The Crystal Structure of Mg-Al-CO3 Layered Double Hydroxide

The crystal structure of quintinite, Mg4Al2(OH)(12)(CO3).3H(2)O, from the Jacupiranga alkaline complex (Cajati, Sao Paulo, Brazil), was refined for two samples (91002 and C7029) using single-crystal X-ray diffraction data. The mineral crystallizes in the P-3c1 space group, alpha = 5.246/5.298, c = 15.110/15.199 angstrom for samples 91002/C7029. The crystal structure consists of octahedral sheets with Mg and Al ordering according to a root 3 x root 3 superstructure. The Mg and Al atoms are coordinated by six hydroxylated oxygen atoms; the average and bond distances are in the ranges 2.022-2.053 angstrom and 1.974-1.978 angstrom, respectively. The interlayer structures are identical (in contradiction to the previous assumptions), and consist of disordered (CO3)(2-) groups and (H2O)(0) molecules. The samples from Jacupiranga can be identified as quintinite-2T, which is the second finding of this polytype after the Kovdor alkaline complex (Kola peninsula, Russia). The powder X-ray diffraction pattern of quintinite-2T contains weak superstructure reflection at 4.57 angstrom (010), indicative of Mg and Al ordering. An important crystal-chemical criterion of quintinite is the interlayer distance (d(00n)-value) of similar to 7.56 angstrom, which is steady among natural specimens from various findings worldwide.