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

Abstract

The crystal structure of quintinite, Mg4Al2(OH)12(CO3)·3H2O, from the Jacupiranga alkaline complex (Cajati, São 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, a = 5.246/5.298, c = 15.110/15.199 Å for samples 91002/C7029. The crystal structure consists of octahedral sheets with Mg and Al ordering according to a 3 × 3 superstructure. The Mg and Al atoms are coordinated by six hydroxylated oxygen atoms; the average <Mg–O> and <Al–O> bond distances are in the ranges 2.022–2.053 Å and 1.974–1.978 Å, 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 Å (010), indicative of Mg and Al ordering. An important crystal-chemical criterion of quintinite is the interlayer distance (d00n-value) of ~7.56 Å, which is steady among natural specimens from various findings worldwide.

Mineralogical Crystallography Volume III

“…Compared to CuKα, the MoKα source demonstrated a 50% improvement in XRD data modeling using Hirshfeld atom refinement, both in terms of overall refinement parameters and more precise H-atom ellipsoids assignment. The crystal structure of layered double-hydroxide quintinite, Mg4Al2(OH)12(CO3)•3H2O from the Jacupiranga (São Paulo, Brazil) was refined in [11], and was thus identified as the 2T polytype of quintinite from Kovdor (Kola peninsula, Russia). This example demonstrates the high probability of known quintinite polytypes being found in other localities, and it shows that despite the differing conditions, the crystal structures of quintinites form in the same way.…”