The crystal structure of guilleminite, Ba(UO₂)₃(SeO₃)₂O₂(H₂O)₃, orthorhombic, a 7.084(1), b 7.293(1), c 16.881(4) Å, V 872.1(3) ų, P2₁nm, Z = 2, has been solved by direct methods and refined to an R index of 4.7% based on 975 observed (5phi) reflections measured with MoK alphaX-radiation on a single-crystal diffractometer. This has resulted in a revision of the chemical formula and space group. There is one unique Ba atom, coordinated by seven oxygen atoms and three H₂O groups in a capped tri-augmented trigonal prismatic arrangement. There is one unique Se atom coordinated by three oxygen atoms, and these four atoms form a triangular pyramid with Se in the apical position, characteristic of stereoactive lone-pair behavior. There are two distinct U atoms: U(1) is coordinated by a hexagonal bipyramidal arrangement of oxygen atoms and U(2) is coordinated by a pentagonal bipyramidal arrangement of oxygen atoms. In both polyhedra, the apical oxygen atoms constitute the uranyl oxygen atoms at a U­O distance of ~1.8 Å. The (UO₇) groups form edge-sharing dimers that are linked into chains of the form [U₃O₁₄] by sharing edges with (UO₈) monomers. The chains extend along [100] and are cross-linked in the [001] direction by (SeO₃) groups to form sheets of the form [U₃(SeO₃)₂O₈] parallel to (010). The sheets are linked along [010] by interstitial Ba atoms and by hydrogen bonding involving interstitial (H₂O) groups. The sheets are topologically identical to the sheets in phosphuranylite, KCa(H₃O)₃[{(UO₂)₃(PO₄)₂O₂}₂(UO₂)] (H₂O)₈, except for the fact that (SeO₃) groups proxy for (PO₄) groups in guilleminite.

Guilleminite is a hydrated selenite of barium and uranium first described by Pierrot et al. (1965) from the oxidized zone of the copper-cobalt deposit of Musonoi, Katanga. It occurs as coatings and silky masses, and as small canary-yellow orthorhombic crystals in geodes.

The coordination polyhedra around the cations in guilleminite are illustrated in Figure 1. Ba is surrounded by seven oxygen atoms and three H₂O groups at distances from 2.73 to 2.97 Å, with a mean distance of 2.89 Å; this is a very restricted range of distances for such a large coordination polyhedron, and hence there is not the usual problem of determining whether the longest distances represent significant bonds. The oxygen atoms form a capped trigonal prism (Fig. 1c), augmented by three H₂O groups in a triangular arrangement.

Selenium is coordinated to three oxygen atoms arranged in a triangle with the central cation significantly displaced from the plane of the oxygen atoms (Fig. 1d). This triangular pyramidal coordination is typical for Se⁴⁺ and is indicative of stereoactive lone-pair behavior. The observed <Se­O> distance of 1.68 Å is within the range of <Se­O> distances observed in inorganic structures (Hawthorne et al. 1987).

The U(1) cation is surrounded by six equatorial oxygen atoms with U(1)­O distances in the range 2.27­2.62 Å and by two `uranyl' (apical) oxygen atoms 1.79 and 1.80 Å from the central cation, the anions forming a hexagonal bipyramid around the central U(1) atom; the <U(1)­O> distance is 2.33 Å. The U(2) cation is surrounded by five equatorial oxygen atoms with U(2)­O distances in the range 2.24­2.42 Å and by two apical oxygen atoms at 1.78 and 1.81 Å from the central cation, the anions forming a pentagonal bipyramid around the central U(2) atom; the <U(2)­O> distance is 2.19 Å. The larger <U­O> distance for the U(1) cation is in accord with the higher coordination number for U(1) relative to U(2). The bond-valence sums around the U cations (Table 4) are significantly larger than their ideal values of 6.0 vu (valence units), a fairly typical situation for U⁶⁺ that suggests that the bond-valence curve for U⁶⁺­O is somewhat inaccurate.

The Uphi_n ( phi= unspecified anion) polyhedra polymerize to form an important chain component of the guilleminite structure. Two Uphi₇ pentagonal bipyramids share an edge to form a [U₂phi₁₂] dimer. These dimers link to form a chain of the type [U₃phi₁₄] by sharing edges with Uphi₈ hexagonal bipyramids (Fig. 2). These chains extend along [100] and are cross-linked in the [001] direction by SeO₃ groups, with adjacent [U₃phi₁₄] chains staggered such that they pack as densely as possible in the (010) plane (Fig. 2). This strongly bonded sheet is the structural unit of guilleminite, with Ba and H₂O as the interstitial species. The [U₃(SeO₃)phi₂₈] sheets are linked in the [010] direction by Ba atoms and by hydrogen bonds. As is apparent in Figure 3, the sheets are modulated in the [001] direction, and the modulations of adjacent sheets intermesh. The fact that the structural unit is a sheet parallel to (010) accounts for the perfect {010} cleavage in guilleminite.

The guilleminite structure has marked Pmnm pseudo-symmetry. The aspect of the structure that breaks Pmnm symmetry is the position of Ba and the positions of the three equatorial H₂O groups. The Ba atom is displaced ~1.2 Å from the pseudo-(100)-mirror plane at x=±½, and the three H₂O groups are slightly displaced from positions required by true mirror symmetry; Figure 4 shows both these aspects of the structure in the upper interstitial layer. In the lower interstitial layer, the Ba atom is shown on the pseudo-mirror plane; note that the coordination of the Ba atom changes quite markedly. In the real structure (upper layer), the Ba coordination is a tri-augmented monocapped triangular prism, whereas in the idealized Pmnm structure (lower layer) the Ba coordination is a gable disphenoid (Johnson 1966). This displacement is due possibly to the bond-valence requirements of the O(1) and O(4) anions. In the real structure, the Ba­O(1) and Ba­O(4) distances are 2.90 and 2.97 Å, respectively. If Ba were to occur on the mirror plane, these Ba­O(1) and Ba­O(4) distances would lengthen to ~3.9 Å, well beyond any significant bonding interaction; this would result in a drastic incident bond-valence deficiency at the Ba atom (with a sum of ~1.63 vu, Table 4) and less pronounced deficiencies at O(1) and O(4). Thus the incident bond-valence requirements of the interstitial Ba atom seems reasonable for the P2₁nm (rather than Pmnm) symmetry of guilleminite. However, it is possible that guilleminite might transform to this symmetry at higher temperature, provided dehydration could be suppressed.

The structural unit in guilleminite is topologically identical to the sheet in phosphuranylite, KCa(H₃O)₃[{(UO₂)₃(PO₄)₂O₂}₂(UO₂)](H₂O)₈ (Demartin et al. 1991), except that, in the latter structure, the cross-linkage between the [U₃phi₁₄] chains occurs via (PO₄) groups rather than (SeO₃) groups (Fig. 5), and the sheets in phosphuranylite are cross-linked into a framework by Uphi₆ octahedra. The similarity of the sheets in each mineral is directly apparent from their cell dimensions: a_g x 2 b_p and c_g c_p. In guilleminite, the SeO₃ groups flanking the [U₃phi₁₄] chain are identical as a result of translational symmetry (Fig. 2), whereas in phosphuranylite, the corresponding tetrahedra alternately point up and down the a axis (Fig. 5).