Akopovaite, Li₂Al₄(OH)₁₂(CO₃)(H₂O)₃, a new Li member of the hydrotalcite supergroup from Turkestan Range, Kyrgyzstan

Abstract

Akopovaite, ideally Li2Al4(OH)12(CO3)(H2O)3, is a new hydrotalcite-supergroup mineral from the Karasu–Karavshinskoye Sn deposit, Turkestan Range, Kyrgyzstan. It occurs as white or pale yellowish rosette-like aggregates that are composed of tiny curved plates up to 20–30 μm. Akopovaite is associated with gibbsite, quartz, albite, microcline, muscovite, montebrasite, siderite, schorl and birnessite-like Fe–Mn oxides. Akopovaite has a perfect cleavage along {001}, the mineral is transparent and very soft (VHN = 24 that corresponds to Mohs hardness of ca. 1). Dmeas = 2.12(2) g/cm3 and Dcalc = 2.106 g/cm3. The Raman spectra contain bands of carbonate groups and bands of O–H stretching vibrations. The chemical composition (wt.%, electron microprobe for Al and Fe; ICP-OES for Li; CHN method for CO2 and H2O) is Li2O 6.43, Al2O3 45.79, Fe2O3 0.27, CO2 10.09, H2O 36.1, total 98.68. The empirical formula based on (Li + Al + Fe) = 6 apfu is Li1.94(Al4.05Fe0.02)Σ4.07(OH)12(CO3)1.03(H2O)3.03. The crystal structure was refined by the Rietveld method with RB = 0.006 and Rwp = 0.014. Akopovaite is monoclinic, C2/m, a = 5.0953(6), b = 8.877(1), c = 7.806(1) Å, β = 102.572(6)°, V = 344.61(8) Å3 and Z = 1. The polytype should be denoted as 1M. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.66(100)(001), 4.397(27)(020), 3.821(45)(002,021), 2.4881(27)(200), 2.2273(16)(201) and 1.9027(18)(202). Akopovaite is the first naturally occurring hydrotalcite-supergroup carbonate species of Al and Li; its synthetic analogue is known.

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

“…All of them, excluding mössbauerite, are composed of di-and trivalent cations, whereas mössbauerite contains only Fe 3+ [20]. The new monoclinic mineral akopovaite, recently approved by the IMA−CNMNC (International Mineralogical Association, Commission on new minerals, nomenclature and classification) (IMA2018-095, [21]), has the chemical formula Li2Al4(OH)12CO3·3H2O, however, its full description has not yet been published. Thus, here we provide the first description of this natural LiAl2-based LDH.…”

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

“…Despite the wide chemical variability of LDHs, their crystal structures show a certain structural commonality, in which one structural type extends to compounds of different compositions. For example, quintinite is isotypic to the Li-Al gibbsite-based mineral akopovaite, Al 4 Li 2 (OH) 12 CO 3 •3H 2 O [32], following the substitution scheme [Mg 4 Al 2 (OH) [34]. Comparison of the crystal structures of minerals shows that they are isotypic to their synthetic analogues, which is well shown by Li-Al LDHs [35][36][37].…”

Kaznakhtite, Ni6Co3+2(CO3)(OH)16⋅4H2O, a new natural layered double hydroxide, the member of the hydrotalcite supergroup

“…Fourty-three of them, including several questionable species, are mentioned in the most recent nomenclature report for the hydrotalcite supergroup (Mills et al , 2012a). Since that time, four new species have been described [akopovaite (Karpenko et al , 2020), dritsite (Zhitova et al , 2019), erssonite (Zhitova et al , 2021) and luidongshengite (Yang et al , 2021)]; whereas jamborite was redefined as a mineral species which does not belong to the hydrotalcite supergroup (Bindi et al , 2015). Here we report on a new member of the supergroup, kaznakhtite (pronounced kǝz nah tait; Cyrillic – казнахтит), named after its type locality – Kaznakhtinskiy ultrabasic massif, Ust’-Koksinskiy District, Altai Republic, SW Siberia, Russia.…”