1. } Tungstates, Molybdates and Chromates
The XO42- anionic complexes are built around Cr6+, Mo6+ and W6+,
all of which are in Group VIb of the Periodic Table. The ionic
radii of Mo6+ and W6+ are identical at 0.62Å and that of Cr6+ is
similar at 0.52Å
Two isostructural groups exist:
Wolframite (monoclinic) Scheelite (tetragonal)
Ferberite FeWO4 Scheelite CaWO4
Huebnerite MnWO4 Powellite CaMO4
Stoltzite PbWO4
Wulfenite PbMO4
WO4 is actually in distorted CaO8 or PbO8 + MoO4 or
Octahedral coordination as WO4 with less distortion
There are two next nearest so symmetry is higher.
Neighbour oxygens. Thus,
WO4+2 }
Wolframite
series

2. The only chromate is crocoite PbCrO4 and there only nine molybdates and tungstates. In all of these, the tetrahedral XO4 complexes are flattened with respect to a regular tetrahedron.

3. wolframite

4. wolframite
Distorted WO4 tetrahedra linked to Fe or Mn octahedra

5. scheelite

6. wulfenite

7. crocroite