Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 52

Fig. 15 Most typical
coordination types of
tetrahedral oxoanions

3.3.1 Orthosilicates and Orthogermanates

g ¼

1:1. pcu-b and nia topological types, being quite common for most classes of
oxoacid salts, have not been found in orthosilicates or orthogermanates; the higher-
coordinated underlying nets are more typical for them. The crystal structure of
ZrSiO 4 (zircon) illustrates a rare case of contradictions in the topology of the
underlying net and cation array [ 16 ]. O'Keeffe and Hyde [ 4 ] found the topological
similarity between the ZrSi cation array and the 10-coordinated array of metal
atoms in rutile or -Hg. However, the orthosilicate groups are not condensed in

zircon, and the contacts Si-Si existing in the array disappear in the underlying net
that has the 6-coordinated uninodal msw topology (Fig. 16a ; Table 9 ). The under-
lying net in ThSiO 4 (huttonite) is 7-coordinated bcu-7- P 2 1 / c (Fig. 16b ); it occurs in
monoclinic RhSi [ 64 ] and in the monazite-type orthophosphates (see below), while
the orthosilicates and orthogermanates of the scheelite type contain topologically
dense 8-coordinated lsz underlying net (Fig. 16c ).
g ¼

b

-Ca 2 SiO 4 ) structure type was related to PbCl 2
(cotunnite) or Ni 2 In-type cation arrays. In a later review [ 16 ], it was shown that the
Ca 2 Si trinodal underlying net in olivine has the same 4,5,9-coordination as the
cotunnite net, but starting from the second coordination shell the coordination
sequences are different. This is a good illustration of the advantages of the computer
approach: in most cases, it is impossible to recognize the differences in outer
coordination shells by visual analysis. Instead of olivine, the

2:1. In [ 4 , 7 ] , the olivine ( g

b

-Ca 2 SiO 4 (larnite)

structure type has the Ni 2 In-type underlying net [ 16 ] (Fig. 6 ) .