Crystal-chemistry of sulfates from the Apuan Alps, Tuscany, Italy. VIII. New data on khademite, Al(SO₄)F(H₂O)₅

Abstract

Khademite, 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.

Raman spectroscopy of anhydrous and hydrated aluminum sulfates: Experience from burning coal heaps

“…They are classified as the fluorine-and hydroxyldominant members, [62] and both minerals were supposedly identified at the Anna dump. [55] The aforementioned spectrum agrees not only with the spectra of welldescribed specimens of khademite [62,63] but also with the spectrum of a phase attributed to rostite, [64] whose chemical composition was, however, only poorly described by the authors. Therefore, the Raman spectrum of the confirmed hydroxyl member is not available for comparison.…”

“…Also, interestingly, we identified additional spectra assigned to khademite that show the same band positions within experimental uncertainties, but intensities of the ν 3 bands vary, which is documented by the intensity ratio between the 1130 and 1081 cm −1 bands (shown in Figure S4). However, this type of variation is commonly found in the Raman spectra reported for khademite, [ 62–64 ] and, in this respect, the observed differences are probably not related to variations in the chemical compositions, but the intensity of the bands changes could be due to different orientations of the crystals during measurements.…”

Effect of Micro-Nanobubbles on Arsenic Removal by Trichoderma atroviride for Bioscorodite Generation

“…This change in absorbance could be attributed to the presence of oxygen and the incorporation of varying amounts of Fe. Furthermore, it is inferred that römerite is not the only mineral forming during the reaction, as there are other minerals, such as coquimbite (Fe 2 (SO 4 ) 3 ·9H 2 O), halotrichite (FeAl 2 (SO 4 ) 4 ·22H 2 O), and copiapite (MgFe[(OH)(SO 4 ) 3 ] 2 ·218H 2 O) [ 75 ].…”