Equilibrium lithium isotope fractionation in Li-bearing minerals

Lithium isotopes are important geochemical tracers for many geological processes. Knowledge of Li isotope fractionation factors is essential for understanding the behavior of Li isotopes. On the basis of density functional perturbation theory (DFPT), we calculate Li isotope fractionation parameters of α-eucryptite (LiAlSiO4), β-eucryptite (LiAlSiO4), lithiophilite (LiMnPO4), montebrasite (LiAlPO4OH), petalite (LiAlSi4O10), bikitaite [Li2(Al2Si4O12)·2H2O], spodumene (LiAlSi2O6), lithiophosphate (Li3PO4) and amblygonite (LiAlPO4F). The reduced partition function ratios of 7Li/6Li (103 ln β7-6) for these minerals decrease in the order petalite > lithiophosphate > bikitaite > α-eucryptite > β-eucryptite > montebrasite > amblygonite > lithiophilite > spodumene, agreeing with the observations on granitic pegmatites. Li isotope fractionations in these Li-rich minerals have a notable linear correlation with the average LiO bond lengths, and are significantly influenced by Li coordination number. Furthermore, we calculate Li isotope fractionation factors of Li-bearing forsterite (Mg2SiO4) and diopside (CaMgSi2O6). It is found that forsterite is enriched in heavy Li relative to diopside, and both minerals' 103 ln β7-6 values depend on Li contents. These results allow for better understanding Li isotope fractionations in natural systems and provide a theoretical basis for Li isotope applications to understand geological processes.