Crystal-chemistry of sulfates from the Apuan Alps, Tuscany, Italy. VIII. New data on khademite, Al(SO4)F(H2O)5. (30th August 2020)

• Record Type: Journal Article
• Authors: Mauro, Daniela; Biagioni, Cristian; Pasero, Marco; Zaccarini, Federica
• Is Part Of: Mineralogical magazine. Volume 84:Number 4(2020)
• Page Start: 540
• Page End: 546
• Publication Date: 2020-08-30
• Subjects: khademite, -- fluorine, -- fluo-sulfate, -- crystal structure, -- hydrogen bonds, -- Raman spectroscopy, -- Monte Arsiccio mine, -- Apuan Alps; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2020.51
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: Khademite, ideally Al(SO4 )F(H2 O)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, Al2 O3 21.27, F 6.92, H2 Ocalc 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 H2 O groups and 1 (F+OH) atom per formula unit, its chemical formula can be written as Al0.96 S1.02 O4 [F0.84 (OH)0.16 ]Σ1.00 ⋅5H2 O. 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) Å 3 and Z = 8. The crystal structure refinement converged to R 1 = 0.0293 on the basis of 2359 unique reflections with F o > 4σ( F o ) 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 H2 O 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.