Epidote: Properties and Occurrences

Epidote is a calcium aluminum iron sorosilicate mineral. The color is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green. It is an abundant rock-forming mineral, but one of secondary origin. It is a lustrous yellow-green crystalline mineral, common in metamorphic rocks.

The perfectly transparent, dark green crystals from the Knappenwand and from Brazil have occasionally been cut as gemstones.

General Information

  • Category: Sorosilicates
  • Formula:{Ca2}{Al2Fe3+}[O|OH|SiO4|Si2O7]
  • Crystal system: Monoclinic
  • Crystal class: Prismatic (2m) (same H-M symbol)

Properties

Clinozoisite is green, white or pale rose-red group species containing very little iron, thus having the same chemical composition as the orthorhombic mineral zoisite. It is a common mineral, occurring in prismatic crystals belonging to the monoclinic system, also massive. It displays strong pleochroism, the pleochroic colors being usually green, yellow and brown.

  • Color: Pistachio-green, yellow-green, greenish black
  • Crystal habit: Prismatic with striations, fibrous, massive
  • Fracture: Flat regular to uneven
  • Mohs scale hardness: 6–7
  • Luste: Vitreous to resinous
  • Streak: Greyish white
  • Diaphaneity: Transparent to nearly opaque
  • Specific gravity: 3.3–3.6

Occurrence

It occurs in marble and schistose rocks of metamorphic origin. It is also a product of hydrothermal alteration of various minerals (feldspars, micas, pyroxenes, amphiboles, garnets, and others) composing igneous rocks. A rock composed of quartz and epidote is known as epidosite.

Well-developed crystals are found at many localities: Knappenwand, near the Großvenediger in the Untersulzbachthal in Salzburg, as magnificent, dark green crystals of long prismatic habit in cavities in epidote schist, with asbestos, adularia, calcite, and apatite.

Epidote has no significant use as an industrial mineral and has only minor use as a gemstone. High-quality transparent crystals are sometimes cut into faceted stones.

 

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