Summary: Bahianite, Sb3Al5O14(OH)2-Sb3Al5(Al, Be, Si)<2016, monoclinic, a 9·406(6), b11·541(8), c 4·410(3) Å, β 90·94(3)°, Z = 2, space group C2/m, is a new species from the Paramirim region, south central Bahia state, Brazil. It occurs as water-worn pebbles (‘favas’) with quartz, andalusite, kyanite, diaspore, eskolaite, cassiterite, etc. Colour tan to cream, crystals in vugs colourless, tan or pale violet. Specific gravity ranges from 4·89 to 5·46 (5·26 g cm−3 for Sb3Al5O14(OH)2); hardness 9, cleavage (100) perfect, lustre adamantine, biaxial (−), 2V large, perfect, lustre adamantine, biaxial (−), 2V large, α = 1·81, β = 1·87, γ = 1·92, dispersion r > v.
The crystal structure represents a new type and is based on hexagonal close-packed oxygens. It is related to the structure of simpsonite Ta3Al14O13(OH).