Abstract
Koenenite 4NaCl · 4(Mg,Ca)Cl2 · 5Mg(OH)2 · 4Al(OH)3 is built up of two trigonal substructures: [Na4(Ca,Mg)2 Cl12]4− and [Mg7Al4 (OH)22]4+ (space group R[unk]m and P[unk]m1 respectively). In direction of the common c axis (c = 3 · 10.88 Å or c = 10.88 Å respectively) NaCl-like chlorine double layers (acl = 4.072 Å) alternate with brucite-like OH double layers (aOH = 3.052 Å). All possible octahedral sites are randomly occupied by the metal ions. The Cl double layers have the following rhombohedral sequence: –BC–AB–CA–BC–. Evidence of a common supercell could not be found. The [100] directions of both subcells form an angle of 28.3 ± .1°. The distances Me–Cl and Me–OH are 2.79 Å and 2.02 Å respectively, the distances between the layers are Cl–Cl = 3.02 Å, OH–OH = 1.99 Å and Cl–OH = 2.94 Å. The double layers of both lattices are held together by their opposite charge and by hydrogen bonds. The reliability index for 215 independent structure factors is R = 0.056.
