Epidote in Quartz

Mineral aggregate

Ca2(Al,Fe)3(SiO4)3(OH) in SiO2

Also known as: Epidote Quartz, Green Epidote Quartz

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Description

Epidote in Quartz refers to a mineral aggregate where crystals or masses of epidote are embedded within a quartz matrix. Epidote is a sorosilicate mineral, typically green to yellowish-green, often forming prismatic crystals. Quartz, a tectosilicate, is generally colorless, white, or translucent, providing a contrasting background for the epidote inclusions. The aesthetic appeal of this material comes from the vibrant green epidote contrasting with the often clear or milky quartz.

How to Identify

Color: The epidote inclusions are typically pistachio-green, yellowish-green, or dark green. The quartz matrix can be colorless, white, milky, or smoky.
Luster: Epidote exhibits a vitreous to resinous luster. Quartz has a vitreous luster.
Texture: The texture is typically granular to crystalline, with distinct epidote crystals or aggregates embedded within the quartz. The quartz itself can be massive or crystalline.
Crystal Form: Epidote often forms prismatic, elongated crystals, sometimes acicular, or granular aggregates. Quartz forms hexagonal prisms with pyramidal terminations, or massive anhedral grains.
Cleavage: Epidote has one perfect cleavage parallel to {001}. Quartz has no true cleavage but exhibits conchoidal fracture.
Geological Environment: Found in regionally metamorphosed rocks (greenschist to amphibolite facies), contact metamorphic skarns, and hydrothermally altered igneous rocks, particularly in veins and replacement bodies.

Key Facts

Hardness: Epidote: 6-7 (Mohs); Quartz: 7 (Mohs). The aggregate will scratch glass.
Specific Gravity: Epidote: 3.3-3.5; Quartz: 2.65. The overall specific gravity will be an average depending on the proportions.
Crystal System: Epidote: Monoclinic; Quartz: Trigonal.
Color: Green (epidote) in colorless/white/milky (quartz).
Luster: Vitreous to resinous (epidote), Vitreous (quartz).
Transparency: Epidote: Translucent to opaque; Quartz: Transparent to translucent.
Fracture: Epidote: Uneven to conchoidal; Quartz: Conchoidal.
Cleavage: Epidote: Perfect on {001}; Quartz: None.
Composition: Epidote: Calcium aluminum iron sorosilicate hydroxide (Ca2(Al,Fe)3(SiO4)3(OH)); Quartz: Silicon dioxide (SiO2).

Quick Check

Color: Pistachio-green to dark green epidote inclusions in colorless, white, or milky quartz.
Luster: Vitreous to resinous for epidote, vitreous for quartz.
Streak: White to grayish-white for epidote; quartz has no streak.

Physical Characteristics

Crystal Habit: Epidote: Prismatic, acicular, granular, massive. Quartz: Prismatic, massive, granular.
Cleavage Type: Epidote: Perfect, one direction; Quartz: Absent.
Fracture Type: Epidote: Uneven to conchoidal; Quartz: Conchoidal.
Tenacity: Epidote: Brittle; Quartz: Brittle.
Luster Type: Vitreous to resinous (epidote), Vitreous (quartz).

Formation

Epidote in Quartz forms primarily through hydrothermal alteration processes, often associated with regional or contact metamorphism of mafic igneous rocks (like basalt, gabbro) or calcareous sedimentary rocks. During these processes, calcium-rich plagioclase feldspars, pyroxenes, and amphiboles are altered to epidote. The epidote crystals then grow within or are encapsulated by concurrently forming or later-stage quartz veins or masses. The iron content in epidote (Fe3+) is responsible for its characteristic green color. The presence of quartz indicates silica-rich fluids were involved in the formation.

Usage

Primarily used as an ornamental stone, for lapidary purposes (cabochons, spheres, carvings), and in jewelry. It is also collected by mineral enthusiasts. It has no significant industrial uses beyond its aesthetic appeal.

Age Distribution

Varies widely depending on the geological setting of formation, ranging from Precambrian to Cenozoic.

Where to Find

Austria

Significant deposits, particularly in the Knappenwand area, Hohe Tauern, known for well-formed epidote crystals in quartz.

Norway

Various localities produce epidote in quartz, often associated with metamorphic terrains.

United States

California (e.g., Sierra Nevada), Alaska, and other western states have occurrences in metamorphic and hydrothermally altered rocks.

Mexico

Known for some occurrences, often associated with skarn deposits.

Pakistan

Certain regions yield attractive specimens, particularly in pegmatitic and metamorphic environments.

Finding Tips

Geological Context

Look for areas with evidence of regional or contact metamorphism, especially where mafic igneous rocks or calcareous sediments have been altered. Skarn deposits are also prime locations.

Vein Systems

Epidote in quartz often forms in hydrothermal veins. Examine quartz veins cutting through host rocks for green inclusions.

Associated Minerals

Epidote is commonly associated with actinolite, tremolite, garnet, calcite, and various feldspars. The presence of these minerals can indicate a favorable environment.

Color and Hardness

The distinctive green color of epidote against the harder quartz matrix is a key visual identifier. Test hardness to confirm quartz (7 on Mohs) and epidote (6-7 on Mohs).

Similar Rocks

Prasiolite

SiO2 (green variety)

Also known as: Green Amethyst

Aventurine

SiO2 with Fuchsite or other mica inclusions

Also known as: Green Aventurine

Serpentine

Mg3Si2O5(OH)4

Also known as: Serpentinite

Scientific Classification

Mineral Class: Sorosilicate (Epidote), Tectosilicate (Quartz)
Group: Epidote Group (Epidote), Quartz Group (Quartz)
Crystal System: Monoclinic (Epidote), Trigonal (Quartz)
Chemical Formula: Ca2(Al,Fe)3(SiO4)3(OH) (Epidote) in SiO2 (Quartz)
Composition: Calcium, Aluminum, Iron, Silicon, Oxygen, Hydrogen (Epidote); Silicon, Oxygen (Quartz)

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