Uranyl-oxide hydroxy-hydrate minerals: their structural complexity and evolution trends

Uranyl-oxide hydroxy-hydrates (UOH's) represent one of the most structurally and chemically complex families of naturally occurring U⁶⁺ phases. To date, about 28 members are known as mineral species and a dozen others have been synthesized under laboratory conditions. The majority of these have been structurally characterized, showing an extraordinary complexity despite their relatively simple chemical composition; U, Pb, O and H are mostly the major constituents. Both the structural and chemical complexity of UOH minerals were determined with the program TOPOS, using complexity parameters that provide Shannon information content per atom and per unit cell and per formula unit. The average content per unit cell, 1159 bits/cell shows that UOH minerals are extraordinary complex; this is in line with (1) the high coordination diversity, considering the transition of U⁴⁺ to U⁶⁺ during oxidation-hydration weathering of uraninite that leads to the formation of UOH phases; (2) incorporation of a large amount of H⁺, in the form of H₂O and (OH)⁻; (3) incorporation of other elements during weathering, mostly K⁺, Pb²⁺ or Ca²⁺. The simplest UOH mineral is vandenbrandeite, triclinic, Cu(UO₂)(OH)₄, with 86 bits/cell. The most complex UOH mineral is vandendriesscheite, orthorhombic Pb_1.5[(UO₂)₁₀O₆(OH)₁₁](H₂O)₁₁, with 4296 bits/cell, which owes its structural complexity to the presence of a large proportion of H₂O/OH and several symmetrically unique U sites in the structure. The general trend of the early-forming UOH minerals is a high molar proportion of U and H₂O/OH and lower proportion of metal cations. This leads in general to their complex character. The increase in complexity (both structural and chemical) is also related to the presence of metal cations in the structure with distinct stereochemistry, such as K⁺, Fe²⁺, Ca²⁺ or Pb²⁺. The overall trend is a steady increase of complexity (both structural and chemical) during continuous alteration. A discussion of occurrence trends and UOH complexity is given. One of the most complex structures among UOHs is schoepite, [(UO₂)₈O₂(OH)₁₂](H₂O)₁₂ (~2593 bits/cell), which is reported from various localities worldwide. The relatively common occurrence of schoepite is probably because it is a very early alteration product, which is (1) complex and (2) easily distinguishable.