TRACE-ELEMENT DISTRIBUTION IN GIANT SPODUMENE CRYSTALS IN GRANITIC PEGMATITES FROM THE BLACK HILLS, SOUTH DAKOTA AND NEWRY, MAINE

Lithium (Li) is an important element required for high-tech applications. Lithium-rich granitic pegmatites are highly evolved magmatic rocks that are of strategic importance. Within granitic pegmatites, the mineral spodumene (LiAlSi₂O₆) can occur, and in some cases form giant meter-scale long single crystals. Spodumene can incorporate up to 3 wt % iron (Fe) in its structure, and the Fe content of Li ore dictates its industrial use and value. The effects of Fe and other trace elements on spodumene nucleation, crystallization, and stability are poorly understood.

To better document and understand the distribution of trace elements within spodumene single crystals, we used portable X-ray Fluorescence (pXRF) to assess their concentrations across growth zones in euhedral crystals. Sample preparation consists only of local grinding with a diamond blade and cleaning of the ~8 mm spots before analyzing with the pXRF to remove surface weathering. We calibrated the pXRF results against the elemental composition of a set of gem-quality, homogeneous spodumene crystals free of inclusions and alteration from world-class localities using both Laser Ablation (LA) and total acid digestion followed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Our preliminary in situ pXRF results show inversely related spatial variations of Fe and tin (Sn) within single crystals. For example, within a single crystal section (Φ ~0.3 m) measured in situ from the Tin Mountain pegmatite, Black Hills, SD, Fe varies from ~2000 to 600 ppm core to rim, while Sn varies ~200 to 300 ppm core to rim. Meanwhile, manganese (Mn) and gallium (Ga) are relatively homogeneous across the same crystal section (~400 ppm and ~90 ppm, respectively).

This study demonstrates the ability to rapidly quantify and map trace element variations along crystallographic planes of single in situ giant crystals of spodumene. However, several factors may adversely affect this method including hydrothermal and metasomatic alteration, weathering, and the presence of microscopic inclusions which need to be carefully accounted for. Despite these obstacles our results may aid in determining mechanisms for giant crystal growth and elucidate other pegmatitic processes.