Epidote
Epidote is a sorosilicate mineral with the general formula Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH). It is a common rock-forming mineral, occurring in a wide variety of metamorphic and igneous rocks.
Properties:
Epidote is typically green, ranging from pistachio green to yellowish-green and brownish-green. The color is due to the iron content. It can also be colorless, grey, brown, or black. It has a vitreous (glassy) luster and a hardness of 6 to 7 on the Mohs scale. Its specific gravity ranges from 3.2 to 3.5. Epidote crystals are commonly prismatic, often elongated and striated. It exhibits perfect cleavage in one direction and uneven fracture. It is pleochroic, meaning it displays different colors when viewed from different angles with polarized light.
Occurrence:
Epidote is found in metamorphic rocks, particularly those that have undergone regional or contact metamorphism. It is often associated with altered igneous rocks and hydrothermal veins. Common occurrences include:
- Regional Metamorphic Rocks: Formed during regional metamorphism of calcium-rich sediments or igneous rocks. It’s frequently found in schists and gneisses.
- Contact Metamorphic Rocks: Formed at the contact between an igneous intrusion and surrounding rocks. It can be present in skarns.
- Hydrothermal Veins: Deposited from hot, aqueous fluids circulating through rocks.
- Altered Igneous Rocks: Formed as a secondary mineral during the alteration of plagioclase feldspar in igneous rocks such as basalts and andesites.
Uses:
Epidote has limited industrial uses. It is sometimes used as a minor gemstone when of sufficient clarity and color. Its primary significance is as a petrologic indicator mineral, providing valuable information about the conditions under which rocks formed or were altered. Its presence and composition can help geologists understand the temperature, pressure, and fluid composition involved in metamorphic and hydrothermal processes.
Varieties:
Several varieties of epidote exist, based on chemical composition or physical characteristics:
- Pistacite: An iron-rich variety of epidote, characterized by its distinctive pistachio green color.
- Clinozoisite: An aluminum-rich variety of epidote, with a lower iron content. Clinozoisite is often lighter in color than typical epidote.
- Allanite: A cerium-bearing variety of epidote, often black or dark brown in color and weakly radioactive.
- Tawmawite: A chromium-bearing variety of epidote, exhibiting a deep green color.