Oxy-foitite, □(Fe²⁺Al₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, a new mineral species of the tourmaline supergroup

Oxy-foitite, □(Fe²⁺Al₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, is a new mineral of the tourmaline supergroup. It occurs in high-grade migmatitic gneisses of pelitic composition at the Cooma metamorphic Complex (New South Wales, Australia), in association with muscovite, K-feldspar and quartz. Crystals are black with a vitreous luster, sub-conchoidal fracture and gray streak. Oxy-foitite has a Mohs hardness of ∼7, and has a calculated density of 3.143 g/cm³. In plane-polarized light, oxy-foitite is pleochroic (O = dark brown and E = pale brown), uniaxial negative. Oxy-foitite belongs to the trigonal crystal system, space group R3m, a = 15.9387(3) Å, c = 7.1507(1) Å and V = 1573.20(6) ų, Z = 3. The crystal structure of oxy-foitite was refined to R1 = 1.48% using 3247 unique reflections from single-crystal X-ray diffraction using MoKα radiation. Crystal-chemical analysis resulted in the empirical structural formula:

^X(□_0.53Na_0.45Ca_0.01K_0.01)_Σ1.00 ^Y(Al_1.53Fe²⁺_1.16Mg_0.22Mn²⁺_0.05Zn_0.01Ti⁴⁺_0.03)_Σ3.00 ^Z(Al_5.47Fe³⁺_0.14Mg_0.39)_Σ6.00 [(Si_5.89Al_0.11)_Σ6.00O₁₈] (BO₃)₃ ^V(OH)₃ ^W[O_0.57F_0.04(OH)_0.39]_Σ1.00.

Oxy-foitite belongs to the X-site vacant group of the tourmaline-supergroup minerals, and shows chemical relationships with foitite through the substitution ^YAl³⁺ + ^WO²⁻ → ^YFe²⁺ + ^W(OH)^1–.