New data on khademite from Monte Arsiccio mine

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Marco E. Ciriotti
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New data on khademite from Monte Arsiccio mine

Messaggio da Marco E. Ciriotti » mar 30 giu, 2020 10:03

Prossima pubblicazione.

Referenza:
▪ Mauro, D., Biagioni, C., Pasero, M., Zaccarini, F. (2020): Crystal-chemistry of sulfates from the Apuan Alps (Tuscany, Italy). VIII. New data on khademite, Al(SO4)F(H2O)5. Mineralogical Magazine, 84, (in press).

Abstract:
Khademite, ideally Al(SO4)F(H2O)5, from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy) has been characterized through quantitative electron microprobe analysis, micro-Raman spectroscopy, and single-crystal X-ray diffraction. Khademite occurs as colorless to whitish tabular crystals, up to 5 mm across, or as sugary aggregates formed by colorless and shiny individuals less than 0.5 mm in size. Electron microprobe analysis (in wt% - average of twenty 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(F0.84OH0.16)Σ1.00·5H2O. The Raman spectrum of khademite is characterized 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, 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 characterized by the alternation, along b, of two distinct kinds of {010} layers, one formed by [001] rows of isolated Al-centered 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 characterization, allowing a comparison with previous results.

NdR: Il Raman di Frost et al. (2013), certamente non è stato fatto su khademite, anche se dichiarato come tale. Nei lavori di Frost et al. non sempre il materiale su cui è stato fatto fatto il Raman è stato in precedenza accuratamente identificato.
▪ Frost, R.L., Scholz, R., Lòpez, A., Xi, Y. (2013): Vibrational spectroscopic characterization of the sulphate mineral khademite Al(SO4)F·5(H2O). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 116, 165–169.
Marco E. Ciriotti

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Massimo Russo
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Iscritto il: mar 29 giu, 2004 12:00
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Re: New data on khademite from Monte Arsiccio mine

Messaggio da Massimo Russo » mar 30 giu, 2020 11:56

La khademite, in associazione con heklaite e metavoltine, è stata anche rinvenuta nelle fumarole del Vesuvio del 1895.

Massimo
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Massimo Russo
INGV - Osservatorio Vesuviano

"Un minerale senza nome è solo un inutile pezzo di pietra" (Massimo Russo)

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