Lithium isotope fractionation during uptake by gibbsite

Abstract

The intercalation of lithium from solution into the six-membered l 2 -oxo rings on the basal planes of gibbsite is well-constrained chemically. The product is a lithiated layered-double hydroxide solid that forms via in situ phase change. The reaction has well established kinetics and is associated with a distinct swelling of the gibbsite as counter ions enter the interlayer to balance the charge of lithiation. Lithium reacts to fill a fixed and well identifiable crystallographic site and has no solvation waters. Our lithium-isotope data shows that 6 Li is favored during this intercalation and that the solid-solution fractionation depends on temperature, electrolyte concentration and counter ion identity (whether Cl À , NO 3 À or ClO 4 À ). We find that the amount of isotopic fractionation between solid and solution (DLi solid-solution ) varies with the amount of lithium taken up into the gibbsite structure, which itself depends upon the extent of conversion and also varies with electrolyte concentration and in the counter ion in the order: ClO 4 À < NO 3 À < Cl À . Higher electrolyte concentrations cause more rapid expansion of the gibbsite interlayer and some counter ions, such as Cl À , are more easily taken up than others, probably because they ease diffusion. The relationship between lithium loading and DLi solid-solution indicates two stages:(1) uptake into the crystallographic sites that favors light lithium, in parallel with adsorption of solvated cations, and (2) continued uptake of solvated cations after all available octahedral vacancies are filled; this second stage has no isotopic preference. The two-step reaction progress is supported by solid-state NMR spectra that clearly resolve a second reservoir of lithium in addition to the expected layered double-hydroxide phase.

Experimental determination of Li isotope behaviour during basalt weathering

“…Therefore, this confirms that clay formation strongly affects Li isotope fractionation (Bouchez et al, 2013;Wimpenny et al, 2015;Wimpenny et al, 2010), but also adds to the mass balance argument made above, that it is secondary mineral formation, rather than sorption, that is largely controlling Li isotope fractionation in this experiment (84-88% of Li by mass balance) (Hindshaw et al, 2019;Pistiner and Henderson, 2003;Vigier et al, 2008;Wimpenny et al, 2015). here (Pogge von Strandmann et al, 2006;Vigier et al, 2009).…”

“…sorbed Li) and the secondary mineral fraction (i.e. Li substituting into the crystal structures of neoforming minerals) (Chan and Hein, 2007;Millot and Girard, 2007;Pistiner and Henderson, 2003;Vigier et al, 2008;Wimpenny et al, 2015;Wimpenny et al, 2010). For this mass balance, we assume that the Na acetate leach represents the exchangeable fraction, while the subsequent HCl leach represents Li recovered from secondary minerals (including oxyhydroxides and clays or zeolites).…”

“…However, while the general mechanics of Li isotope fractionation during weathering are fairly well understood, there is less information available on the precise processes that d 7 Li traces, such as whether the primary fractionation mechanism is adsorption onto secondary mineral surfaces, uptake into interlayer sites, or via structural or lattice-bound incorporation into neoformed minerals (Hindshaw et al, 2019;Pistiner and Henderson, 2003;Pogge von Strandmann et al, 2010;Wimpenny et al, 2015). Further, fractionation factors and reaction kinetics are also poorly known.…”

Tracing silicate weathering processes in the permafrost-dominated Lena River watershed using lithium isotopes

“…The range of isotopic fractionation factors in Fig. 5 are consistent with experimentally determined values for various secondary minerals, including those predicted to be oversaturated within the Lena River (see section 5.2), with αvermiculite = 0.971, αkaolinite = 0.979, αgibbsite = 0.984-0.993, αferrihydrite ≈ 0.998, and αsmectite = 0.984 (Zhang et al 1998;Pistiner and Henderson, 2003;Millot and Girard, 2007;Vigier et al 2008;Wimpenny et al 2015). The range of α is also comparable to that those observed in other global rivers (e.g., Amazon (Dellinger et al, 2015); Ganges (Bagard et al, 2015;Pogge von Strandmann et al, 2017)).…”

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

“…A mass balance approach can potentially provide quantitative information on the biogeochemical processes that fractionate the tracer of interest (Georg et al, 2006a;Bouchez et al, 2013). However, the associated uncertainties are large due to poor constraints on elemental and isotopic fractionation factors, which can be highly dependent on mineralogy, climate, ecosystem dynamics, or other aspects of the studied watershed (e.g., Wimpenny et al, 2015;Frings et al, 2016). As a result, most isotope weathering studies to date have focused on empirically calibrating isotopic fractionation factors, rather than using the isotopic information to assess the underlying biogeochemical processes (e.g., Hughes et al, 2013;Baronas et al, 2017a).…”