Ferro-bosiite, NaFe3+3(Al4Fe2+2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It was found in a giant collapsed cavity discovered in the Marina granitic pegmatite, at the Mavuco locality, Alto Ligonha, Mozambique. Ferro-bosiite occurs as a black acicular late-stage overgrowth at the analogous pole of a multicoloured fluor-elbaite crystal. The black crystals, with a vitreous lustre, have a brown streak, conchoidal fracture and a Mohs hardness of ∼7. Ferro-bosiite is uniaxial (–), with refractive indices ω = 1.675(5) and ε = 1.645(5). It has trigonal symmetry, space group R3m, a = 16.0499(5) Å, c = 7.2977(2) Å, V = 1628.03(11) Å3, Z = 3 and calculated density = 3.216 g/cm3. The crystal structure was refined to R1 = 2.55% using 1547 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula: X(Na0.99K0.02)Σ1.01Y(Fe3+1.56V3+0.02Mg1.01Fe2+0.20Mn2+0.03Ti0.16Li0.02)Σ3.00Z(Al4.32Fe3+0.41Fe2+1.22Mg0.05)Σ6.00T[(Si5.99Al0.01)Σ6.00O18] (BO3)3O(3)(OH)3O(1)[O0.62(OH)0.34F0.04]Σ1.00.

Ferro-bosiite is an oxy-species belonging to alkali group 3 of the tourmaline supergroup. It is related to bosiite by the substitution ZFe2+ ↔ ZMg. The new mineral has been approved by the International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification (IMA 2020-069). Ferro-bosiite forms part of a continuous solid-solution trend from Fe3+-rich oxy-dravite to dutrowite, driven by increasing Fe3+ and Ti, and X-site vacancies. At its type locality, ferro-bosiite formed during late-stage interaction with B-rich hydrothermal fluids that became enriched in Fe and Mg, probably due to a distinct fluid phase active after the collapse of a giant cavity.