Akopovaite, Li 2 Al 4 (OH) 12 (CO 3 )(H 2 O) 3 , a new Li member of the hydrotalcite supergroup from Turkestan Range, Kyrgyzstan

Akopovaite, ideally Li 2 Al 4 (OH) 12 (CO 3 )(H 2 O) 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). 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 CO 2 and H 2 O) is Li 2 O 6.43, Al 2 O 3 45.79, Fe 2 O 3 0.27, CO 2 10.09, H 2 O 36.1, total 98.68. The empirical formula based on (Li + Al + Fe) = 6 apfu is Li 1.94 (Al 4.05 Fe 0.02 ) Σ4.07 (OH) 12 (CO 3 ) 1.03 (H 2 O) 3.03 . The crystal structure was refined by the Rietveld method with R B = 0.006 and R wp = 0.014. Akopovaite is monoclinic, C 2/ m , a = 5.0953(6), b = 8.877(1), c = 7.806(1) Å, β = 102.572(6)°, V = 344.61(8) Å 3 and Z = 1. The polytype should be denoted as 1 M . 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.