Apatite (Ca5(PO4)3(OH,F,Cl)) is the most abundant phosphate mineral, a common reservoir for phosphorus and fluorine in rocks. Like many minerals with complex anions, the geometry is dominated by the complex anions and the cations are stuffed into the remaining spaces in odd-shaped polyhedra. With these minerals, the coordination polyhedron approach can actually make it harder to visualize the structure rather than easier. The most common apatite variety is fluorapatite.
Below is the fluorapatite structure with PO4 tetrahedra in green, fluorine in purple and calcium in blue and yellow. The yellow calcium atoms are coordinated to the oxygen atoms in the phosphate tetrahedra and lie at altitudes 0, 1/2 and 1 in the unit cell. In this case, the plane of the figure is at elevation 1 and the opposite unit cell face is at 0. All other atoms and the phosphate ions are centered at elevation 1/4 and 3/4, with light colors representing 1/4.
Shown below are the coordination poyhedra for the yellow calcium atoms. The coordination is ninefold, with three phosphate groups sharing a single apical oxygen atoms, and three others sharing two (edge sharing).
The other kind of coordination polyhedra. These share a fluorine atom.
The two kinds of calcium coordination polyhedra. The yellow variety has nine-fold coordination and is stacked in columns sharing the top and bottom triangular faces. The blue variety is an elongated pentagonal dipyramid. These form three-fold rosettes around the fluorine atoms, and the rosettes stack in columns. the principal value of irregular coordination polyhedra like these is enabling us to visualize how the cations are confined by surrounding atoms.
A side view of the yellow coordination polyhedra. Mirror image polyhedra, shown as yellow and gold, alternate in vertical columns.
Below is the same view, but showing only the calcium coordination polyhedra at top.
A side view of the pentagonal dipyramids, which cluster around central fluorines and join vertices to the rosettes above and below.
Same view as above, but with the phosphates removed at top to show only the dipyramids.
A side view showing both kinds of polyhedra.
Same view as above, but with phosphates removed on top to display only the Ca polyhedra.
Created 17 December, 1999, Last Update 31 Aug 2011
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