Stilbite is a zeolite with composition NaCa4(Si27Al9)O72·28(H2O). Once regarded as a distinct mineral, now considered a series. The proportions of Ca and Na are variable, with charge balance maintained by varying the proportions of Si and Al, as in the plagioclase feldspars.
Above: the basic unit of the stilbite structure can be considered a group of eight silica tetrahedra. Each unit can be thought of as a pair of four-sided rings of silica tetrahedra with three tetrahedra in each ring joined and the remaining two not joined. Alternatively, it can be pictured as a six-sided ring with four of its tetrahedra linked by an additional pair. Units link to each other and also are linked diagonally by isolated tetrahedra. The view above shows two views of the unit. Tetrahedra in each unit are magenta and the isolated linking tetrahedra are violet. Background tetrahedra are lighter in tone.
View down the  direction showing how units link. They link diagonally via the isolated tetrahedra, and also laterally. The outward-pointing tetrahedra of one unit link to the cross-linking pair of its neighbor.
View down the  direction showing the well known "molecular seive" nature of the zeolites.
View down the  direction showing sodium atoms (orange) and the coordination polyhedra of calcium atoms surrounded by water (green).
Above: coordination of water molecules (oxygen atoms in blue) around a calcium atom (darker green). The coordination is nine-fold. The negative (oxygen) portion of the water molecules is attracted to the calcium ions and the positive hydrogens point away, so this structure can actually be considered a complex cation.
View down the 001 direction. Note that the tunnels in this direction are a bit too small for the Ca-water polyhedra (green) to slip through. Of course, the bonding between the Ca and water is weak and on heating, individual molecules can slip through easily.
Created 22 April 2013, Last Update 16 Jun 2013
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