In situ experimental study on the solubility and crystallization of zabuyelite (Li2CO3) in aqueous solution under igneous conditions

Abstract

Zabuyelite (Li2CO3) was discovered in sedimentary environments, but it can form in igneous environments (e.g., rare-element pegmatites). This experiment extends the solubility and crystallization behaviors of Li2CO3 in an aqueous solution to igneous conditions (up to 783 °C and 8.24 kbar) from sedimentary conditions using a Bassett-type hydrothermal diamond-anvil cell. The solubility of Li2CO3 in an aqueous solution increases with temperature and pressure above the vapor-liquid critical pressure–temperature (P–T) condition. This is in contrast to its negative temperature coefficient in vapor-saturated water. The solubility data are fitted with the eq. R × T × lnxLi2CO3 = −3.0328 × 104–16.6486 × T + 3.3252 × 103 × P (r2 = 0.9643), where xLi2CO3 is the mole fraction (mol/mol) of Li2CO3, 714 ≤ T (K) ≤ 1056, 3.28 ≤ P (kbar) ≤ 8.24, and R is the gas constant. Zabuyelite grows quickly when nucleated from complete Li2CO3-dissolved aqueous solutions during cooling (at average rates of 0.2–3.0 μm2/s), especially in the initial exponential growth stage with approximately 18.7–34.2 μm2/s. This is orders of magnitude faster than other pegmatite minerals (e.g., spodumene), forming tabular crystals with the pseudo-hexagonal outlines of monoclinic systems in an aqueous solution. Nevertheless, zabuyelite forms microcrystals in Li2CO3 melt. In addition, zabuyelite crystallization (exhibiting significant temperature dependence) dominantly occurs at ~450–600 °C. The aforementioned crystallization features are identical with the features of zabuyelite contained in crystal-rich fluid inclusions from pegmatite, proving the zabuyelite's daughter-mineral nature in the fluid inclusions, hence indicating the primary origin of the crystal-rich inclusions in pegmatite. In the future, the solubility model of Li2CO3 in an aqueous solution can infer pegmatite formation conditions based on the analysis of the Li2CO3 content dissolved in the pegmatite crystallization media.