Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
In contrast to andalusite and sillimanite, which have fairly straightforward structures of chains of aluminum octahedra linked by silica tetrahedra, the structure of kyanite is very complex. It consists of "staircases" of aluminum octahedra linked by silica tetrahedra, but the staircases are full of holes, making the structure extremely hard to visualize. Below is a diagram showing staircases end on. The staircases consist of zigzag chains of octahedra, separated by additional octahedra. In the diagram below, the zigzag chains are in yellow and light green, and the linking octahedra in blue. Aqua blue and yellow are in the plane of the diagram, and green and lavender are one level lower.
The diagram below is a face-on view of a single staircase. The zigzag strips are yellow and the linking octahedra are blue. The c axis runs along the continuous chains. The face is  as befits the 3:2 proportions of the steps. Note that the strips seen above are oblique to the staircase.
Below is a side view of the staircases, showing silica tetrahedra in purple.
Face on view of a staircase, showing silica tetrahedra in purple. Only tetrahedra on the facing side are shown. There are corresponding tetrahedra on the far side.
A face-on view of the staircase strips, as shown above, does not look along the staircase because the strips are oblique. This view looks down the c-axis, along the staircases. Aluminum octahedra are yellow and silica tetrahedra in purple. One unit cell is outlined in red.
Created 22 April 2011, Last Update 31 May 2020