Lithium Isotopic Fractionation in Minerals from Pegmatites: Perspective of Crystal Chemistry

Abstract

Lack of information regarding lithium (Li) crystal chemistry in numerous minerals, especially those containing trace amounts of Li (ranging from a few to tens of ppm), limits our understanding of Li isotopic fractionation in pegmatites. In this study, we examined the Li isotopic composition and Li content in various Li-poor (e.g., quartz or feldspar) together with Li-rich (sopdumene or lepidolite) mineral phases within granitic pegmatites. We compiled a comprehensive dataset, encompassing a broad spectrum of Li contents (ranging from a few to tens of thousands of ppm) and Li isotopic values (−8‰ to 41‰). The minerals exhibit distinct Li isotopic signatures. Specifically, elbaite and beryl show the highest values, while biotite displays a negative average. Compared to individual minerals, whole rocks demonstrate lower Li isotopic values, with pegmatites exhibiting the highest and non-granitic pegmatite wall rocks showing the lowest. Our study also uncovers a clear “V”. shape relationship between Li isotopic values and logarithm of Li contents, with different mineral groups occupying specific regions within this shape. Furthermore, a significant correlation was observed between average Li isotopic values and Li-O (OH, F) bond lengths in various minerals. These discoveries underscore the crucial role of crystal chemistry in shaping the Li isotopic behavior in pegmatites from a statistical perspective.