Kayrobertsonite, MnAl2(PO4)2(OH)2·6H2O, is a new secondary phosphate mineral from the Hagendorf Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany and the Foote Lithium Company mine, Kings Mountain district, Cleveland County, North Carolina, USA. Kayrobertsonite crystals occur as intergrown masses of snow-white, soft, finely fibrous needles, less than 5 μm in diameter and no more than 100 μm in length. Quantitative analysis of Foote mine kayrobertsonite gave the empirical formula: Mn0.97Ca0.03Fe0.02Al1.87(PO4)2(OH)1.62F0.03(H2O)0.38·6H2O; and for Hagendorf Süd kayrobertsonite: Mn0.92Ca0.06Fe0.02Al1.87(PO4)2 (OH)1.19F0.42(H2O)0.39·6H2O. Foote mine kayrobertsonite crystals are biaxial (−), with indices of refraction α = 1.530(1), β = 1.554(1), γ = 1.566(1), measured in white light; 2V(meas.) is 70.3(5)°, while 2V(calc.) is 69.6°. The mineral is nonpleochroic. The orientation of the crystals is Zc (length slow). Kayrobertsonite is triclinic, space group P1¯, with the unit-cell parameters: a = 10.049(2), b = 10.205(2), c = 6.083(1) Å, α = 91.79(3), β = 99.70(3), γ = 98.02(3)°, V = 607.9(2) Å3 and Z = 2 (Foote mine). The polyhedral framework in kayrobertsonite has the same topology as that in nordgauite, but with replacement of F by OH at the bridging anion sites. The main crystal chemical change from nordgauite to kayrobertsonite is a doubling of the number of water molecules in the [001] channels.

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