Monday, March 20, 2017

Epidote (the coating form)

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.

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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