Hydrothermal Synthesis of Lithium-Bearing Tourmaline, and the Elusive Stability Field of Elbaite

Tourmaline was synthesized by hydrothermal methods from 450 to 750 °C, pressures from 150 to 260 MPa H2O, and pH from 3 to 6 of aqueous solutions on quench in an attempt to form the lithium-rich tourmaline elbaite. Tourmaline grew in 91 experiments, of which 78 were analyzed in detail. The normative endmember components of natural and synthetic tourmalines were calculated from electron microbeam analyses that included boron. Analyzed lithium contents of thirteen samples showed good agreements with the calculated values. In a series of experiments termed ElbK, the calculated normative components in mole fractions were olenite (0.44–0.49) > rossmanite (0.36–0.29) > elbaite (0.09–0.15). In a composition termed ElbKQ, rossmanite (0.35–0.52) and olenite (0.44–0.35) were the principal normative components, and elbaite was nil (0–0.05). Boron in excess of 3 atoms per formula unit (apfu) was assigned to the tetrahedral site, B(T), which increased with decreasing temperature. A foitite component increased below 500 °C and above 650 °C, but only at the higher pressures. The addition of HF to the ElbKQ mix yielded tourmaline of the lowest Li content, the highest X site (Na) occupancy, B(T) near zero, and with appreciable Fe and Mg. In contrast, experiments with HCl produced the highest Li content (0.97 Li apfu), X site occupancy near zero, B(T) = 0.83 apfu, and no mafic constituents. The normalized compositions of the HCl products and other synthetic tourmalines with ElbKQ at 250 MPa deviated from the stoichiometry of the calculated tourmaline endmember components in other respects. In some overgrowths, lithium likely occupied the 9-coordinate X site. In the compositions of ElbK and ElbKQ, the lithium content of the synthetic tourmaline increased modestly with increasing pressure. Pressure, however, was not a deciding factor for the incorporation of lithium. The results indicate that the precipitation of tourmaline rich in a rossmanite-like component is not narrowly defined by pressure, temperature, or pH of a coexisting aqueous solution. However, the chemical nature of acidic components exhibited a large effect on the compositions of the synthetic tourmaline. Tourmaline with a predominant component of elbaite was not synthesized with either starting composition, nor was it credibly identified in prior experiments that reported it. A reconnaissance survey of natural lithium tourmalines revealed a large fraction of rossmanite and olenite components, as did the experimental products. Future attempts to synthesize elbaite should employ a reagent composition more closely aligned with the pocket-forming medium in natural pegmatites.