Dutrowite, Na: the first member of the tourmaline supergroup with Ti as a species-forming chemical constituent

The new tourmaline supergroup mineral dutrowite, Na(Fe2.52+Ti.sub.0.5 )Al.sub.6 (Si.sub.6 O.sub.18 )(BO.sub.3).sub.3 (OH).sub.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) Ã, V=1597.68(18) Ã.sup.3, and Z=3. The crystal structure was refined to R.sub.1 =0.0257 for 1095 unique reflections with F.sub.o >4Ï (F.sub.o) and 94 refined parameters. Electron microprobe analysis, coupled with Mössbauer spectroscopy, resulted in the empirical structural formula .sup.X (Na.sub.0.81 Ca.sub.0.20 K.sub.0.01).sub.Σ1.02 .sup.Y (Fe1.252+Mg.sub.0.76 Ti.sub.0.56 Al.sub.0.42).sub.Σ3.00 .sup.Z (Al.sub.4.71 Fe0.273+V0.023+Mg.sub.0.82 Fe0.182+).sub.Σ6.00 .sup.T [(Si.sub.5.82 Al.sub.0.18).sub.Σ6.00 O.sub.18 ] (BO3)3O(3)(OH)3O(1)[O.sub.0.59 (OH).sub.0.41 ].sub.Σ1.00, which was recast in the empirical ordered formula, required for classification purposes: .sup.X (Na.sub.0.81 Ca.sub.0.20 K.sub.0.01).sub.Σ1.02 .sup.Y (Fe1.432+Mg.sub.1.00 Ti.sub.0.56).sub.Σ3.00 .sup.Z (Al.sub.5.13 Fe0.273+V0.023+Mg.sub.0.58).sub.Σ6.00 .sup.T [(Si.sub.5.82 Al.sub.0.18).sub.Σ6.00 O.sub.18 ] (BO.sub.3 )3V(OH).sub.3 .sup.W [O.sub.0.59 (OH).sub.0.41 ].sub.Σ1.00 . 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.sup.3+ = Ti.sup.4+ + (Fe,Mg).sup.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.