Armellinoite-(Ce), ideally Ca4Ce4+(AsO4)4⋅H2O, is a new mineral discovered in Fe–Mn ore in metaquartzites of the Montaldo mine, Corsaglia Valley, Cuneo Province, Piedmont, Italy. It occurs as very small and rare, pale yellow to brown–yellow pseudo-octahedral translucent crystals hosted by a matrix of quartz, hematite, cryptomelane/hollandite, tilasite, muscovite, braunite and montmorillonite. The mineral is translucent, with white streak and has a resinous to vitreous lustre. It is brittle with irregular fracture and fair cleavage parallel to {110} and {100}. Estimated Mohs hardness is ~3–3.5. Calculated density is 4.29 g⋅cm–3. Armellinote-(Ce) is uniaxial (–), ω = 1.795(5), ɛ = 1.765(5) (white light), non-pleochroic and non-fluorescent. Chemical point analyses by WDS-EPMA yielded the empirical formula (based on 17 O+F anions): A(Ca3.89Th0.08Sr0.02La0.03)Σ4.02B(Ce4+0.76Nd0.13Y0.08Gd0.03Sm0.02Pr0.01Dy0.01Ho0.01)Σ1.05[(As4.00P0.01)Σ4.01O4]4⋅(H2O0.85F0.15)Σ2.00. The presence of H2O was confirmed by Raman spectroscopy. The mineral is tetragonal, I41/a, with single-crystal unit-cell parameters a = 10.749(2), c = 12.030(2) Å and V = 1390.0(6) Å3, with Z = 4. The eight strongest X-ray powder diffraction lines are [d Å (Irel; hkl)]: 7.983 (36; 101), 4.443 (23; 2̄11), 2.957 (100; 3̄12), 2.398 (14; 420), 1.875 (22; 424, 325), 1.728 (19; 3̄16), 1.612 (13; 613) and 1.475 (26; 712, 552). The crystal structure (R1 = 0.0284 for 1275 unique reflections) has isolated TO4 (T = As5+) tetrahedra that link Ca2+- or Ce4+-centred polyhedra via common oxygen ligands to form 2D blocks or double-layered (DL) structural units parallel to (001). Armellinoite-(Ce) is isostructural with pottsite, ideally (Pb3Bi)Bi(VO4)4⋅H2O, and closely related to a larger number of anhydrous synthetic compounds. The mineral is named after the mineral collector Gianluca Armellino (b. 1962), who collected the discovery sample.

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