Twinning is a phenomenon complicating structure analysis of single crystals of standard as well as modulated structures. Jana2006 as a software for advanced structure analysis contains tools for recognition and refinement of twins including most complicated cases of modulated and magnetic structures. In order to efficiently use the tools of Jana2006 related to twinning, we explain the basic terminology and the underlying theory, especially the symmetry of the diffraction patterns affected by twinning. We present typical diffraction patterns of twins and show how twinning can be recognized or detected by various tools and described with twinning matrices. Data processing of twins and ways of how they can be imported to Jana2006 are also discussed. Two examples demonstrate the solution of typical twins: twinning by metric merohedry and twinning by reticular merohedry, followed by the third example demonstrating twinning in a commensurately modulated structure. The relationship between the dimensionality of the structure and twinning is discussed, too.

Uranyl-oxide hydroxy-hydrates (UOH) represent one of the most structurally and chemically complex families of naturally occurring U6+ phases.To date, about 28members 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.