Quartz with Epidote

Mineral assemblage

SiO2 (Quartz) with Ca2(Al,Fe)3(SiO4)3(OH) (Epidote)

Also known as: Epidote in Quartz, Epidote Quartz

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Description

Quartz with Epidote refers to an occurrence where crystals or masses of epidote are intergrown with or included within quartz. Quartz (SiO2) is a tectosilicate mineral, one of the most abundant minerals in the Earth's crust. Epidote (Ca2(Al,Fe)3(SiO4)3(OH)) is a sorosilicate mineral belonging to the epidote group, characterized by its distinctive pistachio-green to yellowish-green color. The combination creates a visually appealing specimen, often with green epidote needles or granular aggregates embedded in or coating quartz crystals.

How to Identify

Color: Quartz is typically colorless, white, or milky. Epidote ranges from pistachio-green to yellowish-green, brownish-green, or blackish-green. The combination will show clear to milky quartz with green inclusions or intergrowths.
Luster: Quartz: Vitreous. Epidote: Vitreous to resinous.
Texture: Can vary from massive to crystalline. Epidote often forms prismatic crystals, acicular (needle-like) crystals, or granular aggregates within the quartz matrix.
Crystal Form: Quartz: Hexagonal prisms with pyramidal terminations. Epidote: Monoclinic, typically forming prismatic crystals, often striated parallel to the length. When intergrown, epidote crystals may be embedded randomly or aligned within the quartz.
Cleavage: Quartz: None (conchoidal fracture). Epidote: Perfect on {001}, good on {100}.
Geological Environment: Metamorphic rocks (schists, gneisses, amphibolites), hydrothermal veins, contact metamorphic zones, and altered igneous rocks.

Key Facts

Hardness: Quartz: 7 (Mohs). Epidote: 6-7 (Mohs).
Specific Gravity: Quartz: 2.65. Epidote: 3.35-3.47.
Crystal System: Quartz: Trigonal. Epidote: Monoclinic.
Color: Colorless, white, or milky (quartz) with pistachio-green to dark-green (epidote).
Luster: Vitreous.
Transparency: Quartz: Transparent to translucent. Epidote: Translucent to opaque.
Fracture: Quartz: Conchoidal. Epidote: Uneven to subconchoidal.
Cleavage: Quartz: None. Epidote: Perfect on {001}, good on {100}.
Composition: Quartz: Silicon dioxide (SiO2). Epidote: Calcium aluminum iron sorosilicate hydroxide (Ca2(Al,Fe)3(SiO4)3(OH)).

Quick Check

Color: Clear to milky white (quartz) with pistachio-green to dark-green (epidote) inclusions/intergrowths.
Luster: Vitreous.
Streak: White (quartz), grayish-white (epidote).

Physical Characteristics

Crystal Habit: Quartz: Prismatic, massive, granular. Epidote: Prismatic, acicular, granular, massive.
Cleavage Type: Quartz: Absent. Epidote: Perfect basal cleavage on {001}, good on {100}.
Fracture Type: Quartz: Conchoidal. Epidote: Uneven to subconchoidal.
Tenacity: Brittle for both minerals.
Luster Type: Vitreous for both minerals, epidote can also be resinous.

Formation

Quartz with Epidote typically forms in metamorphic environments, particularly in greenschist to amphibolite facies conditions, where calcium-rich rocks (like basalts, gabbros, or limestones) undergo regional or contact metamorphism. Epidote is a common product of the alteration of plagioclase feldspar, pyroxenes, and amphiboles. Quartz, being a ubiquitous mineral, co-crystallizes or is present in the host rock during these metamorphic processes. It can also form in hydrothermal veins where silica-rich fluids interact with calcium-aluminum-iron-rich rocks.

Usage

Primarily used as an ornamental stone, for lapidary purposes (cabochons, carvings), and in jewelry. Due to its attractive green color and the contrast with clear or milky quartz, it is popular among mineral collectors. It has no significant industrial applications beyond its aesthetic value.

Age Distribution

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

Where to Find

Austria

Significant occurrences of well-formed epidote crystals, often associated with quartz, particularly in the Knappenwand area.

Pakistan

Known for high-quality epidote specimens, sometimes found with quartz.

United States

Various localities, including California (e.g., Sierra Nevada), Alaska, and North Carolina, where metamorphic and hydrothermal environments are prevalent.

Mexico

Occurrences in mining districts where hydrothermal alteration has taken place.

Norway

Epidote is found in various metamorphic terrains.

Finding Tips

Geological Context

Look in areas known for regional or contact metamorphism, especially where calcium-rich rocks have been altered. Hydrothermal vein systems are also good targets.

Associated Minerals

Often found with other metamorphic minerals such as actinolite, garnet, chlorite, and various feldspars.

Visual Identification

Search for clear or milky quartz specimens containing distinctive pistachio-green to dark-green inclusions or intergrowths of prismatic or acicular crystals.

Hardness Test

Quartz (7 on Mohs) will scratch glass. Epidote (6-7 on Mohs) will also scratch glass, but may be slightly softer than quartz.

Similar Rocks

Prasiolite

SiO2 (green variety of Quartz)

Also known as: Green Amethyst

Aventurine

SiO2 with Fuchsite or other mica inclusions

Also known as: Green Quartzite

Serpentine

Mg3Si2O5(OH)4

Also known as: Serpentinite

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Epidote: Sorosilicate.
Group: Quartz Group, Epidote Group
Crystal System: Quartz: Trigonal. Epidote: Monoclinic.
Chemical Formula: SiO2 (Quartz) and Ca2(Al,Fe)3(SiO4)3(OH) (Epidote)
Composition: Quartz is composed of silicon and oxygen. Epidote is composed of calcium, aluminum, iron, silicon, oxygen, and hydrogen.

Explore Quartz with Epidote

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