Sinkankasite, ideally H2MnAl(PO4)2(0H) · 6H2O, is a new mineral occurring as an alteration product of triphylite at the Barker pegmatite, Keystone, South Dakota. It is triclinic, space group P1 or P1, with cell parameters a = 9.58(4), b = 9.79(5), c = 6.88(4)Å, α = 108.1(2), β = 99.6(3), γ = 98.7(3)°, V = 590(7)Å3, and Z = 2. Crystals are elongated on [001], flattened on {100} and twinned on {100}. Sinkankasite is transparent, colorless, has hardness 4 and a good {100} cleavage. The observed and calculated densities are 2.27 and 2.25 g/cm3, respectively. Optical parameters include: biaxial negative; 2V (calc.) = 84°; α = 1.511(2), β= 1.529(2), γ = 1.544(2); dispersion r < v; c Λ Y ≃ 11°. The strongest lines in the powder diffraction pattern are (d, I/Io, hkl): 9.2, 100, 100; 5.41, 50, 111; 5.06, 60, 111; 4.58, 40, 200, 111; 2.834, 40, 230; 2.701, 40, 130. The name is in honor of Dr. John Sinkankas.

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