Synthesis and crystal structure of V-rich tourmaline

Abstract Tourmaline is the most abundant natural borosilicate, an important indicator of its host rock chemistry and formation conditions, as well as a valuable material for various industrial applications. In this work, V-rich tourmaline crystals were grown (T = 650 °C, P = 100 MPa) for the first time in two systems: (V1) V2O3–Al2O3–SiO2–B2O3–H2O and (V2) Na2O–MgO–V2O3–Al2O3–SiO2–B2O3–H2O. Newly formed overgrowth layers of tourmaline on an elbaite seed are dark green, up to 0.7 mm in thickness, and contain up to ∼12 wt% V2O3 [∼1.6 atoms per formula unit (apfu)] and ∼41 wt% V2O3 (∼6 apfu) for the system V1 and V2, respectively. Synthetic tourmalines V1 [a = 7.0988(1), b = 9.4274(2), c = 9.4279(2) Å, α = 113.920(2)°, β = 104.551(2)°, γ = 104.515(2)°] and V2 [a = 7.3390(1), b = 9.6075(2), c = 9.6077(2) Å, α = 113.742(2)°, β = 104.721(2)°, γ = 104.786(2)°] are triclinic (P1) due to cation ordering and could be considered as dimorphs of the V-dominant analog of alumino-oxy-rossmanite (V1) and oxy-vanadium-dravite (V2). Formation of V-rich tourmalines does not require high temperatures or pressures and is constrained by the simultaneous abundance of boron and vanadium in the mineral-forming medium. The stability of the crystal structure of V-rich tourmaline is increased by the incorporation of Mg cations, thereby reducing the disproportion of the octahedra in V,Al-rich species.