Abstract: Wupatkiite (Co,Mg,Ni)Al₂(SO₄)₄.22H₂O is a new member of the halotrichite group. It occurs 8 miles ESE of Gray Mountain, Arizona, near the prehistoric pueblo dwelling Wupatki. It occurs as cross-fibre veinlets with fibres up to 8mm long. Colour: empire rose (RHS48C), streak white, H. = 1 ½. G = 1.92, D(calc) = 1.87g/cm³. Nonpleochroic with α = 1.477, γ = 1.484, Z∧fibre axis = 12°. Wet chemistry gave MgO 1.63, CaO 0.10, MnO 0.17, FeO 0.15, NiO 0.52, CuO 0.12, CoO 3.41, Al₂O₃ 11.30, SO₃ 35.97, H₂O 42.26 (total 95.63%); rem. = montmorillonite and quartz. Parameters refinedo from the powder pattern are close to those of halotrichite. Wupatkiite is monoclinic P2₁/c with a = 6.189 Å, b = 24.23 Å, c = 21.20 Å, β = 100.33°. Strongest lines are 4.790, 100 (024); 4.295, 27 (140); 3.945, 26 (025); 3.768, 33 (062); 3.494, 92 (124, 063).