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the tetrahedral coordination. Once more, the cation subarray in an oxide behaves as
a Zintl polyanion.
4.2 BCT-Type Zeolites
work type BCT. Its subarray of aluminium and phosphorus atoms forms a three-
dimensional framework in which each Al atom is connected to four P atoms and
Fig. 16 (a) Tetrahedral skeleton in metavariscite AlPO
4
·2H
2
O[
72
]. [AlO
4
] and [PO
4
] tetrahedra
are depicted as
unshaded
and
shaded tetrahedra
, respectively. [TO
4
] tetrahedra share all their
corners with adjacent tetrahedra and give rise to a T substructure similar to that of the II-VI
compound
b
