Finch, Hawthorne, and Ewing reported in 1998 that schoepite, [(UO2)8O2(OH)12](H2O)12, transforms slowly in air at ambient temperature to metaschoepite, UO3*nH2O (n=2), and crystals commonly contain an intergrowth of both minerals. The transformation may be due to the loss of one-sixth of the interlayer H2O groups in schoepite, and a possible structural formula for metaschoepite is [(UO2)8O2(OH)12](H2O)10. The transformation of schoepite (a 14.337, b 16.813, c 14.731 A, P21ca) to metaschoepite (a 13.99, b 16.72, c 14.73 A, Pbna) is characterized by a 2% decrease 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 reflect 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 schoepite" [a 6.86, b 4.26, c 10.20 A, Abcm (?)l when partly altered crystals are exposed to an external stress (e.g., heat, sunlight or mechanical pressure).
Metaschoepite is apparently stable in air; canary yellow altered crystals commonly consist of a polycrystalline mixture of "dehydrated schoepite" and
metaschoepite [Related to Finchite?]. 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 configuration that may be similar to that of metaschoepite, and
3) further rearrangement to a defect a-UO2(OH)2-type sheet. The complete reaction is [(UO2)8O2(OH)12 ⇒ 8[(UO2)O0.25(OH)1.5]+12H2O.
The authors proposed that "dehydrated schoepite" forms an omission solid solution over the compositional range UO3*0.75H2O to UO3*H2O, represented by the general formula (UO2)O0.25 -x(OH)1.5+2x (0≤x≤0.25).
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