Structure cristalline de la khadémite Al(SO4)F.5 H2O

Abstract

La khadémite est orthorhombique, groupe spatial Pcab, avec a = 11,181(4) ; b = 13,048(5) ; c = 10,885(4) Å et Z = 8. Les intensités difîractées ont été mesurées avec un diffractomètre à quatre cercles Philips 1100 et la structure a été partiellement résolue par la méthode directe, puis complétée à l'aide de séries de Fourier et de séries différences. Un dernier affinement avec 1 126 réflexions indépendantes a conduit à un facteur R final égal à 0,052. La structure est constituée de tétraèdres SO₄ et d'octaèdres Al(Ow)₆ et Al(Ow)₄F₂, tous isolés, reliés entre eux par un réseau de liaisons hydrogène. Les atomes Al sont dans les positions spéciales 4(a) et 4 (b) et les octaèdres correspondants forment des couches parallèles à (100) ; celles-ci alternent avec des couches de tétraèdres SO₄. Les distances moyennes S-O et Al-Ow sont respectivement égales à 1,477 et 1,872 Å. Les cinq molécules d'eau sont directement liées aux atomes Al mais l'examen attentif des séries différences n'a pas permis de localiser le proton du groupement OH ; diverses considérations ont conduit à remplacer ce groupe par un atome de fluor : la formule de la khadémite serait alors Al(SO₄)F • 5 H₂O et non Al(SO₄)(OH) • 5 H₂O.

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

“…The F atom is the acceptor of a very strong H bond from Ow(5). The role of F as an acceptor of H bonds agrees with the previous structural model proposed by Bachet et al (1981) and was observed in other rare sulfate minerals (e.g. wilcoxite, MgAl(SO 4 ) 2 F⋅17H 2 O; Mauro et al, 2019).…”

“…This observation suggests that H bonds play a key role in the stabilisation of the crystal structure of khademite. The geometrical features of H bonds given in Table 6 match those found by Bachet et al (1981). On the basis of the O⋅⋅⋅O distances, d, the ten H bonds can be grouped into three types: (1) very strong Brown and Altermatt (1985) as well as the relationship of Ferraris and Ivaldi (1988), giving BVS for these strong H bonds ranging between 0.23 and 0.27 vu.…”

“…Taking into account the results of the electron microprobe analysis, the site scattering at the Al, S, F, O and H sites was modelled using neutral site-scattering curves taken from the International Tables for Crystallography (Wilson, 1992). Although the coordinates of H atoms were given by Bachet et al (1981), their positions for the sample from Monte Arsiccio have been sought in the difference-Fourier maps. Soft restraints were applied to O-H bonds, in order to avoid unrealistically short distances.…”

“…Khademite and its (OH)-analogue rostite have a troubled history, as summarised by Košek et al (2019). These authors stated that the crystal structure study of khademite performed by Bachet et al (1981) "found that the atomic positions in the structure were so small that they require the presence of fluorine, and that the presence of OH in the structure was not possible." It is not clear what Košek et al (2019) mean when they state that "the atomic positions […] were so small".…”

“…The current definition of khademite is the result of a long and debated history summarised by Košek et al (2019). As a matter of fact, a modern and full characterisation of khademite is still lacking, as Bachet et al (1981) gave no chemical analysis and no further crystal-structure refinements were reported. The finding at the Monte Arsiccio mine has permitted the collection of a new set of good-quality data, integrating electron microprobe, spectroscopic, and single-crystal X-ray diffraction analyses, along with a thoughtful discussion of previous available data.…”