Dutrowite, Na(Fe-2.5(2+) Ti-0.5)Al-6(Si6O18)(BO3)(3)(OH)(3)O, a new mineral from the Apuan Alps (Tuscany, Italy): the first member of the tourmaline supergroup with Ti as a species-forming chemical constituent

The new tourmaline supergroup mineral dutrowite, Na(Fe-2.5(2+) Ti-0.5)Al-6(Si6O18)(BO3)(3)(OH)(3)O, has been discovered in an outcrop of a Permian metarhyolite near the hamlet of Fornovolasco, Apuan Alps, Tuscany, Italy. It occurs as chemically homogeneous domains, up to 0.5 mm, brown in colour, with a light-brown streak and a vitreous lustre, within anhedral to subhedral prismatic crystals, up to 1 mm in size, closely associated with Fe-rich oxy-dravite. Dutrowite is trigonal, space group R3m, with a = 15.9864(8), c = 7.2187(4) angstrom, V = 1597.68(18) angstrom(3), and Z = 3. The crystal structure was refined to R-1 = 0.0257 for 1095 unique reflections with F-o > 4 sigma (F-o) and 94 refined parameters. Electron microprobe analysis, coupled with Mossbauer spectroscopy, resulted in the empirical structural formula (X)(Na0.81Ca0.20K0.01)(sigma 1.02) (Y) (Fe-1.25(2+) Mg0.76Ti0.56Al0.42)(sigma 3.00) (Z)(Al4.71Fe0.273+V0.023+Mg0.82Fe0.182+)(sigma 6.00) (T) [(Si(5.8)2Al(0.18))(sigma 6.00)O-18] (BO3)(3)(O(3)) (OH)(3)(O(1))[O-0.59(OH)(0.4)](sigma 1.00), which was recast in the empirical ordered formula, required for classification purposes: (X)(Na0.81Ca0.20K0.01)(sigma 1.02) Y (Fe1.432+Mg1.00Ti0.56)(sigma 3.00) (Z)(Al5.13Fe0.273+V0.023+Mg0.58)(sigma 6.00) (T) [(Si5.82Al0.18)(sigma 6.00)O-18] (BO3)(3)(V)(OH)(3) (W)[O-0.59(OH)(0.41)](sigma 1.00). Dutrowite is an oxy-species belonging to the alkali group of the tourmaline supergroup. Titanium is hosted in octahedral coordination, and its incorporation is probably due to the substitution 2Al(3+) = Ti4+ + (Fe,Mg)(2+). Its occurrence seems to be related to late-stage high-T/low-P replacement of biotite during the late-magmatic/hydrothermal evolution of the Permian metarhyolite.