Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Epidote is easily the most common mineral with paired silica tetrahedra. In the diagram below, single tetrahedra are dark blue and the slightly kinked pair of tetrahedra are light blue. The silica tetrahedra link chains of aluminum octahedra. Corners of the octahedra not joined to silica tetrahedra are occupied by hydroxyl ions.
Ferrous iron and aluminum atoms occupy distorted octahedra adjoining the aluminum chains. These atoms are shown in orange, with some of the octahedra indicated. Calcium atoms (green) fill larger voids, surrounded irregularly by eight oxygen atoms. Unit cell edges are purple.
Note that there are continuous chains of aluminum octahedra and silica groups joined only by iron and calcium ions. These planes of weakness account for epidote's single perfect cleavage.
The diagram above shows octahedra and tetrahedra in their true symmetrical orientations, but at the price of many oxygen atoms coinciding. The symmetry of epidote is 2/m. Alternate chains of aluminum octahedra and silica units differ in elevation by half a unit cell, thus the space group symmetry is P21/m. The elevation difference accounts for some calcium atoms being in front of the paired tetrahedra, others behind.
In the diagram below, the octahedra and tetrahedra are slightly skewed to reveal the superimposed oxygens. This view also indicates how successive layers of the structure are connected.
Created 26 February, 2001, Last Update 31 May 2020