The crystal chemistry of the uranyl silicate minerals

The uranyl silicate minerals have been divided into three groups on the basis of their uranium to silicon ratios. The 1:1 group includes uranophane, beta-uranophane, boltwoodite, sodium boltwoodite, kasolite, sklodowskite, and cuprosklodowskite. A structure refinement of uranophane, a structure determination of boltwoodite, and previously reported structure determinations of most of these minerals indicate that they are composed of uranyl silicate chains made of edge-shared uranium pentagonal bipyramidal groups and silicate tetrahedra. These chains have the composition $[(UO2)(SiO4)]n−2n$ and are crosslinked by a bridging oxygen atom to form a uranyl silicate sheet. These sheets are crossbonded by the additional cations in the structure. The uranyl minerals with a uranium to silicon ratio of 1:3 include week-site and haiweeite. A partial structure analysis of weeksite suggests that the structure type for this group consists of uranyl silicate chains, similar to those found in the 1:1 group, that are crosslinked by the additional silicate tetrahedra in the structure. The uranyl mineral group with a uranium to silicon ratio of 2:1 contains only the mineral soddyite. This structure is composed of uranyl silicate chains that are crossbonded by sharing a common silicon to give a three-dimensional framework structure. A new triclinic uranyl silicate mineral was discovered during this study, although there is not enough sample to describe it adequately. The locations of the uranium atoms in this structure indicate that it may not be composed of uranyl silicate chains such as those found in all the other uranyl silicate minerals.