Crystal Chemistry of Alkali Thorium Silicates Under Hydrothermal Conditions

An
investigation into the crystal chemistry of the alkali thorium silicates
resulted in the formation of five novel materials by hydrothermal
synthesis. Of these, Rb4Th2Si12O30, and K2ThSi4O10F2 are new structure types crystallizing in space groups P1̅ and Pnma, respectively. The structure of
Rb4Th2Si12O30 reveals
a silicate sheet structure with eight-, six-, five-, and four-membered
silicate rings. Crystals of a related composition, Cs2ThSi6O15, were also synthesized. They crystallize in
space group Cmc21 and have a silicate
sheet containing eight-, six-, and four-membered silicate rings. The
structure of K2ThSi4O10F2 consists of a tubular silicate column comprised of eight-, six-,
and four-membered silicate rings and features chains of fluorine-bridged
Th atoms. Two new wadeite analogues Cs2ThSi3O9 and Rb2ThSi3O9 were
also discovered and constitute the first actinide representatives
in this mineral family. All products were crystallized from reactions
of ThO2 and silica with alkali fluorides or hydroxides
in hydrothermal fluids at 575 °C. They were all characterized
by single crystal X-ray diffraction and infrared spectroscopy, and
detailed structural comparisons are provided. The work significantly
expands the field of synthetic thorium silicates.