Crocoite (PbCrO4) and monazite (Rare Earths-PO4 ) are radically different in appearance and occurrence, but are isostructural. Crocoite is famous for its brilliant orange color and requires the rare juxtaposition of lead mineralization and ultramafic rocks (to supply chromium). Monazite is a brown pegmatite mineral. Both have tetrahedra containing chromium or phosphorus, with the other cations in asymmetrical nine-fold coordination.
The 9-fold coordination polyhedra can be visualized in two ways. At top is a pentagonal ring of oxygen atoms with an additional oxygen on top forming a pyramid, and a basal triangle of additional oxygens. At bottom, we can also picture a pentagonal ring of oxygens with skewed pairs of oxygen atoms above and below. It all depends which set of atoms one chooses to consider the pentagonal ring.
Above are views of the 9-fold coordination polyhedron from a variety of angles.
Above we see a central 9-fold coordination polyhedron (yellow) coordinated with others (blue) and tetrahedra (green). The coordination is far from simple. 9-fold coordination polyhedra share edges with neighbors and tetrahedra share opposing edges with 9-fold coordination polyhedra.
View down the a-axis. 9-fold coordinated cations are shown individually ate left. Foreground atoms are darker.
View down the c-axis. At left are tetrahedra and 9-fold cations, in the center are tetrahedra and 9-fold coordination polyhedra, and at right is the next lower layer of polyhedra.
Created 10 Oct 1997, Last Update 16 December 1999
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