Abstract
Abstract. Monteneveite, ideally Ca3Sb25+(Fe23+Fe2+)O12, 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 oxycalcioroméite. Monteneveite occurs as black, subhedral crystals with adamantine lustre. They are equidimensional and up to 400 µm 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 kg mm−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)Σ=3.00 (Sb1.735+Sn0.104+Fe0.173+)Σ=2.00(Fe2.433+Fe0.372+Zn0.20)Σ=3.00O12. The most significant chemical variations encountered in the sample are related to a substitution of the type YSn4++ZFe3+→YSb5++ZFe2+. Mössbauer data obtained at RT and 77 K 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 (Å), 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-3‾d, with a=12.6093(2) Å, V=2004.8(1) Å3, and Z=8. The crystal structure was refined up to R1=0.0197 for 305 reflections with Fo>4σ(Fo) 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 ∘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.
Highlights
The garnet supergroup, with the general formula {X3} [Y2](Z3)φ12, includes all minerals isostructurally with garnet regardless of what elements occupy the four atomic sites; i.e. the supergroup includes several chemical classes (Grew et al, 2013)
In recent years, several new members of the garnet supergroup that are nominally Si-free and contain elements such as Sb and Sn have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA)
During an examination of roméite group specimens from various localities, a sample from the Monteneve (Schneeberg) mine, Passiria Valley, Bolzano Province, Alto Adige (South Tyrol), Italy (46◦53 46 N, 11◦10 46 E, ∼ 2300 m above sea level), was found to contain a new mineral belonging to the garnet supergroup
Summary
The garnet supergroup, with the general formula {X3} [Y2](Z3)φ12, includes all minerals isostructurally with garnet regardless of what elements occupy the four atomic sites; i.e. the supergroup includes several chemical classes (Grew et al, 2013). Sjögren’s collection was donated to the Swedish Museum of Natural History in 1901 This new member of the garnet supergroup, with the ideal formula Ca3Sb52+(Fe32+Fe2+)O12, was named monteneveite after the type locality, and both the mineral and mineral name have been approved by the IMA CNMNC (2018–060). The sample could potentially be from Bockleitner Halde (“Bockleitner dump”), from where Brezina (1880) described a very similar mineral assemblage in his work on Schneebergit; he reported his specimen as being associated with anhydrite and gypsum, which are not found in the present material This was later corrected by von Elterlein (1891), who pointed out that the supposed sulfates are calcite. The sample is quite vuggy, but we suspect that it has been treated with dilute acid at some point in order to dissolve the carbonates (mainly calcite) to reveal the oxycalcioroméite crystals
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