Hubbardite, a new magnesium uranyl-sulfate mineral closely related to zippeite

Hubbardite (IMA 2025-041), Mg(H 2 O) 6 [(UO 2 ) 2 O(OH)(SO 4 )] 2 8H 2 O, is a new zippeite-like mineral from the Hubbard Homestead mine, Mesa County, Colorado, USA.The new mineral occurs on sandstone matrix in close association with gypsum and an unknown Al-bearing uranyl-oxide hydroxy-hydrate.Hubbardite is orthorhombic, space group Fddd (#70), with unit cell parameters a = 8.8698( 17), b = 34.183(7),c = 39.377(9), V = 11939(4) 3 and Z = 16.Crystals are plates up to about 0.3 mm in diameter, commonly forming subparallel or divergent aggregates.Crystals are flattened on {001} and exhibit the forms {100}, {001} and {140}.Hubbardite is yellow in color but sometimes appears dark brown, especially on edges.It has a pale-yellow streak and is nonfluorescent.It is transparent with vitreous luster.The mineral is flexible, but not elastic, and has a curved and stepped fracture.The Mohs hardness is about 2.The measured density is 3.59(5) g/cm 3 .The mineral is optically biaxial (-), with = 1.570(5), = 1.625( 5), = 1.646(5) (white light).The mea-and the pleochroism is X light yellow, Y yellow, Z brownish yellow; X < Y < Z.The empirical formula is {(Mg 0.92 0.08 ) 1.00 (H 2 O) 6 [(NH 4 ) 0.69 (H 2 O) 7.31 ] 8.00 } {(U 0.98 O 2 ) 2 [O 1.02 (OH) 0.98 ] 2.00 (SO 4 )]} 2 based on O + N = 34 and S = 2 apfu.The crystal structure of hubbardite (R 1 = 0.0616 for 1348 reflections with I > 2 I ) contains zippeite-type sheets between which are Mg(H 2 O) 6 octahedra and isolated H 2 O groups.