The experimental determination of REE partition coefficients in the water-calcite system

Abstract

To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~ 6 and 25 °C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates.

REE Incorporation into Calcite Individual Crystals as One Time Spike Addition

“…Our calculations show that more than 99% of the total dissolved REE carbonate species is in the REECO 3 + aqueous complex. This is in agreement with the calculations reported in the literature [10], which eliminates the difference in the speciation control of REE incorporation while comparing our data.…”

“…No REE "hot spots" were observed with LA-ICP-MS, suggesting the absence of precipitation of REE-rich mineral phases with calcite. The Na in the fluids was analysed together with Ca, showing that the Na concentration in solutions varies between 4.1 × 10 −3 mol/kg and 8.8 × 10 −3 mol/kg; this Na content is more than an order of magnitude lower than in other studies, where NaCl = 0.1 M to 0.6 M [7][8][9][10]. The fluid concentrations of Ca for experiments at 21.7 • C and 24.6 • C were reported by Gabitov et al [15] and are shown in Table 1.…”

“…For example, the proposed incorporation mechanisms for Eu 3+ involve: (i) the formation of vacancies (Eu 2x (CO 3 ) 3 ) solid , e.g., [3]; (ii) the incorporation of OH − (EuOHCO 3 ) solid , e.g., [3]; and (iii) the incorporation of Na + with a formation of EuNa(CO 3 ) 2 [2,11]. Spectroscopic data [11,12], as well as a recent experimental study [10], suggest the incorporation of REE into calcite accompanied by the incorporation of Na + . The reported values of K REE vary by more than an order of magnitude and depend on the concentration of REE in solution and the saturation state of the fluid with respect to calcite (Ω calcite ).…”

“…Large calcite crystals (up to 1.5 mm in size) grew, while CO 2 and NH 3 (decomposition products of ammonium carbonate) diffused into non-stirred growth media (Figure 1). In all other experimental works, the fluids were continuously stirred, resulting in the precipitation of small calcite crystals (a few µm in size) [8][9][10], with an exception of a study by Terakada and Masuda [7]. REE elements were introduced into the fluid one time each in the following order: Sm, La, Nd, Tb, Pr using 60 mL syringe with luerlock tubing.…”

“…The existing data in the literature showed a large range of apparent REE partition coefficients between calcite and fluid at K REE = (REE/Ca) calcite /(REE/Ca) fluid [7][8][9][10]. The reason for normalizing to Ca is that trace elements often substitute Ca in the calcite lattice.…”