Summary
The crystal structure of alunogen, space group\(P\bar 1\),a=7.425(2),b=26.975(2),c=6.0608(5) Å, α=90.03(1), β=97.66(2), γ=91.94(1)°, was determined by means of photographic X-ray data collected from a twinned crystal. The structure was solved by direct methods and refined by least-squares to anR index of 0.064. The dominant structural feature is the three-dimensional hydrogen bonding system which connects the discrete Al(H2O)6 octahedra with the SO4 tetrahedra (two and three respectively in the asymmetric unit). As to the water molecules, twelve behave as ligands coordinated to the Al atoms; four lie in a channel running parallel to thec axis; a fifth one with partial occupancy also lies in the channel and forms still weaker bonds. The crystal chemical formula of the investigated sample is therefore [Al(H2O)6]2(SO4)3·4.4H2O. An explanation of twinning, with (010) as the twin plane, is given.
Zusammenfassung
Die Kristallstruktur von Alunogen, Raumgruppe\(P\bar 1\),a=7,425(2),b=26,975(2),c=6,0608(5) Å, α=90,03(1), β=97,66(2), γ=91,94(1)°, wurde aus den photographisch gesammelten Röntgendaten eines verzwillingten Kristalls bestimmt. Die Struktur wurde mit direkten Methoden aufgeklärt und mit der Methode der kleinsten Quadrate aufR=0,064 verfeinert. Das Hauptmerkmal der Struktur ist ein dreidimensionales Gerüst von Wasserstoffbrücken, das pro asymmetrische Einheit zwei Al(H2O)6-Oktaeder mit drei SO4-Tetraedern verknüpft. Von den Wassermolekülen sind zwölf als Liganden an die Al-Atome koordiniert, vier liegen in einem parallel zurc-Achse verlaufenden Kanal und ein weiteres liegt ebenfalls in diesem Kanal, aber mit nur partieller besetzung und mit noch schwächeren Bindungen. Die kristallchemische Formel der untersuchten Probe lautet folglich [Al(H2O)6]2 (SO4)3·4,4H2O. Eine Erklärung der Verzwillingung, bei der (010) Zwillingsebene ist, wird gegeben.
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Menchetti, S., Sabelli, C. Alunogen: Its structure and twinning. TMPM Tschermaks Petr. Mitt. 21, 164–178 (1974). https://doi.org/10.1007/BF01081029
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DOI: https://doi.org/10.1007/BF01081029