Abstract: A number of minerals and compounds of the torbernite group are represented by the formula A(UO2)(As,P)O4·3H2O where A = K+, NH4+, and/or H3O+. Crystal-structure analyses of K(UO2AsO4)·3H2O, NH4(UO2AsO4)·3H2O, and K0.45(H3O)0.55(UO2AsO4)·3H2O recently completed by Ross and Evans (1964) reveal the exact nature of their atomic arrangements. All three compounds have interlayer structures formed by hydrogen bonding of water molecules into infinite sheets composed of four- and eight-membered rings, isostructural with the [Si8O20]n8n− layers of apophyllite. The interlayer K+, NH4+, and H3O+ ions, instead of entering in between the [UO2AsO4]nn− layers co-ordinated by a hydration sphere of water molecules, are randomly distributed over the water molecule sites. The formula of the interlayer structure is written [(H2O)3A]nn+.
A similar structural scheme may apply to the expanding layer silicates, such as the montmorillonites and vermiculites. For these minerals, particularly those with low to moderate cation-exchange capacity, the potassium and ammonium analogs usually do not fully contract to the 10 Å basal spacing on immersion in K+ or NH4+ solutions, but rather obtain a spacing of 11–14 Å. In these structures it is proposed that a single or double layer of water molecules, arranged in a manner related to one of the silica or ice polymorphs, enters between the 2:1 layers, the charge being balanced by random distribution of K+, NH4+, and perhaps H3O+ ions over the H2O sites. The formula of the interlayer structure is written [(HaO)b−yAy]nny+ where A = K+, NH4+, and/or H3O+ and ny+ is the charge required to balance that on the 2:1 layer. Other Group IA or Group IIA cations may also be randomly distributed over water-molecule sites.