Heulandite is a zeolite with formula (Ca,Na)2-3 Al3(Al,Si)2Si13O36· 12H2O. Like many other zeolites, it is now considered a series rather than a single mineral, with the name modified by the dominant cation. A randomly oriented view of the Heulandite structure looks like a chaotic jumble of silica tetrahedra impossible to visualize. There is a way to make sense of it, but even so, it's not an easy structure to see. Brewsterite (Sr,Ba,Ca)[Al2Si6O16]·5H2O is isostructural.
The structure of heulandite can be described as strips of pentagonal rings of silica tetrahedra, as shown above. There are two sets, mirror images of each other.
The strips overlap each other and link to form thick sheets. In the view above, the foreground strips are purple and the background strips are light green. The strips are parallel to the c-axis and the layers are in the a-c plane.
Above is a view looking along the c-axis. The strips are not flat but arched. Outward pointing tetrahedra in each layer link to similar tetrahedra in adjoining layers, enclosing tunnels. In this figure, water molecules are blue and calcium atoms orange. As is typical for zeolites, a wide variety of ionic substitution and water content is possible.
Above, a view down the b-axis, showing silica strips, water molcules, and calcium atoms.
Created 22 April 2013, Last Update 26 Jun 2013
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