Abstract
AbstractThe new mineral bianchiniite, Ba2(Ti4+V3+)(As2O5)2OF, has been discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as brown {001} tabular crystals, up to 1 mm across, with a vitreous lustre. It is brittle, with a perfect {001} cleavage. Streak is brownish. In reflected light, bianchiniite is grey, with orange–yellow internal reflections. It is weakly bireflectant, with a very weak anisotropy in shades of grey. Minimum and maximum reflectance data for COM wavelengths [Rmin/Rmax(%), (λ, nm)] are: 5.0/5.8 (470), 5.7/6.5 (546), 5.7/7.0 (589) and 5.2/6.3 (650). Electron microprobe analyses gave (wt.% – average of 10 spot analyses): TiO210.34, V2O33.77, Fe2O33.76, As2O344.36, Sb2O30.22, SrO 0.45, BaO 34.79, PbO 0.28, F 1.77, sum 99.74, –O = F –0.75, total 98.99. On the basis of 12 anions per formula unit, the empirical formula of bianchiniite is (Ba2.00Sr0.04Pb0.02)Σ2.06(Ti4+1.14V3+0.44Fe3+0.42)Σ2.00[(As3.96Sb0.02)Σ3.98O10](O1.18F0.82)Σ2.00. Bianchiniite is tetragonal, space groupI4/mcm, with unit-cell parametersa= 8.7266(4),c= 15.6777(7) Å,V= 1193.91(12) Å3andZ= 8. Its crystal structure was refined from single-crystal X-ray diffraction data toR1= 0.0134 on the basis of 555 unique reflections withFo> 4σ(Fo) and 34 refined parameters. The crystal structure shows columns of corner-sharing [Ti/(V,Fe)]-centred octahedra running alongc, connected alongaandbthrough (As2O5) dimers. A {001} layer of Ba-centred [10+2]-coordinated polyhedra is intercalated between (As2O5) dimers. Bianchiniite has structural relations with fresnoite- and melilite-group minerals. The name honours the two mineral collectors Andrea Bianchini (b. 1959) and Mario Bianchini (b. 1962) for their contribution to the knowledge of the mineralogy of pyrite ± baryte ± iron-oxide ore deposits from the Apuan Alps.
Highlights
Arsenic mineralogy is a very interesting topic, owing to the complex crystal-chemistry of this element, occurring in more than 400 mineral species, and its environmental significance (e.g. Majzlan et al, 2014)
For the last ten years, the Monte Arsiccio mine has enriched the mineral systematics with several new mineral species belonging to different crystal-chemical classes, i.e. sulfosalts, oxides and sulfates
A few other minerals having Ba as a primary constituent are currently known: benstonite, cymrite and mannardite – all identified from the Monte Arsiccio mine (Biagioni et al, 2009; Biagioni and Orlandi, 2010)
Summary
Arsenic mineralogy is a very interesting topic, owing to the complex crystal-chemistry of this element, occurring in more than 400 mineral species, and its environmental significance (e.g. Majzlan et al, 2014). Arsenic mineralogy is a very interesting topic, owing to the complex crystal-chemistry of this element, occurring in more than 400 mineral species, and its environmental significance In addition to isolated (As3+Φ3) groups, (As2O5) dimers, (As4O8) tetramers and (As5O11) pentamers are known in minerals. Pentamers and infinite chains are rare, having been found in only four mineral species. Rings formed by four (AsO3) groups occur in stenhuggarite, ideally CaFeSbAs2O7 (Coda et al, 1977), whereas pentamers were observed in ludlockite, PbFe4As10O22 (Cooper and Hawthorne, 1996). Fe2+Fe33+As5O13, (As2O5) dimers occur along with isolated (AsO3) groups (Hawthorne, 1985; Cooper and Hawthorne, 2016), whereas karibibite displays (As2O5) dimers and (AsO2) chains (Colombo et al, 2017)
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