PREDICTION OF CRYSTAL MORPHOLOGY OF COMPLEX URANYL-SHEET MINERALS. II. OBSERVATIONS

Abstract

The morphology of basal faces of uranyl-sheet minerals is predicted by considering the bond-valence deficiency of the chains of polyhedra, the shift between the layers, and the arrangement of the interstitial complexes between the layers. The sheet structural units of schoepite, [(UO2)8O2(OH)12](H2O)12, and fourmarierite, Pb[(UO2)4O3(OH)4](H2O)4, have the same underlying anion-topology. The stabilities of their edges differ because adjacent layers in schoepite are shifted by one chain of polyhedra, whereas adjacent layers are not shifted in fourmarierite. The minerals becquerelite, [7] Ca(H2O)4[(UO2)3O2(OH)3]2 (H2O)4, compreignacite, [7] K2(H2O)3 [(UO2)3O2(OH)3]2(H2O)4, billietite, [10] Ba(H2O)4[(UO2)3O2(OH)3]2 (H2O)3, protasite, [10] Ba(H2O)3[(UO2)3O3(OH)2], and masuyite, [10] Pb(H2O)3[(UO2)3O3(OH)2], all have structural units with the same basic aniontopology. The stabilities of their edges differ because of the different arrangements of interstitial species. In becquerelite, the interstitial cations are arranged in rows parallel to [010], whereas in billietite and compreignacite, the interstitial cations are arranged in rows parallel to [100]. For masuyite and protasite, the stability of their edges is defined by large differences in the bond-valence deficiencies of the chains of polyhedra with left and right terminations. The stabilities of edges in curite, Pb3[(UO2)8O8(OH)6](H2O)3, are dictated by the low bond-valence deficiencies of the chains of polyhedra parallel to [001] and [011], and by the arrangement of cations in rows parallel to [001]. Minerals of the carnotite group contain the structural unit [(UO2)2(V2O8)] 2‐ , and the stability of the edges of their sheets is determined by the low bond-valence deficiency of chains of polyhedra parallel to [010 and [110]. Minerals of the uranophane group are based on the [(UO2)SiO3(OH)] ‐ sheet, and the stabilities of their edges are more strongly affected by the bond-valence deficiency of the chains of polyhedra than by the arrangement of the interstitial cations, and by the shift in the sheets. All predictions are compared with the corresponding morphologies of crystals recorded by Atomic Force Microscopy (AFM), on images of crystals, and on crystal drawings from the (mainly older) mineralogical literature. All predictions are in good agreement with the observed morphology of the basal faces.