It is not an uncommon rock in certain conditions. I have written about massive pyrite and umber, which are characteristic rocks of complete ophiolites (like the Troodos Ophiolite in Cyprus). These rocks are enriched in metals (copper, zinc, manganese). Epidosite, on the other hand, is very much depleted in these elements, although they are present in mafic dikes (parent rock of epidosite).
A sample from the Løkken ophiolite in Norway. Width of sample 12 cm.
It seems logical to assume that seawater that circulates in the oceanic crust near the spreading zones heats up and alters the dikes by turning them into epidosite. By doing that it carries away not only metals but also sodium and magnesium and leaves behind rocks enriched in calcium and silicon. Hence, epidosite is an important piece of the puzzle, which helps us understand where this material is coming from that is carried to the seafloor by the hydrothermal vents called black smokers.
Sheeted dikes in Cyprus, which were sometimes more or less greenish in color. It is mineral epidote that gives them this bright green tone.
Some dikes are almost apple-green, which indicates that they are composed mostly of epidote and quartz. Original mafic minerals are completely replaced.
Weathered slope of an epidositic roadcut.
A sample from Cyprus. Width of sample 10 cm.