The structural change is
14 Finch & Hanchar
accomplished through rotations of ThO4
e
and SiO4 tetrahedra and formation of an additional bond
between an O atom of the fifth SiO4 tetrahedron and the Th site. The transformation of thorite to
huttonite results in a structure with greater overall connectivity than zircon, which, as noted,
consists of two independent arrays of Si and Zr tetrahedra. These arrays both exist in huttonite, but
they are not independent, being connected through the additional Th-O-Si linkage. The structure
collapses slightly due to the rotation of polyhedra and the additional Th-O bond. The transformation
of thorite to huttonite is sluggish, presumably due to formation of an additional Th-O bond.
Details of the transformation remain unclear, as no high-temperature structural studies of the
thorite-huttonite transformation have, to our knowledge, been reported. The fact that a structural
transformation similar to the thorite-huttonite transformation is not observed in zircon before
decomposition may be due to the greater tetrahedral rotation required to increase the coordination
number around the small Zr4+ cation