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Armellinoite-(Ce), CaCe(AsO)⋅HO, a new mineral species isostructural with pottsite, (PbBi)Bi(VO)⋅HO

Mineralogical Magazine, ISSN: 1471-8022, Vol: 85, Issue: 6, Page: 901-909
2021
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Armellinoite-(Ce), ideally CaCe(AsO)⋅HO, 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. 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): (CaThSrLa)(CeNdYGdSmPrDyHo)[(AsP)O]⋅(HOF). The presence of HO was confirmed by Raman spectroscopy. The mineral is tetragonal, I4/a, with single-crystal unit-cell parameters a = 10.749(2), c = 12.030(2) Å and V = 1390.0(6) Å, with Z = 4. The eight strongest X-ray powder diffraction lines are [d Å (I; 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 = 0.0284 for 1275 unique reflections) has isolated TO (T = As) tetrahedra that link Ca- or Ce-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 (PbBi)Bi(VO)⋅HO, 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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