Crystal chemistry of chlormagaluminite, Mg4Al2(OH)12Cl2(H2O)2, a natural layered double hydroxide

Abstract

Chlormagaluminite is the only Cl-dominated hydrotalcite-supergroup mineral species with M2+:M3+ = 2:1. The holotype sample of chlormagaluminite from the Kapaevskaya volcanic pipe (Irkutsk Oblast, Siberia, Russia) has been chemically and structurally characterized. The average chemical composition of the mineral is (electron microprobe, OH content is calculated by stoichiometry and H2O from the crystal-structure data, wt. %): MgO 33.85, FeO 1.09, Al2O3 22.07, Cl 14.72, H2Otot 30.96, Cl=O −3.39, total 99.30. The empirical formula based on Mg + Al + Fe = 6 atoms per formula unit (apfu) is [Mg3.91Fe2+0.07Al2.02(OH)12]Cl2.02(H2O)2.0(2). The crystal structure has been solved from single-crystal X-ray diffraction data in the space group P63/mcm, a = 5.268(3), c = 15.297(8) Å and V = 367.6(4) Å3. The refinement converged to R1 = 0.083 on the basis of 152 unique reflections with I > 2σ(I) collected at room conditions. The powder pattern contains standard reflections of a 2H polytype and two additional reflections [(010), d010 = 4.574 Å; (110), d110 = 2.647 Å] indicative of Mg and Al ordering according to the 3-√×3-√ superstructure. The structure is based upon brucite-type octahedral layers with an ordered distribution of Mg and Al over octahedral sites. The Cl− anions and H2O molecules reside in the interlayer, providing a three-dimensional integrity of the structure.