Abstract
AbstractKhademite, ideally Al(SO4)F(H2O)5, from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy, has been characterised through quantitative electron microprobe analysis, micro-Raman spectroscopy and single-crystal X-ray diffraction. Khademite occurs as colourless to whitish tabular crystals, up to 5 mm. Electron microprobe analysis (in wt.%, average of 20 spot analyses) gave: SO3 35.43, Al2O3 21.27, F 6.92, H2Ocalc 39.73, sum 103.35, –O = F 2.92, total 100.43. On the basis of 10 anions per formula unit, assuming the occurrence of 5 H2O groups and 1 (F+OH) atom per formula unit, its chemical formula can be written as Al0.96S1.02O4[F0.84(OH)0.16]Σ1.00⋅5H2O. The Raman spectrum of khademite is characterised by the occurrence of vibrational modes of SO4 groups and by broad and strong bands due to the O–H stretching modes. Khademite is orthorhombic, space group Pcab, with unit-cell parameters a = 11.1713(2), b = 13.0432(3), c = 10.8815(2) Å, V = 1585.54(5) Å3 and Z = 8. The crystal structure refinement converged to R1 = 0.0293 on the basis of 2359 unique reflections with Fo > 4σ(Fo) and 152 refined parameters. The crystal structure of khademite is characterised by the alternation, along b, of two distinct kinds of {010} layers, one formed by [001] rows of isolated Al-centred octahedra, connected to each other through H bonds, and the other showing isolated SO4 groups. Along b, oxygen atoms belonging to SO4 groups act as acceptor of H bonds from H2O groups coordinating Al atoms. The new data improved the description of the H bonds in khademite and led us to discuss about the possible existence of its (OH)-analogue, rostite. In addition, Raman spectroscopic data were collected on the same crystal used for the crystal-chemical characterisation, allowing a comparison with previous results.
Highlights
Sulfate minerals play a central role in governing the release and transport of acidity and potential environmentally critical elements following pyrite oxidation (e.g. Jerz and Rimstidt, 2003; Hammarstrom et al, 2003, 2005)
The occurrence of fluorine in the crystal structure of khademite has been debated for a long time
The occurrence of khademite at the Monte Arsiccio mine allowed the collection of high-quality single-crystal X-ray diffraction data and electron microprobe analyses, improving the crystal-chemical knowledge of this rare fluo-sulfate and confirming previous results reported by Bachet et al (1981)
Summary
Sulfate minerals play a central role in governing the release and transport of acidity and potential environmentally critical elements following pyrite oxidation (e.g. Jerz and Rimstidt, 2003; Hammarstrom et al, 2003, 2005). The presence of sulfate assemblages related to the weathering of pyrite ores has been known since the second half of the 19th Century (e.g. D’Achiardi, 1872). Modern mineralogical data have only been collected in the last decade, with the identification of well-crystallised sulfates in the Fornovolasco and Monte Arsiccio mines. In the former locality, the new mineral species volaschioite, Fe4(SO4)O2(OH)6⋅2H2O, as well as Tl-bearing varieties of alum-(K) and voltaite, were found (Biagioni et al, 2011, 2020). Monte Arsiccio proved to be an extraordinary laboratory for the study of sulfate minerals, providing the mineral systematics with three new K–Fe sulfates (giacovazzoite, scordariite and magnanelliite – Biagioni et al, 2019a; 2019b; 2019c), and several other species, among which were world-class specimens of coquimbite (Mauro et al, 2020)
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