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Minerals 2019, 9, 766 13 of 21 the OH of the lepidolite. An important process could be the reaction of NaCl to NaOH in the NaCl solutions and from KCl to KOH in the KCl solutions. In contrast to the 0.03 to 4.26 molal MgCl2 solutions, a very weak decrease in the pH was detected in the 5.51 molal (mol/kg H2O) MgCl2 solution (sample 14) and seawater (sample 15). A more pronounced decrease in the pH was measured in the solutions Q and R (samples 12 and 13), up to a pH of 3.3. As one of the results of the lepidolite corrosion, for example, Si was released into the reaction solution, which would affect the crystal lattice. However, microstructural investigations concerning these effects have not been performed yet. Changes in the K concentrations between the initial solutions and those after reaction were observed in double distilled H2O, the NaCl and in most of the MgCl2 solutions (with the exception of the 3.50 molal MgCl2 solution). The two highest concentrated KCl solutions (initially 0.60 and 4.29 molal KCl), showed a K release from the mineral into the solution in comparable low quantities in the first year. After three years, the release increased, also in the 0.37 molal KCl solution. The KCl solutions with an initial KCl content of 0.01 and 0.02 mol/kg H2O showed no significant change. In comparison, the development of the K content in the evaporating seawater at different evaporation levels (indicated by the first occurrence of typical salt minerals, based on EQ3/6 [41] modelling), is displayed in Table 2. Additional information is given in terms of the Li, Rb, Si and Cs concentrations. The strongest effect of Li leaching was observed in the interaction reaction with the Mg-bearing solution with the lowest concentration (0.069 wt. % Mg; sample 11) and with modern seawater (0.113 wt. % Mg, 0.036 wt. % K; sample 15). It is notable that leaching is more effective in solutions with lower concentrations compared to solutions with higher concentrations with respect to Mg. In addition, especially the K solutions with lower concentrations are still actively leaching, although not as effectively as the MgCl2 solutions. Depending on the coordination number (CN) 6 in the crystal, the ionic radius of Mg2+ (0.72 Å [46]) is comparable with the ionic radius of Li+ (0.76 Å [46]), which might be the reason for a certain exchange, certainly persisting a charge difference. Because in the lepidolite both Li+ and Al3+ share the same position, a coupled exchange of Li+ and Al3+ (0.54 Å [46]) with Mg2+ is possible. According to Reference [51], the substitution of Al + Li = 2 Mg (a coupled exchange) between the phlogopite-trilithionite solid solutions series and a Li+ exchange for the muscovite-zinnwaldite solid solutions series are documented. While this was detected at high temperatures between 500 ◦C and 700 ◦C at 2 kbar, it is ambiguous if this is performed at ambient P-T-conditions. However, higher Mg concentrations are not effective, probably due to the higher salinity and the corresponding increase, e.g., of OH groups that attach the Mg ions, so that the activity of Mg and the capability for incorporation/exchange decreases. The experiments document that the reactions continue, and after three years, no equilibrium was reached. These reaction times are quite different to other rock types, such as basalt. For example, the reaction between basalt sand and river water seams to reach equilibrium after 126 days at 20 ◦C in closed experiments [52]. The experiments show that solutions with low Mg concentrations are more effective in leaching than those with higher Mg concentrations. However, the highest Li concentrations in natural brines are measured in solutions with high Mg content (Figure 7). This implies that the concentration mechanism during leaching by naturally occurring brines might have occurred later, because high Mg concentrations are less effective in leaching Li. In addition, higher temperatures and pressures favour rock–water interaction processes. Due to a maximum depth of the Zechstein evaporites of maximum 3500 m to 4000 m (Gorleben [30], Morsleben [33]) in relation to the Zechstein basis, a temperature increase of up to ca. 150 ◦C [38] and a lithostatic pressure of maximum 80 MPa was estimated. At present, the lithostatic pressure in Gorleben varies between 17.6 MP and 19.3 MPa at the mining level, corresponding to a rock temperature of 30–38 ◦C [53]. Transferring these geological conditions to the experiment, a higher Li release can be expected. During the precipitation of salt minerals, Li salts do not form, and no Li is incorporated in the crystal lattice of naturally formed salt minerals. Despite the same charge (1+), Li+ (0.76 Å) does notPDF Image | Lithium Occurrences in Brines from Two German Salt Deposits
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