Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 85 (2012) 314–325 www.elsevier.com/locate/gca
Partitioning of lithium between smectite and solution: An experimental approach Alain Decarreau a, Nathalie Vigier b,⇑, Helena Pa´lkova´ a,b,c, Sabine Petit a, Philippe Vieillard a, Claude Fontaine a b
a Universite´ de Poitiers, CNRS UMR 7285 IC2MP, 6 rue Michel Brunet, F86022 Poitiers, France CRPG-CNRS, Universite´ de Lorraine, 15 rue Notre Dame des pauvres, F54501 Vandoeuvre le`s Nancy Cedex, France c Institute of Inorganic Chemistry, Slovak Academy of Sciences, Du´bravska´ cesta 9, 845 36 Bratislava, Slovakia
Received 24 March 2011; accepted in revised form 13 February 2012; available online 23 February 2012
Abstract Lithium is an element currently used as a tracer of water–rock interactions in continental and oceanic systems. Li is essentially concentrated in clays, and most often associated with Mg. However, there are to date few data on the partitioning of Li between Mg-clays and water that are required for developing appropriate weathering models. In this study, Mg–Li smectites (hectorites) were synthesized at 70, 90 and 150 °C, during periods ranging from 1 day to 4 months, and from solutions with various Li contents (typically from 2 to 190 mg/L). Pure hectorite fractions collected at the end of the syntheses were successively saturated three times with 1 N CaCl2 in order to completely eliminate exchangeable and adsorbed Li and Mg. The results show that the structural (octahedral) Li content of hectorites increases with Li concentration in solution, with temperature and with synthesis duration. A steady-state is reached after 4 weeks at 150 °C and after 2 months at 70 and 90 °C. Calculated partition coefficients, expressed either as DLi = [Li(clay)]/[Li(aq)] (ppm/ppm) or as D0Li=Mg = (Li/Mg)clay/(Li/ Mg)solution (ppm/ppm), increase with the synthesis temperature which is consistent with a true Li–Mg substitution within the clay lattice. DLi also increases when the Li concentration in solution decreases, and reaches 22 at 150 °C (using a 1.5 mg/L Li solution). A general expression for DLi can be established: logDLi = 1319/T(K) + 5.5 ([Li(aq)])0.0806. The main conclusion is that neoformed smectites which crystallize during water–rock interactions can be efficient Li sinks in the external geochemical cycle. Quantitative applications to several natural systems are assessed. Ó 2012 Elsevier Ltd. All rights reserved.
1. INTRODUCTION Recently, the geochemistry of Li and Li isotopes has been used to trace low and high temperature weathering processes of silicate rocks, either in oceanic or in continental environments (e.g. Chan et al., 1992, 2002; James and Palmer, 2000; Vils et al., 2008, 2009; Pistiner and Henderson, 2003; Rudnik et al., 2004; Kisaku¨rek et al., 2004). The scientific objectives of these studies are first to determine the rates and conditions of past and present-day silicate chemical erosion, as it represents a major sink of atmospheric CO2 (e.g. ⇑ Corresponding author.
E-mail address:
[email protected] (N. Vigier). 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.02.018
Gaillardet et al., 1999; Dessert et al., 2001), and second to resolve the apparent lack of Li mass balance in the ocean. In both cases, during water–rock interactions, the geochemical behavior of Li is intimately linked to the formation of secondary phases, mainly clay minerals. In clayey rocks, Li can occur in the crystal structure of clay minerals and as adsorbed cations (Brindley and Brown, 1980). The latter are more or less exchangeable, depending on the chemical composition of the successive fluids in contact with clays, and therefore may not reflect the primary conditions of clay formation. Due to the similarity of their ionic radius (Shannon, 1976; Vieillard, 1987), Li+ often substitutes for Mg2+ in octahedral sites of clay minerals, and their external
A. Decarreau et al. / Geochimica et Cosmochimica Acta 85 (2012) 314–325
geochemical cycles are thought to be closely related (Tardy et al., 1972; Stoffyn-Egli and Mackenzie, 1984). Nevertheless, there is no simple correlation between Mg and Li concentrations in solid phases. For example, hectorites are enriched in Li and Mg but palygorskite, which is also an Mg-rich clay, has low Li contents. For a better quantification of the Li geochemical cycle, it is necessary to determine more precisely the rates at which Li substitutes for Mg in smectite-like clay structures. There are numerous data showing that, during low temperature seawater–seafloor interactions, Li is taken up by clay mineral assemblages dominated by Mg-smectites (Seyfried et al., 1984 and references therein). However, few papers deal with experimental measurements of Li fractionation between clays and fluids. Seyfried et al. (1984) experimentally quantified Li uptake by secondary smectite during basalt weathering by seawater at 150 °C. Berger et al. (1988) investigated partitioning of Li, Rb and Cs between seawater and synthesized smectites formed on a basaltic glass. However their data reflected exchange reactions rather than a true substitution. In this study we have experimentally measured the partitioning of Li between synthesized Mg-rich smectites (hectorites) and Li solutions at various temperatures.
315
exchangeable cation mainly located in the interlayer sites. Exchangeable cations compensate the negative structural layer charge linked to the Li+–Mg2+ heterovalent substitution process. During synthesis, exchangeable cations may be Li+, Na+ or Mg2+. Syntheses were performed at 75, 90 and 150 °C under equilibrium water pressure, during 1, 2, 7, 14, 28, 60 and 120 days. Below 100 °C, syntheses were performed in polyethylene sealed containers, using about 600 mg of starting gel and 90 ml of solution. At 150 °C, 200 mg of starting gel and 30 ml of solution were put in sealed Teflon coated metallic bombs. The resulting “water/solid” ratios were high (near 150). At each given temperature, experiments were systematically performed with Li concentrations in solution of 1.9, 9, 19, 90 and 190 mg/L, respectively. At the end of synthesis, solid products were separated by filtration. The synthesized clays were then successively washed three times using 1 N CaCl2 in order to completely eliminate Li in any remaining solution and to remove all the exchangeable Li located in the interlayer sites. The efficiency of these saturations has been previously demonstrated (Vigier et al., 2008). Synthesized clays were then dried overnight using a lyophilization technique, before analysis by X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR).
2. METHODS 2.2. Li, Mg and Si content measurements 2.1. Hectorite synthesis Mg–Li smectites were synthesized using the procedure described in detail in Decarreau (1980, 1983, 1985) and recently used for the experimental determination of Li isotope fractionation between solution and clay minerals (Vigier et al., 2008). Synthesis consists of a closed system hydrothermal treatment of a starting gel (i.e. a high specific area disordered but not completely amorphous solid (Decarreau, 1980, 1985)), previously produced by mixing sodium silicate (SiO2 Na2O) with MgCl2: 4SiO2 Na2 O þ 3MgCl2 þ 2HCl ! Si4 Mg3 O11 ; nH2 OðStarting gelÞ þ 8NaCl
ð1Þ
Lithium is introduced using LiCl solutions. Two different procedures are used: in procedure (A), LiCl is mixed with MgCl2 during the gel precipitation (1); in procedure (B) LiCl is added to the gel after its precipitation. NaCl produced as shown by Eq. (1) is not removed before the syntheses in order to prevent non-stoichiometric dissolution of the starting gel during removal of NaCl by rinsing. NaCl is thus the main dissolved salt in the synthesis solutions, yielding a relatively constant ionic strength for all experiments. The hectorite synthesis associated with Li incorporation can be described by the following reaction (Decarreau, 1985):
3. RESULTS 3.1. Hectorite crystallization
½Si4 Mg3 O10e ðOHÞ2þe þ þ 2x Liþ ðStarting gelÞ
! ðMg3x Lix ÞSi4 O10 ðOHÞ2 Liþexch þ x Mg2þ þ e Hþ x
Li, Mg and Si contents in Ca saturated clays and in the final solutions were measured with a VARIAN220 FS Atomic Absorption Spectrometer, at the SARM (Service d’Analyses des Roches et des Mine´raux du CNRS) national facility. Clays were dissolved using a mixture of concentrated hydrofluoric and nitric acids. Detection limits in the solutions are 0.01 mg/L for Li and Mg, and 0.2 mg/L for Si. Standard deviations are in the following ranges; for Si: from 15% for 0.2 mg/L to 1% for >10 mg/L; for Mg: 5% for 50 mg/L; for Li: 10% for 100 mg/L. For clays, data are expressed as weight% and ppm. Detection limits are: 0.1% Mg and 1 ppm Li. Corresponding relative standard deviations are 1–2% for Mg and 1– 3% for Li.pH (±0.1) was measured with PHM220 Meterlab equipment calibrate with Titrinorm buffer solutions (±0.01 at 20 °C). In the following, aqueous Li concentrations are expressed as [Li(aq)] (in mg/L) and Li concentrations in clays as [Li(clay)] (in ppm), and similarly for Mg.
ð2Þ
ðHectoriteÞ
In synthesized hectorites, reaction (2) shows that Li can occupy two different sites: Li+ that substitutes for Mg2+ in the structural octahedral sites, and Li+exch which is an
More than 150 hectorite syntheses were performed using either procedure (A) or (B) (see Section 2.1). The crystallization mechanisms of the synthesized hectorites and their properties have been previously extensively described (Decarreau, 1980, 1983). Each synthesized hectorite was characterized by XRD and FTIR. Data were consistent
316
A. Decarreau et al. / Geochimica et Cosmochimica Acta 85 (2012) 314–325
with those given in Vigier et al., (2008). The main features of the synthesized clays are found to be: (1) Pure hectorites of structural formula (Mg3xLix)Si4O10(OH)2 Mþexch are systematically obtained when x using these experimental procedures (Decarreau, 1980, 1983; Vigier et al., 2008). (2) Actual Li for Mg substitution in the octahedral sheet of hectorite can be demonstrated using 7Li NMR spectroscopy (Conard, 1975; Decarreau, 1983) (3) Crystallinity of hectorites increases when the synthesis temperature increases.
3.2. Hectorite Li contents At 75 and 90 °C, most of the syntheses were performed using both procedures, (A) and (B) (see Fig. 1 and Section 2.1.), and both give similar results. For example, the
Li contents of the hectorite synthesized at similar conditions (temperature, solution, run duration) are the same. The synthesis procedure is therefore highly reproducible, as previously shown for M2+/Mg chemical partitioning (Decarreau, 1985), and also for Li isotope fractionation (Vigier et al., 2008). The Li concentrations measured in synthesized hectorites, after washing with CaCl2, correspond strictly to Li-Mg substitution into the octahedral sites of the clay. Indeed, Vigier et al. (2008) have shown that all the exchangeable and macroporosity Li (coming from the solution) was removed using three washings with 1 N CaCl2. Vigier et al. (2008) used highly concentrated Li solutions (3 M LiCl, i.e. 27 000 mg/L Li), while Li concentrations are considerably lower (
