Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Anthony R. Kampf, Jakub Plášil, Travis A. Olds, Christopher Emproto, Chi Ma, Jiří Sejkora, Paul M. Adams, Joe Marty

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

Journal of Geosciences, volume 70 (2025), issue 3, 151 - 161

DOI: http://doi.org/10.3190/jgeosci.0040.25

Hubbardite (IMA 2025-041), Mg(H₂O)₆[(UO₂)₂O(OH)(SO₄)]₂·8H₂O, 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) Å, V = 11939(4) ų 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 2½. The measured density is 3.59(5) g/cm³. The mineral is optically biaxial (-), with α = 1.570(5), β = 1.625(5), γ = 1.646(5) (white light). The measured 2V is 62(2)°. Dispersion is r < v strong. The optical orientation is X = c, Y = b, Z = a, and the pleochroism is X light yellow, Y yellow, Z brownish yellow; X < Y < Z. The empirical formula is {(Mg_0.92□_0.08)_Σ1.00(H₂O)₆[(NH₄)_0.69(H₂O)_7.31]_Σ8.00}{(U_0.98O₂)₂[O_1.02(OH)_0.98]_Σ2.00(SO₄)]}₂based on O+N = 34 and S = 2 apfu. The crystal structure of hubbardite (R₁ = 0.0616 for 1348 reflections with I > 2σ_I) contains zippeite-type sheets between which are Mg(H₂O)₆ octahedra and isolated H₂O groups.