STRUCTURAL RELATIONS AMONG SCHOEPITE, METASCHOEPITE AND DEHYDRATED SCHOEPITE

Schoepite, [(UO2)(8)O-2(OH)(12)](H2O)(12), transforms slowly in air at ambient temperature to metaschoepite, UO3. nH(2)O (n congruent to 2), and crystals commonly contain an intergrowth of both minerals. The tr ansformation may be due to the loss of one-sixth of the interlayer H2O groups in schoepite, and a possible structural formula for metaschoep ite is [(UO2)(8)O-2(OH)(12)](H2O)(10). The transformation of schoepite (a 14.337, b 16.813, c 14.731 Angstrom, P2(1)ca) to metaschoepite (a 13.99, b 16.72, c 14.73 Angstrom, Pbna) is characterized by a 2% decre ase in the a cell dimension, a slight decrease in the b dimension, and little or no change in the c dimension. Unit-cell changes probably re flect the reorganization of H-bonds. Differences in unit-cell volumes induce strain in crystals in which the transformation to metaschoepite is incomplete, and stored strain energy may be sufficient to rapidly drive the transformation of the remaining schoepite to ''dehydrated sc hoepite'' [a 6.86, b 4.26, c 10.20 Angstrom, Abcm (?)] when partly alt ered crystals are exposed to an external stress (e.g., heat, sunlight or mechanical pressure). Metaschoepite is apparently stable in air; ca nary yellow altered crystals commonly consist of a polycrystalline mix ture of ''dehydrated schoepite'' and metaschoepite. The alteration of schoepite to ''dehydrated schoepite'' occurs in three steps: (1) loss of all interlayer H2O from schoepite, causing collapse of the layers, (2) atomic rearrangement within the structural sheets to a configurati on that may be similar to that of metaschoepite, and (3) further re-ar rangement to a defect alpha-UO2(OH)(2)-type sheet. The complete reacti on is [(UO2)(8)O-2(OH)(12)](H2O)(12) double right arrow 8 [(UO2)O-0.25 (OH)(1.5)] + 12H(2)O. We propose that ''dehydrated schoepite'' forms a n omission solid-solution over the compositional range UO3. 0.75H(2)O to UO3. H2O, represented by the general formula (UO2)O0.25-x(OH)(1.5 2x) (0 less than or equal to x less than or equal to 0.25).