Fanfaniite, Ca4Mn2+Al4(PO4)6(OH,F)4·12H2O, is a new secondary phosphate mineral from the Foote spodumene mine, North Carolina, USA and the Hagendorf-Süd pegmatite, Bavaria, Germany. At the Foote mine, it forms radial aggregates up to 0.5 mm in diameter of colourless, transparent, thin blades, flattened on {010} and elongated on [001], associated with whiteite-(CaMnMn). At Hagendorf-Süd, the mineral occurs as isolated very thin laths on the surface of fibrous spheroids of kayrobertsonite and is associated with altered triplite–zwieselite and whiteite-(CaMnMn). The measured density (Foote mine) is 2.58(2) g cm−3. Optically, fanfaniite (Foote mine) is biaxial (–), with α = 1.573(2), β = 1.582(2), γ = 1.585(2) and 2V(meas) = 57(1)°. Dispersion was not observed. The optical orientation is: Z = b, X ^ c ≈ 40° in β obtuse. Pleochroism was not evident. Electron microprobe analyses gave the empirical formulas Ca3.91Mn0.772+Mg0.10Zn0.02Al3.89Fe0.213+(PO4)6(OH)3.90(H2O)12.10 (Foote mine) and Ca3.73Mn0.762+Mg0.25Zn0.08Al3.89Fe0.293+(PO4)6F1.10(OH)3.08(H2O)11.82 (Hagendorf-Süd). Fanfaniite has monoclinic symmetry, space group C2/c, with a = 10.021(4) Å, b = 24.137(5) Å, c = 6.226(3) Å, β = 91.54(2)° and V = 1505(1) Å3. The crystal structure was refined to Robs = 0.043 for 1909 unique reflections to a resolution of 0.7 Å. Fanfaniite is the Mn2+-dominant analogue of montgomeryite. The name honours Luca Fanfani who structurally characterised many phosphate minerals including montgomeryite.

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