Adding complexity to the garnet supergroup: monteneveite, Ca3Sb25+(Fe23+ Fe2+)O12, a new mineral from the Monteneve mine, Bolzano Province, Italy

Monteneveite, ideally Ca3Sb25+ (Fe-2(3+) Fe2+)O-12, is a new member of the garnet supergroup (IMA 2018-060). The mineral was discovered in a small specimen belonging to the Swedish Museum of Natural History coming from the now abandoned Monteneve Pb-Zn mine in Passiria Valley, Bolzano Province, Alto Adige (South Tyrol), Italy. The specimen consists of mainly magnetite, sphalerite, tetrahedrite-(Fe) and oxycalcioromeite. Monteneveite occurs as black, subhedral crystals with adamantine lustre. They are equidimensional and up to 400 mu m in size, with a subconchoidal fracture. Monteneveite is opaque, grey in reflected light, and isotropic under crossed polars. Measured reflectance values (%) at the four COM wavelengths are 12.6 (470 nm), 12.0 (546 nm), 11.6 (589 nm) and 11.4 (650 nm). The Vickers hardness (VHN100 / is 1141 kgmm(-2), corresponding to H = 6 :5-7, and the calculated density is 4.72(1) g cm(-3). A mean of 10 electron microprobe analyses gave (wt %) CaO 23.67, FeO 3.75, Fe2O3 29.54, Sb2O5 39.81, SnO2 2.22, ZnO 2.29, MgO 0.15, MnO 0.03 and CoO 0.03. The crystal chemical formula calculated on the basis of a total of eight cations and 12 anions, and taking into account the available structural and spectroscopic data, is (Ca2 :97Mg0 :03) 6D3 :00 (Sb1.735+Sn0.104+Fe0.173+)(Sigma)=2.00(Fe2.433+Fe0.372+Zn0.20)(Sigma)=3.00O(12). The most significant chemical variations encountered in the sample are related to a substitution of the type (Y) Sn4+ + Fe3+ ->(Y) Sb5+Fe2+. Mossbauer data obtained at RT and 77K indicate the presence of tetrahedrally coordinated Fe2+. Raman spectroscopy demonstrates that there is no measurable hydrogarnet component in monteneveite. The six strongest Bragg peaks in the powder X-ray diffraction pattern are [d (angstrom), I (%), (hkl)]: 4.45, 100, (220); 3.147, 60, (400); 2.814, 40, (420); 2.571, 80, (422); 1.993, 40, (620); 1.683, 60, (642). Monteneveite is cubic, space group Ia-3d, with a D 12:6093(2) angstrom, V D 2004:8(1) angstrom(3), and Z D 8. The crystal structure was refined up to R 1 D 0:0197 for 305 reflections with Fo > 4 sigma(F-o) and 19 parameters. Monteneveite is related to the other Ca-, Sb- and Fe-bearing, nominally Si-free members of the bitikleite group, but it differs in that it is the only known garnet species with mixed trivalent and divalent cations (2 : 1) at the tetrahedral Z site. Textural and mineralogical evidence suggests that monteneveite formed during peak metamorphism (at ca. 600 degrees C) during partial breakdown of tetrahedrite-(Fe) by reactions with carbonate, under relatively oxidizing conditions. The mineral is named after the type locality, the Monteneve (Schneeberg) mine.