Ferro-bosiite, NaFe ³⁺ ₃ (Al ₄ Fe ²⁺ ₂ )(Si ₆ O ₁₈ )(BO ₃ ) ₃ (OH) ₃ O, a new mineral species of the tourmaline supergroup

Abstract Ferro-bosiite, NaFe 3+ 3 (Al 4 Fe 2+ 2 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, 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 R 3 m , a = 16.0499(5) Å, c = 7.2977(2) Å, V = 1628.03(11) Å 3 , Z = 3 and calculated density = 3.216 g/cm 3 . The crystal structure was refined to R 1 = 2.55% using 1547 unique reflections collected with Mo K α X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula: X (Na 0.99 K 0.02 ) Σ1.01 Y (Fe 3+ 1.56 V 3+ 0.02 Mg 1.01 Fe 2+ 0.20 Mn 2+ 0.03 Ti 0.16 Li 0.02 ) Σ3.00 Z (Al 4.32 Fe 3+ 0.41 Fe 2+ 1.22 Mg 0.05 ) Σ6.00 T [(Si 5.99 Al 0.01 ) Σ6.00 O 18 ] (BO 3 ) 3 O(3) (OH) 3 O(1) [O 0.62 (OH) 0.34 F 0.04 ] 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 Z Fe 2+ ↔ Z Mg. 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 Fe 3+ -rich oxy-dravite to dutrowite, driven by increasing Fe 3+ 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.