Synthesis and crystal structure of V-rich tourmaline

Tourmaline is the most abundant natural borosilicate, an important indicator of its host rock chemistry and formation condition, as well as 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 analogue of aluminooxy-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