Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Apart from being the major ore of tungsten and famous for its fluorescence, scheelite ( CaWO4) is of interest bevcause of its interesting coordination. Scheelite consists of WO4 tetrahedra surrounding Ca atoms with eight-fold coordination. Some references describe the Ca coordination polyhedron as a square antiprism, and it is approximately that shape.
A square antiprism consists of two parallel square faces at 45 degrees to each other, with a girdle of triangles separating them. The figure above shows a regular square antiprism. The girdle of triangles is yellow, exposed square faces are blue and hidden square faces are green as if seen through the yellow triangles. Exposed edges are black and hidden edges are gray.
The Ca coordination polyhedron in scheelite is fairly similar in shape except the square faces aren't entirely regular and they are slightly bent along one diagonal. The figure above shows a Ca coordination polyhedron in scheelite.
In the figure above, a central Ca-O unit (red) is shown coordinated to four other units (yellow) and six W-O tetrahedra (blue). In this view it is obvious that the W-O tetrahedra are linked only at their vertices.
Another view of the coordination of a Ca-O unit. In this view it looks like the tetrahedra share edges but that is only a perspective illusion.
Ca-O units link to make oval six-membered rings. Two W-O tetrahedra link across the opening. Again, note that the tetrahedra are vertex-linked only.
A view down the c-axis (4-fold axis). At lower right some W-O tetrahedra are shown.
A view perpendicular to the 4-fold axis, which is horizontal in this view. W-O tetrahedra are shown at right.
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Created 22 April 2013, Last Update