Abstract
Alunogen is triclinic, space group Pl with a = 7.420(6)Å; b = 26.97(2)Å; c = 6.062(5)Å; α = 89°57(5)′; β = 97°34(5)′; γ = 91°53(5)′; and Z = 2. A computer-controlled Buerger-Supper two-circle goniometer (Weissenberg geometry) was utilized for collection of X-ray intensity data. The structure was partially solved by the symbolic addition method and completed by use of Fourier and difference Fourier syntheses. Full-matrix least-squares refinement, with 1675 reflections, led to a final R index of 0.077.
The structure is built of discrete SO4 and Al(Ow)6 polyhedra interconnected by a network of hydrogen bonds. Four of the five independent polyhedra are spatially arranged in a manner which gives rise to pseudo-double sheets parallel to (010), thereby accounting for the (010) cleavage and twinning. The fifth polyhedron (an SO4 group) is situated between pseudo-sheets and is completely surrounded by the five zeolitic water molecules. Mean Al–Ow and S–O distances are 1.887 and 1.467 Å, respectively.
Maximum H2O content in the alunogen formula was found to be 17 molecules as opposed to the heretofore accepted value of 18. Partial dehydration is accompanied by partial occupancies of zeolitic water sites. That there exists no known aluminum sulfate hydrate other than alunogen is interpreted to be a result of the strong tendency for the Al3+ ion to form Al(H2O)3+6 in aqueous solutions.
