The structure of boltwoodite and implications of solid solution toward sodium boltwoodite
The structure of boltwoodite and implications of solid solution toward sodium boltwoodite
The Canadian Mineralogist (August 1998) 36, Part 4: 1069-1075
- bonding
- crystal chemistry
- crystal structure
- crystal systems
- electron microscopy data
- lattice parameters
- least-squares analysis
- monoclinic system
- nesosilicates
- orthosilicates
- polyhedra
- refinement
- silicates
- solid solution
- space groups
- statistical analysis
- tetrahedra
- unit cell
- uranium minerals
- uranophane
- X-ray diffraction data
- boltwoodite
The structure of boltwoodite (space group P2 (sub 1) /m, a 7.0772, b 7.0597, c 6.6479 Aa, beta 104.982 degrees , V 320.9 Aa (super 3) , Z = 2) has been refined to R 3.2%). The structure determination, supported by EPMA, provided the new formula (K (sub 0.56) Na (sub 0.42) )[(UO (sub 2) )(SiO (sub 3) OH)](H (sub 2) O) (sub 1.5) . The structure contains silicate tetrahedra and uranyl pentagonal bipyramids that share edges and corners to form alpha -uranophane-type sheets (100) at x 1/4 and 3/4. There are four distinct partially occupied sites in the interlayer that correspond with one Na, and K and two H (sub 2) O groups. Local arrangements of the interlayer are non-centrosymmetric, although the long-range symmetry of the structure is consistent with space group P2 (sub 1) /m. The structure permits substantial variation of interlayer composition, which is significant for mineral stability. Boltwoodite may be a key phase in determining the future mobility of certain radionuclides in a geological repository containing spent nuclear fuel.
