Crystal chemistry of natural layered double hydroxides: 4. Crystal structures and evolution of structural complexity of quintinite polytypes from the Kovdor alkaline-ultrabasic massif, Kola peninsula, Russia

Abstract

Two quintinite polytypes, 3R and 2T, which are new for the Kovdor alkaline-ultrabasic complex, have been structurally characterized. The crystal structure of quintinite-2T was solved by direct methods and refined to R1 = 0.048 on the basis of 330 unique reflections. The structure is trigonal, P$\bar 3$c1, a = 5.2720(6), c = 15.113(3) Å and V = 363.76(8) Å3. The crystal structure consists of [Mg2Al(OH)6]+ brucite-type layers with an ordered distribution of Mg2+ and Al3+ cations according to the $\sqrt 3 $ × $\sqrt 3 $ superstructure with the layers stacked according to a hexagonal type. The complete layer stacking sequence can be described as …=Ab1C = Cb1A=…. The crystal structure of quintinite-3R was solved by direct methods and refined to R1 = 0.022 on the basis of 140 unique reflections. It is trigonal, R$\bar 3$m, a = 3.063(1), c = 22.674(9) Å and V = 184.2(1) Å3. The crystal structure is based upon double hydroxide layers [M2+,3+(OH)2] with disordered distribution of Mg, Al and Fe and with the layers stacked according to a rhombohedral type. The stacking sequence of layers can be expressed as …=АB = BC = CA=… The study of morphologically different quintinite generations grown on one another detected the following natural sequence of polytype formation: 2H → 2T → 1M that can be attributed to a decrease of temperature during crystallization. According to the information-based approach to structural complexity, this sequence corresponds to the increasing structural information per atom (IG): 1.522 → 1.706 → 2.440 bits, respectively. As the IG value contributes negatively to the configurational entropy of crystalline solids, the evolution of polytypic modifications during crystallization corresponds to the decreasing configurational entropy. This is in agreement with the general principle that decreasing temperature corresponds to the appearance of more complex structures.

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

“…The major elements with Z > 6 are Mg, Al, O, and Cl, with admixed Fe (Table 2) which was considered as Fe 2+ due to the existence of a negative correlation between Mg and Fe. 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 hydroxyl groups were taken as two per cation in agreement with the LDHs stoichiometry, in particular, other quintinite-group minerals [1].…”

Modification of Layered Double Hydroxides Using First-Row Transition Metals for Superior UV-Vis-NIR Absorption and the Influence of the Synthesis Method Used

“…TM-modification of MgAl-LDHu led to the formation of more defined absorption bands in the UV-region and higher absorption intensities in the Vis range, while it led to a reduction in absorbance in the NIR-range ( Figure 2). These structures were of much higher crystallinity than those prepared by CP, and it is thus possible that -analogous to the findings by Yan et al (2010) 15 -the increased absorption could have resulted from an increased cation ordering and more consistent stacking sequence 24,25 -contrasting observations for plain MgAl-LDH. It follows a summary of observations for the modification with each of the TMs.…”

Dritsite, Li2Al4(OH)12Cl2·3H2O, a New Gibbsite-Based Hydrotalcite Supergroup Mineral

“…The arrangement of Cl − anions and H 2 O groups in the interlayer space of dritsite shows statistical disorder ( Figure 5). The general architecture of the interlayer arrangement of dritsite is principally different from that of carbonate hydrotalcite supergroup minerals, namely quintinite, hydrotalcite, pyroaurite, stichtite, takovite, and zaccagnaite [38,[43][44][45][46][47][48][49][50], as shown in Figure 5c,d. In accordance with the approach by Bookin and Drits [23], the interlayer space can be considered as consisting of trigonal prisms formed by H atoms occupying the H1 site of the metal-hydroxide layers (Figure 5e,f).…”