Erionite is a zeolite mineral, a common alteration product produced by weathering of felsic volcanic rocks and tuff. It is inconspicuous because it tends to occur as tiny crystals or fine, fragile needles. It is nevertheless significant because (1) it is a carcinogen, although the mechanism responsible is unknown, and (2) large areas of the High Plains are covered with erionite-bearing formations.
The silicate framework of erionite consists of alternating single and double rings of tetrahedra. Below is a view perpendicular to the ring planes. Single rings are blue and double rings are pink.
This view shows the rings from the side. Since the rings are interconnected, erionite is a tectosilicate. Also note the eight-membered rings perpendicular to the six-membered rings.
This view shows two layers of rings, with the rear rings lighter in color.
Running down the holes formed by the rings are potassium (yellow) and calcium (green) atoms.
The Ca atoms have a complex coordination with oxygen atoms and water molecules. Below is a diagram showing the coordination polyhedra for four calcium atoms. The polyhedra form a dumbbell shape.
Below is the coordination of calcium atoms (green) between two of the single hexagonal rings. On the left is the dumbbell chain of coordination polyhedra for four calcium atoms as shown above. Also shown are the remaining calcium atoms for which coordination polyhedra are hard to define. Above and below the central dumbbell are two more calcium atoms, each confined by the dumbbell and the tetrahedral ring. Around the top and bottom of the dumbbell are sets of three more calcium atoms (only two visible).
The middle diagram shows water molecules (blue). In addition to those coordinated to calcium atoms, there are three others (light blue, only two visible) that are coordinated to the double tetrahedral rings.
On the right, we see the coordination of the outlying calcium atoms, which are located within a rhombus of water molecules and more distantly coordinated with oxygen atoms in the tetrahedral rings.
Here we see a side view of the structure, showing potassium atoms (yellow) and two sets of calcium dumbbells (green)
Looking down the c-axis, showing potassium (yellow), calcium (green) and water molecules not attached to the dumbbells (dark blue).
Created 18 April, 2011, Last Update 19 April, 2011
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