Armellinoite-(Ce), Ca 4 Ce 4+ (AsO 4 ) 4 ⋅H 2 O, a new mineral species isostructural with pottsite, (Pb 3 Bi)Bi(VO 4 ) 4 ⋅H 2 O

Armellinoite-(Ce), ideally Ca 4 Ce 4+ (AsO 4 ) 4 ⋅H 2 O, 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 (Ca 3.89 Th 0.08 Sr 0.02 La 0.03 ) Σ4.02 B (Ce 4+ 0.76 Nd 0.13 Y 0.08 Gd 0.03 Sm 0.02 Pr 0.01 Dy 0.01 Ho 0.01 ) Σ1.05 [(As 4.00 P 0.01 ) Σ4.01 O 4 ] 4 ⋅(H 2 O 0.85 F 0.15 ) Σ2.00 . The presence of H 2 O was confirmed by Raman spectroscopy. The mineral is tetragonal, I 4 1 / a , with single-crystal unit-cell parameters a = 10.749(2), c = 12.030(2) Å and V = 1390.0(6) Å 3 , with Z = 4. The eight strongest X-ray powder diffraction lines are [ d Å ( I rel ; 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 ( R 1 = 0.0284 for 1275 unique reflections) has isolated T O 4 ( T = As 5+ ) tetrahedra that link Ca 2+ - or Ce 4+ -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 (Pb 3 Bi)Bi(VO 4 ) 4 ⋅H 2 O, 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.