} Tungstates, Molybdates and Chromates

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Presentation transcript:

} 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

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.

wolframite

wolframite Distorted WO4 tetrahedra linked to Fe or Mn octahedra

scheelite

wulfenite

crocroite