Castellaroite (IMA2015-071), Mn2+3(AsO4)2·4.5H2O, is a new secondary arsenate mineral from the Monte Nero mine, Rocchetta Vara, La Spezia, Liguria, Italy and the Valletta mine, near Canosio, Cuneo, Piedmont, Italy. It crystallized from As- and Mn-rich hydrothermal fluids in an oxidizing environment. At Monte Nero, it is associated with coralloite, manganohörnesite, rhodochrosite, sarkinite, sterlinghillite, strashimirite and wallkilldellite. At Valletta, it is associated with braccoite, hematite, manganberzeliite, orthoclase and tiragalloite. Castellaroite occurs as thin blades, flattened on [001], striated and elongated parallel to [100] and exhibiting the forms {110}, {012} and {001}. The mineral is colourless and transparent with a vitreous to silky lustre and white streak. Crystals are flexible with a curved fracture, and one perfect cleavage on {001}. The Mohs’ hardness is 2½. The measured and calculated densities are 3.14(2) g·cm−3 and 3.164 g·cm−3, respectively. The mineral is easily soluble in dilute HCl at room temperature. Optically, castellaroite crystals are biaxial (−), with α = 1.644(1), β = 1.662(1) and γ = 1.667(1) (white light); 2V = 57(1)°; dispersion r < v, moderate; the optical orientation Y = b; Za. The Raman spectrum is dominated by features corresponding to the AsO4 group and also confirms the presence of H2O. Electron-microprobe analyses gave the empirical formula Mn2+3.02(As1.94P0.06)Σ2.00O12.5H8.96, based on 12.5 O apfu. Castellaroite is monoclinic, P21/n, with the unit-cell parameters: a = 8.7565(8), b = 13.4683(13), c = 18.652(2) Å, β = 94.876(7)°, V = 2191.7(4) Å3 and Z = 8. The eight strongest lines in the X-ray powder diffraction pattern are [dobs/Å (I) (hkl)]: 10.90(100)(011), 9.27(67)(002), 6.97(42)( 1¯11), 3.323(47)(multiple), 3.043(87) ( 1¯134,204, 2¯232), 2.656(85)(multiple), 2.165(46)(multiple), and 1.5589(32)(multiple). The crystal structure was refined to R1 = 0.118 for 2513 observed reflections [Fo > 4σF]. The structure contains kinked chains of edge-sharing MnO6 octahedra parallel to [100]. The chains are linked to each other by corner-sharing, forming sheets parallel to {001} and AsO4 tetrahedra corner-link with octahedra in the sheet, forming a heteropolyhedral layer. Edge-sharing MnO6–MnO5 dimers share corners with octahedra and tetrahedra in adjacent layers, thereby linking them in the [001] direction. The heteropolyhedral layer is topologically identical to those in the structures of the phosphate minerals: angarfite, bakhchisaraitsevite, mejillonesite, metaswitzerite, rimkorolgite and switzerite. Overall, the structure is most similar to that of metaswitzerite.

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