Autunite structure

Crystal structure studies have only been made on members of the meta-autunite family. Abernathyite and metatorbernite have been studied by Ross and Evans, and Ross and coworkers,meta-uranocircite by Zolensky and Khosrawan-Sazedj, threadgoldite by Piret and co-workers, and hydrogen-meta-autunite by Morosin." These studies verify the general meta-autunite structure proposed by Beintema, but show that there are subtle structural differences among the species. The basic unit is an infinite sheet of 2-4 UOe polyhedra and (P,As)04 tetrahedra, which share corners. Ideally, this sheet has tetragonal symmetry. It is shown in Fig. 13, which also shows how the sheets are proposed to... 

The structure of threadgoldite determined by Piret and co-workers shows a complicated derivative of the autunite structure. The structure consists of [U02P04]n layers that are only slightly distorted from the square array. The stacking of the layers shifts parallel to c (the a-axis corresponds to the c-axis in the tetragonal forms) to accommodate an interlayer AI2O10 double octahedral cluster and to yield a monoclinic structure. 

Appendix C contains the chemical formulae for the minerals used in this book. There are very few minerals that have the ideal crystalline structures discussed above. There are sufficient substitutional impurities, crystal defects, and distortions that make the CBPC structure significantly different from the models discussed above. Several well-established minerals exhibit these features, as are many of those listed in Appendix C. For example, Ca(UO2)2(PO4)2T0H2O is formed by the substitution of Ca in autunite by uranyl (UO2) ions, making the autunite a mineral of radioactive uranium. Similarly, (Ce,Th)P04 is formed by the substitution of the Ce in monazite by Th. Numerous minerals can be formed by substitutions and provide a researcher sufficient degree of freedom to synthesize very complex minerals to produce useful CBPCs. 

Figure 5. The autunite sheet anion-topology (a) and corresponding sheet of vertex-sharing uranyl square bipyramids from the structures of p and y [(U02)(0H)2] (b).

The autunite-t5 e compounds with Cs in the interlayer region adopt lower S5mimetry ordered structures with a slightly different state of hydration (Tables 10 and 11) [110], In the pseudotetragonal (aNl bHl 7.0 A, p = 90.4°) structure of Cs2[(U02)(P04)]2(H20)5, two independent Cs atoms occupy the interlayer in conjunction with five H2O groups (Fig. 30). 

In the structure of autunite, Ca[(U02)(P04)]2(H20)n, (Fig. 31), Ca is in sevenfold coordination by H2O groups and is further coordinated by two O atoms of the uranyl phosphate sheet at somewhat longer distances. There are two additional H2O groups located in the interlayer where they are held in position only by hydrogen bonding [125]. 

Autunite is well-known to dehydrate rapidly to form meta-autunite, for whieh no satisfactory structure model has yet been presented. A method of direet synthesis of meta-autunite has not been reported, and its formation by dehydration of autunite likely induces so much mosaic spread that single crystal structure determinations have not been successful. Autunite may be eonsidered to exhibit pseudosymmetry, as the transformation matrix [- /2OO/OOI/OIO] yields a metrically tetragonal cell. 

Figure 31. The structure of autunite, Ca[(UO2)(PO4)]20T2O)n, projected along [001] at left Ca atoms are shown as black spheres and O atoms in the interlayer as gray spheres, after [125], At right, a portion of the structure of autunite with H atoms omitted, but a possible hydrogen bonding scheme shown.

A wider range of structural variation is exhibited by the autunite-type compounds that contain Ba. The barium uranyl phosphate decahydiate mineral uranocircite, Ba[(U02)(P04)]2(H20)io, is likely isostructural with its arsenate analogue, heinrichite, Ba[(U02)(As04)]2(H20)io, (Tables 12 and 13) [128, 131],... 

The autunite-type compounds with Mg or divalent transition metal cations (Mn, Fe, Co, Ni, Cu, Zn, Cd) in their interlayers are listed in Table 14 (phosphates) and Table 15 (arsenates). In the known structures of these compounds, the divalent interlayer cations are all in sixfold coordination appearing as nearregular octahedra with the exception of Cu, which shows a typically Jahn-Teller distorted sixfold coordination environment appearing as tetragonal dipyramids, and whose structures will be discussed separately. Three states of hydration are observed among these (Mn, Fe, Co, Ni) structures the triclinic dodecahydrates, the monoclinic decahydrates and the triclinic octahydrates [136, 138]. With progressive dehydration, the interlayer spacing decreases, the sheets shift in relative position, and the interlayer octahedra shift positions relative to the sheets (Fig. 34). 

Compounds with the autunite-type sheet and trivalent interlayer ions have been described with both large cations (La, Pr, Nd) and smaller cations (Al, Fe), but structures have been refined only for a few Al-bearing compounds (Table 16). 

The structure of CS2UO2P2O7 consists of uranyl square bipyramids that share equatorial vertices with pyrophosphate groups to form an open sheet (Fig. 47). The Cs atoms are centered below the cavities in the sheets and occupy the interlayer region [181]. The uranyl pyrophosphate sheet in CS2UO2P2O7 can be compared to the uranyl phosphate sheet found in autunite-type compounds (Fig. 28) the pyrophosphate group of the [(U02)(P207)] sheet occupies the same position as the single phosphate tetrahedron in the autunite-type sheet, [(U02)(P04)]. 

Autunite, Ca(U02)2(P04)2 H20, n = 6-10, is representative of a class of minerals and synthetic products of the general formula U02P04 H20 which have layer-like structures and which... 

Chapter 5.7). The uranium compound has a structure consisting of alternate layers of H3O+/H2O and UO2PO4 ions which is related to that of Autunite (Figure 5.40). The high proton conductivity (Table 12.49) is associated with a ready transfer of H from H3O to H2O within the water layers [78,79]. 

Fig. 13 Ideal uranyl phosphate sheet structure as found in autunite and meta-autunite families, (a) (top), structure of [(U02)(T04)] sheet (b) (centre), stacking of sheets in autunite family (c) (bottom), stacking of sheets in meta-autunite family. After Beintema °...

Specimens of trogerite and meta-uranocircite are known to transform from tetragonal to lower symmetry at temperatures near room temperature. For trogerite the transformation is around 25°C for meta-uranocircite it is at 108°C. A slight distortion in the structure results and the crystals show an extensive cross-grid twinning. Similar transformations occur in other minerals also—for example, Ca and Pb meta-autunites, and may explain the reported meta-autunite II, which is orthorhombic. 

The 2 4 compounds appear to have unrelated crystal structures. Walpurgite shows a chain unit of 2-4UO edge-shared polyhedra with attached ASO4 tetrahedra.Pseudo-autunite may be related to the autunite family with extra PO4 tetrahedra. The other compounds probably have a sheet-like structure also. Coconinoite is a mixed phosphate-sulphate and is therefore quite unique. Until the X-ray powder pattern is indexed and a unit cell is determined, no comments on its character can be made. 

---