Ever since I started collecting minerals, I noticed green
splashes of a material coating various pieces of granite and other various rocks. I learned later that the coating consists of the common mineral epidote (pronounced “ep-i-dote”).
Epidote is a calcium, aluminum, iron, hydroxyl-silicate
mineral typically found in metamorphic-rock areas where alteration or
replacement took place in association with hydrothermal fluids. Epidote is especially common in
fractures or joints. Fibrous crystals of epidote can be dark-green, black,
or even yellow.
The epidote I find, however, has a very distinctive
pistachio or pea-green color. It occurs primarily as surface coatings on cobbles and
boulders of biotite-rich granite, which weather out from sedimentary rock
conglomerates, as shown below. The name “epidote” is derived from a Greek word
meaning “increase,” in reference to its crystalline shape.
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| Epidote coating a clast (maximum dimension 5 cm) of granodiorite found on a hiking trail in the Santa Clarita area, Los Angeles County, Southern California. |
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