The scheelite and fergusonite structures

The scheelite structure is named after the mineral with the composition CaW04. Examples of compounds with this structure include  

A different deformed version of the scheelite structure is found for KCrOaCl. One of several structures found for molybdates and tungstates of trivalent metals is the defect scheelite structure, adopted by Eu2(W04)3 and one polymorph of Nd2(Mo04)3. In this structure M ions occupy two-thirds of the sites in the scheelite structure. 

Three structures of this type were described in our discussion of the closest packing of equal spheres, namely, those in which certain proportions of the tetrahedral and/or octahedral holes are occupied  

Phenacite and olivine are orthosilicates, and since the present group includes all compounds B2(A04) containing tetrahedral AO4 ions and ions B of various sizes it is a very large one and there are numerous structures, of which a selection is included in Table 13.9. For example, the alkali-metal tungstates have four different crystal structures  

U2WO4 (phenacite), Na2W04 (modified spinel), K2WO4 (and the isostructural Rb2W04), and CS2WO4 ((3-K2SO4 structure), with respectively 4-, 6-, 8-, and (9 and 10)-coordination of M ions. 

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