Crystal Chemistry of Chlormagaluminite, Mg4Al2(OH)12Cl2(H2O)2, a Natural Layered Double Hydroxide

Elena S Zhitova,Vasiliy O Yapaskurt,Sergey V Krivovichev,Igor V Pekov

doi:10.3390/min9040221

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 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.

Highlights

Chlormagaluminite, Mg4 Al2 (OH)12 Cl2 (H2 O)2, is a member of the quintinite group, which belongs to the hydrotalcite supergroup [1]
This composition is in agreement with the data reported for chlormagaluminite previously [22,23] and with the iron oxidation state in quintinite [34], which is a carbonate analog of chlormagaluminite
The interlayer anion site is entirely occupied by Cl

Summary

Introduction

Chlormagaluminite, Mg4 Al2 (OH) Cl2 (H2 O) , is a member of the quintinite group, which belongs to the hydrotalcite supergroup [1]. Hydrotalcite-supergroup minerals and their synthetic analogs are known as layered double hydroxides (LDHs), which found many applications in modern industry. The structures of LDHs are built of positively charged brucite-type layers consisting of [M2+ (OH)6 ]. The metal-hydroxide layers are interstratified with interlayer anions and water molecules. Species-defining parameters [1] for hydrotalcite-supergroup minerals are as follows: (i) the dominant M2+ cation; (ii) the dominant M3+ cation; (iii) the dominant interlayer anion, and (iv) the M2+ :M3+ ratio with the values 2:1 and 3:1 being the most widespread among natural [1].

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Discussion

Conclusion