TRACE ELEMENT DISTRIBUTION IN BERYL CRYSTALS FROM LCT AND NYF PEGMATITES: A PRELIMINARY PORTABLE-XRF STUDY

Beryl - Be₃Al₂(Si₆O₁₈) - is the predominant mineral of beryllium. It is a cyclosilicate with hexagonal structure and is common in fractionated pegmatites of Li-Cs-Ta (LCT), Nb-Y-F (NYF), and hybrid (NYF-LCT) families. The trace element contents of beryl may provide insight into magmatic fractionation leading to economic concentrations of critical metals such as lithium, beryllium, cesium, tantalum, etc. This study aims to: 1) compare the trace-element geochemistry of coarse beryl crystals from LCT pegmatites, Oxford Co., ME and from Fisher Quarry (FQ), an NYF ± LCT pegmatite, Sagadahok Co., ME, respectively and 2) explore trace element distribution within magmatic, euhedral beryl crystals, to validate their crystallization history and degree of magmatic fractionation.

Several beryl crystals of ~4 – 10 cm in size were collected from magmatic zones of pegmatites. The LCT Rose Quartz (RQ) quarry, Newry, ME in particular, produced pollucite (Cs ore mineral) and spodumene (Li ore mineral) in its core, whereas the NYF ± LCT FQ pegmatite is less differentiated, with late-stage gahnite (Zn mineral) and lepidolite (Li mineral). The euhedral, milky white to vibrant green crystals were sliced perpendicular to the c axis to ensure consistency. Most crystals display 5-10 mm rims of distinct lighter colors, and variable discoloration along fractures.

At this preliminary stage, we conducted portable X-Ray Fluorescence (p-XRF) qualitative spot analysis with a Tracer 5i Bruker Inc. unit optimized for elements of low to medium atomic number, including Al, Si, Fe, and Zn (K lines) and heavy elements Cs and Rh (L lines). Each crystal was measured along two perpendicular traverses of points, spaced 5 mm apart. To correct for daily variations, angle of incidence, and other external factors, we have normalized all data to silicon counts of a reference beryl that was measured during each data collection session. The results were confirmed by preliminary µ-XRF analysis conducted on an M4 Tornado Plus, at Bruker AXS in Madison, WI. Cs and Fe contents are high along the crystal rims and low within the cores, whereas Zn tends to have an opposite distribution. The depletion of Zn and enrichment of Cs ±Fe is stronger in the FQ beryl rims than in the RQ rims, possibly because of late-stage crystallization of gahnite and pollucite in FQ and RQ, respectively.