spectroscopy to identify areas of differing Fe and Mn speciation.

We collected samples from two case-study locations: Tin Mountain (TM) pegmatite in Black Hills, SD and Dibs pegmatite from Winnipeg River pegmatite field, Manitoba. Spodumene from Tin Mountain formed as giant, meter-scale subhedral crystals of primary magmatic origin, while mm-scale spodumene from Dibs formed as a secondary product of petalite (LiAlSi₄O₁₀) breakdown. Compositional zoning was revealed using XRF for the TM giant crystal and CL for the Dibs fine-grained spodumene. Distinct core to rim LA-ICP-MS concentrations for the TM crystal averaged 700 to 350 ppm for Mn and 2000 to 2500 ppm for Fe, respectively. Dibs had lower overall concentrations with high zones averaging 200 ppm Mn and 200 ppm Fe, and low zones averaging 60 ppm Mn and 40 ppm Fe.

Our XAFS measurements from Advanced Photon Source, Argonne National Laboratory indicate that the speciation of Fe and Mn varies within individual crystals, however, this variation is unrelated to concentration zoning. The TM spodumene shows two distinct Fe-spectrum patterns, likely related to late-stage hydrothermal alteration. After preliminary peak-deconvolution, unaltered spodumene contains both Fe(II) and Fe(III), while altered spodumene contains only Fe(III). The Dibs results are more complex, with at least three distinct Fe-spectrum patterns and at least two distinct Mn patterns.

Integrated in situ concentration and speciation analysis of spodumene can be further used to reveal pegmatite crystallization conditions, as well as alteration processes.