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

摘要

Akopovaite, ideally Li2Al4(OH)(12)(CO3)(H2O)(3), is a new hydrotalcite-supergroup mineral from the Karasu-Karayshinskoye 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 um. 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). D-meas = 2.12(2) g/cm(3) and D-calc = 2.106 g/cm(3). 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 Li-1(.9)4(Al4.05Fe0.02)(Sigma 4.07)(OH)(12)(CO3)(1.03)(H2O)(3.03). The crystal structure was refined by the Rietveld method with R-B = 0.006 and R-wp = 0.014. Akopovaite is monoclinic, C2/m, a = 5.0953(6), b= 8.877(1), c = 7.806(1) angstrom, beta= 102.572(6)degrees, V= 344.61(8) angstrom(3) and Z = 1. The polytype should be denoted as 1M. The strongest lines of the powder X-ray diffraction pattern [d, angstrom (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.