Epidote is a structurally complex mineral having both
single silicate tetrahedrons, SiO4, and double silicate
tetrahedrons, Si2O7. The formula of epidote could be
expressed in a such a way so as to reflect this organization;
Ca2(Al, Fe)Al2O(SiO4)(Si2O7)(OH). The two aluminums
represent the parallel chains of AlO6 and AlO4(OH)2
octahedra that are the heart of the epidote structure.
The silicate groups and extra ions connect the chains
together. Since the chains are parallel, the crystals
tend to be prismatic. The chains are arranged in parallel
planes and the perfect cleavage breaks the bonds between
these planes. Epidote, while not general known for exceptional
crystals can produce some magnificent specimens. Its
unique green color which is often described as "pistachio",
is quite striking in well colored specimens.