IMA 2018-095 = akopovaite

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Marco E. Ciriotti
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IMA 2018-095 = akopovaite

Messaggio da Marco E. Ciriotti » gio 13 feb, 2020 11:46

Prossima pubblicazione.

▪ Karpenko, V.Yu., Zhitova, E.S., Pautov, L.A., Agakhanov, A.A., Siidra, O.I., Krzhizhanovskaya, M.G., Rassulov, V.A., Bocharov, V.N. (2010): Akopovaite, Li2Al4(OH)12(CO3)(H2O)3, a new Li-member of the hydrotalcite supergroup from Turkestan Range, Kyrgyzstan. Mineralogical Magazine, 84, (in press).

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 ca. 1). Dmeas = 2.12(2) g/cm3, 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, 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)(OH)12(CO3)1.03(H2O)3.03. The crystal structure was refined by 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) º, V = 344.61(8) Å3, 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) and (021), 2.4881 (27) (200), 2.2273 (16) (201), 1.9027 (18) (202). Akopovaite is the first naturally occurring hydrotalcite-supergroup carbonate species of Al and Li, while its synthetic analogue is known.
Marco E. Ciriotti

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