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Adding complexity to the garnet supergroup: monteneveite, Ca3Sb5+2(Fe3+2Fe2+)O12, a new mineral from the Monteneve mine, Bolzano Province, Italy
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0002-5390-0577
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0003-2109-2277
Dipartimento di Scienze della Terra, Università degli Studi di Firenze.
Dipartimento di Scienze della Terra, Università degli Studi di Firenze.
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2020 (English)In: European journal of mineralogy, ISSN 0935-1221, E-ISSN 1617-4011, Vol. 32, no 1, p. 77-87Article in journal (Refereed) Published
Abstract [en]

Monteneveite, ideally Ca3Sb5+2(Fe3+2Fe2+)O12, is a new member of the garnet supergroup (IMA2018-060). 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 equidimensionaland 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(Sb5+1.73Sn4+0.10Fe3+0.17)Σ=2.00(Fe3+2:43Fe2+0.37Zn0.20)Σ=3.00O12. The most significant chemical variations encounteredin the sample are related to a substitution of the type YSn4+ + ZFe3+  YSb5+ + ZFe2+. Mössbauer 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 (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-3d, 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 ºC) during partial breakdown of tetrahedrite-(Fe) by reactions with carbonate, under relatively oxidizingconditions. The mineral is named after the type locality, the Monteneve (Schneeberg) mine.

Place, publisher, year, edition, pages
2020. Vol. 32, no 1, p. 77-87
National Category
Geology Inorganic Chemistry
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-3720DOI: 10.5194/ejm-32-77-2020OAI: oai:DiVA.org:nrm-3720DiVA, id: diva2:1388668
Available from: 2020-01-27 Created: 2020-01-27 Last updated: 2020-01-27Bibliographically approved

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