Xenophyllite, Na4Fe7(PO4)6, an Exotic Meteoritic Phosphate: New Mineral Description, Na-ions Mobility and Electrochemical Implications

摘要: Xenophyllite, ideally Na4Fe7(PO4)6, is a rare meteoritic phosphate found in phosphide-phosphate assemblages confined to troilite nodules of the Augustinovka iron meteorite (medium octahedrite, IIIAB). The mineral occurs as tiny lamella up 0.15 mm long cross-cutting millimeter-sized grains sarcopside, Fe3(PO4)2, associated with schreibersite, chromite and pentlandite. Xenophyllite translucent, has bluish-green grey-green color vitreous lustre. Moh’s hardness 3.5–4. Cleavage perfect on {001}. Measured density 3.58(5) g/cm3. biaxial (−), 2V 10–20°, refractive indexes: α 1.675(2), β 1.681(2), γ 1.681 (2). Chemical composition holotype specimen (electron microprobe, wt.%) is: Na2O 10.9, K2O 0.4, MnO 5.8, FeO 42.1, Cr2O3 0.8, P2O5 40.7, total 100.7, corresponding empirical formula (Na3.67K0.09)Σ3.76(Fe2+6.12Mn2+0.85Cr0.11)Σ7.08P5.99O24.00. triclinic, P1 or P-1, 9.643(6), b 9.633(5), c 17.645(11) A; 88.26(5), 88.16(5), 64.83(5)°, V 1482(2) A3, Z = 3. toichiome C-centered subcell following dimensions: 16.257(9), 10.318(8), 6.257(9) A, 112.77(9)°, 968(2) 2. structurally related synthetic Kna3Fe7(PO4)6 having channel-type structure, galileiite, NaFe4(PO4)3. variations chemical xenophyllite ranging from Na4Fe7(PO4)6 almost Na2Fe8(PO4)6 are accounted for by Na-ions mobility. latter property makes promising prototype cathode materials used sodium-ion batteries.