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Quartz with Epidote refers to an occurrence where crystals or masses of epidote are intimately associated with or included within quartz. Quartz (SiO2) is one of the most abundant minerals in the Earth's crust, known for its hardness and vitreous luster. Epidote (Ca2(Al,Fe)3(SiO4)3(OH)) is a sorosilicate mineral characterized by its distinctive pistachio-green to yellowish-green color. The combination often results in striking specimens, with green epidote needles or granular aggregates embedded within clear, milky, or smoky quartz. The iron content in epidote determines the intensity of its green color.
How to Identify
- Color
- Quartz can be colorless, white, gray, or smoky. Epidote typically presents as pistachio-green, yellowish-green, or dark green. The combination will show these colors.
- Luster
- Quartz: Vitreous. Epidote: Vitreous to resinous.
- Texture
- Can range from massive to crystalline. Epidote often forms prismatic crystals, acicular (needle-like) inclusions, or granular aggregates within the quartz matrix.
- Crystal Form
- Quartz: Hexagonal prisms with pyramidal terminations. Epidote: Monoclinic, typically forming elongated, prismatic crystals, often striated parallel to the length. Can also be granular or massive.
- Cleavage
- Quartz: None (conchoidal fracture). Epidote: Perfect on {001}, good on {100}.
- Geological Environment
- Metamorphic rocks (schists, gneisses, amphibolites), contact metamorphic zones, hydrothermal veins, 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: Quartz: Colorless, white, gray, smoky. Epidote: Pistachio-green, yellowish-green, dark green.
- Luster: Vitreous to resinous.
- Transparency: Quartz: Transparent to translucent. Epidote: Transparent to translucent.
- Fracture: Quartz: Conchoidal. Epidote: Uneven to subconchoidal.
- Cleavage: Quartz: None. Epidote: Perfect on {001}, good on {100}.
- Composition: SiO2 (Quartz) and Ca2(Al,Fe)3(SiO4)3(OH) (Epidote).
Quick Check
- Color: Green (epidote) within colorless, white, or smoky (quartz)
- 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 {001}, good {100}.
- Fracture Type: Quartz: Conchoidal. Epidote: Uneven to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to resinous.
Formation
Quartz with Epidote forms primarily in metamorphic and hydrothermal environments. Epidote is a common product of low-to-medium grade regional metamorphism of mafic igneous rocks (e.g., basalt, gabbro) and calcareous sedimentary rocks, where plagioclase feldspar, pyroxenes, and amphiboles are altered. It also forms in contact metamorphic aureoles and in hydrothermal veins associated with granitic intrusions, where hot, silica-rich fluids interact with calcium- and iron-bearing rocks. Quartz, being ubiquitous, crystallizes alongside epidote from these metamorphic or hydrothermal fluids, often filling fractures or forming veins.
Usage
Primarily a collector's mineral. When found in sufficient quantities and quality, it can be used for ornamental purposes, lapidary work (cabochons, carvings), and occasionally as a minor gemstone. It is not typically used as an industrial ore, though epidote itself can be an indicator mineral for certain ore deposits (e.g., porphyry copper deposits).
Age Distribution
Varies widely depending on the geological setting of formation, ranging from Precambrian to Cenozoic.
Where to Find
Knappenwand, Untersulzbachtal, Salzburg, Austria
Famous for producing exceptional epidote crystals, often associated with quartz.
Prince of Wales Mine, Ketchikan, Alaska, USA
Known for fine epidote specimens, sometimes with quartz.
Serifos Island, Cyclades, Greece
Produces excellent epidote crystals, frequently found with quartz.
Pakistan (various localities)
Numerous localities yield high-quality epidote, often in association with quartz.
Brazil (various localities)
Known for various quartz varieties, and epidote can be found in metamorphic and hydrothermal settings.
Finding Tips
Geological Context
Focus on areas known for regional or contact metamorphism, especially where mafic igneous rocks or calcareous sediments have been altered. Hydrothermal vein systems are also prime targets.
Color and Habit
Look for the distinctive pistachio-green to dark green color of epidote, often as prismatic crystals or needle-like inclusions within quartz. The quartz itself might be clear, milky, or smoky.
Associated Minerals
Epidote often co-occurs with other metamorphic minerals such as actinolite, chlorite, garnet, plagioclase, and calcite. Finding these can indicate a favorable environment.
Vein Systems
Examine quartz veins cutting through metamorphic or altered igneous rocks. Epidote can line the walls of these veins or be intergrown with the quartz.
Similar Rocks
Prasiolite
SiO2 (Quartz) with iron inclusions
Also known as: Green Amethyst
Actinolite in Quartz
SiO2 (Quartz) and Ca2(Mg,Fe)5Si8O22(OH)2 (Actinolite)
Also known as: Actinolite Quartz
Chlorite in Quartz
SiO2 (Quartz) and (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (Chlorite group minerals)
Also known as: Chlorite Quartz
Scientific Classification
- Mineral Class
- Quartz: Tectosilicate. Epidote: Sorosilicate.
- Group
- Quartz: Quartz group. Epidote: Epidote group.
- Crystal System
- Quartz: Trigonal. Epidote: Monoclinic.
- Chemical Formula
- SiO2 (Quartz) and Ca2(Al,Fe)3(SiO4)3(OH) (Epidote)
- Composition
- Silicon dioxide (Quartz) and Calcium Aluminum Iron Silicate Hydroxide (Epidote).
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