Oxy-foitite, [] (Fe2+ Al2)Al6 (Si6 O18 )(BO3)3 (OH)3 O, a new mineral species of the tourmaline supergroup

Abstract

Oxy-foitite, □(Fe 2+ Al 2 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, 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/cm 3 . 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 R 3 m , a = 15.9387(3) A, c = 7.1507(1) A and V = 1573.20(6) A 3 , Z = 3. The crystal structure of oxy-foitite was refined to R 1 = 1.48% using 3247 unique reflections from single-crystal X-ray diffraction using Mo K α radiation. Crystal-chemical analysis resulted in the empirical structural formula: X ( □ 0.53 Na 0.45 Ca 0.01 K 0.01 ) ∑ 1.00 Y ( Al 1.53 Fe 2 + 1.16 Mg 0.22 Mn 2 + 0.05 Zn 0.01 Ti 4 + 0.03 ) ∑ 3.00 Z ( Al 5.47 Fe 3 + 0.14 Mg 0.39 ) ∑ 6.00 [ ( Si 5.89 Al 0.11 ) ∑ 6.00 O 18 ] ( BO 3 ) 3 V ( OH ) 3 W [ O 0.57 F 0.04 ( OH ) 0.39 ] ∑ 1.00 . Oxy-foitite belongs to the X -site vacant group of the tourmaline-supergroup minerals, and shows chemical relationships with foitite through the substitution Y Al 3+ + W O 2− → Y Fe 2+ + W (OH) 1− .