Dutrowite, Na(Fe²⁺_2.5Ti_0.5)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

Abstract

The new tourmaline supergroup mineral dutrowite, Na(Fe2.52+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) Å3, and Z=3. The crystal structure was refined to R1=0.0257 for 1095 unique reflections with Fo>4σ (Fo) and 94 refined parameters. Electron microprobe analysis, coupled with Mössbauer spectroscopy, resulted in the empirical structural formula X(Na0.81Ca0.20K0.01)Σ1.02 Y(Fe1.252+Mg0.76Ti0.56Al0.42)Σ3.00 Z(Al4.71Fe0.273+V0.023+Mg0.82Fe0.182+)Σ6.00 T[(Si5.82Al0.18)Σ6.00O18] (BO3)3O(3)(OH)3O(1)[O0.59(OH)0.41]Σ1.00, which was recast in the empirical ordered formula, required for classification purposes: X(Na0.81Ca0.20K0.01)Σ1.02 Y(Fe1.432+Mg1.00Ti0.56)Σ3.00 Z(Al5.13Fe0.273+V0.023+Mg0.58)Σ6.00 T[(Si5.82Al0.18)Σ6.00O18] (BO3)3V(OH)3 W[O0.59(OH)0.41]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 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.