THE ATOMIC ARRANGEMENT OF IIMORIITE-(Y), Y-2(SIO4)(CO3)

Iimoriite-(Y) from Bokan Mountain, Prince of Wales island, Alaska has been studied using single-crystal X-ray- diffraction techniques. The m ineral, ideally Y-2(SiO4)(CO3), crystallizes in space group <P(1)over bar>, with a 6.5495(13), b 6.6291(14), c 6.4395(11) Angstrom, alpha 11 6.364(15), beta 92.556(15) and gamma 95.506(17)degrees. The atomic arr angement has been solved and refined to an R value of 0.019. The arran gement of atoms consists of alternating (01(1) over bar) slabs of orth osilicate groups and carbonate groups, with no sharing of oxygen atoms between anionic complexes in adjacent slabs. Y1 atoms separate adjace nt tetrahedra along [100] within the orthosilicate slab, and Y2 atoms separate adjacent carbonate groups along [100] within the carbonate sl ab: Adjacent orthosilicate and carbonate slabs are linked in (100) by bonding Y atoms from each slab to oxygen atoms of adjacent slabs, in t he Form of YOB polyhedra. The Y1 atoms exist in Y1(2)O(14) dimers in t he orthosilicate slab, and the Y2 atoms exist in continuous [011] ribb ons of edge-sharing Y2O(8) polyhedra in the carbonate slab.