Dutrowite, Na(Fe2.52+Ti0.5)Al6(Si6O18)(BO3)3(OH)3O, 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(Fe2+ 2.5 Ti0.5)Al6(Si6O18)(BO3)3(OH)3O, 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) Å, V = 1597.68(18) Å 3 , and Z = 3. The crystal structure was refined to R1 = 0.0257 for 1095 unique reflections with Fo > 4? (Fo) and 94 refined parameters. Electron microprobe analysis, coupled with Mössbauer spectroscopy, resulted in the empirical structural formula X(Na0.81Ca0.20K0.01)61.02 Y (Fe2+ 1.25Mg0.76Ti0.56Al0.42)63.00 Z(Al4.71Fe3+ 0.27V 3+ 0.02Mg0.82Fe2+ 0.18)66.00 T [(Si5.82Al0.18)66.00O18] (BO3) O(3) 3 (OH)O(1) 3 [O0.59(OH)0.41]61.00, which was recast in the empirical ordered formula, required for classification purposes: X(Na0.81Ca0.20K0.01)61.02 Y (Fe2+ 1.43Mg1.00Ti0.56)63.00 Z(Al5.13Fe3+ 0.27V 3+ 0.02Mg0.58)66.00 T [(Si5.82Al0.18)66.00O18] (BO3) V 3 (OH)3 W [O0.59(OH)0.41]61.00. 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 2Al3+ = 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