The new mineral ilirneyite, ideally Mg0.5[ZnMn3+(Te4+O3)3]·4.5H2O, a member of the zemannite structural family, was found at the Sentyabr'skoe Ag-Au deposit, Ilirney ore district, Western Chukotka, North-Eastern Region, Russia. It is associated with gypsum, malachite, azurite, cerussite, anglesite, brochantite, linarite, posnjakite, chlorargyrite, acanthite, gold, goethite, coronadite, paratellurite, raisaite, and xocomecatlite in the oxidation zone of quartz veins with tellurides. Ilirneyite occurs as: (1) long prismatic hexagonal crystals up to 0.15 mm in length and up to 0.01 mm in thickness, typically combined in sprays or open-work chaotic groups; (2) moss-like aggregates consisting of acicular to hair-like crystals up to 0.03 mm in length and up to 2 μm in thickness; and (3) earthy films. Ilirneyite is transparent to translucent, reddish brown, dark brownish-red, brown, or light brown. The luster is adamantine to semi-metallic in crystals and silky in moss-like aggregates. Ilirneyite is brittle; cleavage was not observed. Dcalc = 4.205 g/cm3. In reflected light, ilirneyite is dark grey. Bireflectance is weak, internal reflections are red-brown. The reflectance values [Rmax–Rmin, % (λ, nm)] are: 9.0–7.0 (470), 8.7–7.3 (546), 8.6–7.2 (589), 8.4–7.1 (650). The chemical composition (average of nine analyses, H2O by stoichiometry) is MgO 4.20, MnO 1.25, CuO 0.58, ZnO 11.42, Mn2O3 7.14, Fe2O3 0.06, TeO2 65.06, H2Ocalc 11.01, total 100.72 wt.%. The empirical formula calculated based on Te3O9 with 4.5 H2O molecules pfu is Mg0.56[Zn1.03Mg0.21Mn2+0.13Cu0.05Mn3+0.67Fe3+0.01(Te4+O3)3]·4.5H2O. Ilirneyite is hexagonal, P63/m, a 9.423(1), c 7.669(1) Å, V 589.7(2) Å3, and Z = 2. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 8.18(100)(100), 4.088(61)(200), 3.847(14)(002), 3.087(15)(120, 210), 2.977(16)(112), 2.864(24)(121, 211), and 2.796(52)(202). The crystal structure was solved from single-crystal XRD data, R1 = 0.045. Ilirneyite is isotypic with other zemannite-like minerals. It is named after the discovery locality.

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