Encyclopedia of Crystallographic Prototypes

γ-Alum [AlNa(SO$_{4}$)$_{2}$ $\cdot$ 12H$_{2}$O, $H4_{15}$] Structure: AB24CD20E2_cP192_205_a_4d_b_c3d_c-001

• The alums have the general formula AB(XO$_{4}$)$_{2}$ · 12H$_{2}$O, where A is a monovalent ion, B is a trivalent ion, and X is a chalcogen. In most cases atom B is aluminum and atom X is sulfur, leading to the name alum.
• All alums have their room-temperature form in space group $Pa\overline{3}$ #205, but the bonding between the A and B ions and the XO$_{4}$ complex can be quite different.
• (Lipson, 1935ab) described three general forms of alum based on the sizes of the monovalent ions. Each of these forms was given a Strukturbericht designation by (Gottfried, 1937):
  • $\alpha$-alum, with intermediate sized ions, prototype KAl(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{13}$,
  • $\beta$-alum, with large ions, prototype (NH$_{3}$CH$_{3}$)Al(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{14}$, and
  • $\gamma$-alum, with small ions, prototype NaAl(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{15}$ (this structure).
  This classification scheme is not compete, e.g., (Ledsham, 1968) points out that NaCr(SO$_{4}$)$_{2}$ · 12H$_{2}$O does not fit into any of these categories, and that the actual structure depends on the combination of monovalent and trivalent ions.
• As noted above, the $Pa\overline{3}$ structures of alum are the room temperature form. As the temperature decreases the alum structure may transform. For example, in the temperature range 150-170K the $\beta$-alum (NH$_{3}$CH$_{3}$)Al(SO$_{4}$)$_{2}$ · 12H$_{2}$O transforms into an orthorhombic structure with fully ordered NH$_{3}$CH$_{3}$ ions.

Simple Cubic primitive vectors

References

• D. T. Cromer, M. I. Kay, and A. C. Larson, Refinement of the alum structures. II. X-ray and neutron diffraction of NaAl(SO$_{4}$)$_{2}$$\cdot$12H$_{2}$O, γ-alum, Acta Cryst. 22, 182–187 (1967), doi:10.1107/S0108768199014846.
• H. Lipson, Existence of Three Alum Structures, Nature 135, 912 (1935), doi:10.1038/135912b0.
• H. Lipson, The Relation between the Alum Structures, Proc. Roy. Soc. London 151, 347–356 (1935), doi:10.1098/rspa.1935.0154.
• C. Gottfried and F. Schossberger, eds., Strukturbericht Band III 1933-1935 (Akademische Verlagsgesellschaft M. B. H., Leipzig, 1937).
• A. H. C. Ledsham and H. Steeple, The crystal structure of sodium chromium alum and caesium chromium alum}, Acta Crystallogr. Sect. B 24, 1287–1289 (1968), doi:10.1107/S0567740868004188.
\bibitem{Fletcher_Acta_Cryst_17_290_1964.bibR. O. W. Fletcher and H. Steeple, The crystal structure of the low-temperature phase of methylammonium alum}, Acta Cryst. 17, 290–294 (1964), doi:10.1107/S0365110X64000706.\bibAnnoteFile{Fletcher_Acta_Cryst_17_290_1964.bib

Found in

• R. T. Downs and M. Hall-Wallace, The American Mineralogist Crystal Structure Database, Am. Mineral. 88, 247–250 (2003).

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