Nepheline and Leucite are called feldspathoids because they have chemical compositions akin to the feldspars but form in silica poor rocks where there is insufficient silica to form feldspars (and of course, they never occur with quartz). Chabazite (ideally (Ca,Na2,K2,Mg)Al2Si4O12·6H2O) might be called a "scapolitoid" since, like scapolite, it has a feldspar-like composition with additional anions.
The fundamental structure of chabazite is built of twelve-membered double rings of silica tetrahedra as shown below.
Below is a view of connected silica units.
Below, a view perpendicular to the rings.
The view below shows silica units in purple with water molecules (blue) and other cations (orange).
View parallel to the silica units. There may or may not be a cation at the centers of the silica units. Water, other cations, and other anions occupy the large voids between the silica units. This open structure leads to chabazite being called a "molecular seive." A wide vaiety of constituents in various arrangements may occur in the voids (schematically indicated at lower right)
Created 22 April 2013, Last Update 05 Jun 2013
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