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Epidosite is a metamorphic rock characterized by a high concentration of the mineral epidote, typically exceeding 50% by volume. It often forms from the alteration of mafic igneous rocks (basalts, gabbros) or calcium-rich sedimentary rocks. The rock is generally fine-grained to medium-grained, with a distinctive pistachio-green to yellowish-green color due to the dominant epidote content. Quartz is a common associated mineral, and other minerals like actinolite, chlorite, and albite may also be present. Its formation is indicative of specific pressure-temperature conditions during metamorphism, particularly within the greenschist facies.
How to Identify
- Color
- Pistachio-green, yellowish-green, or olive-green, often with a mottled appearance due to varying epidote concentrations.
- Luster
- Vitreous to resinous, sometimes dull in massive forms.
- Texture
- Typically fine-grained to medium-grained, granoblastic or nematoblastic (if actinolite is present). Can be massive or weakly foliated.
- Crystal Form
- Epidote crystals within the rock can be anhedral to subhedral, sometimes forming radiating aggregates or prismatic crystals visible with a hand lens.
- Cleavage
- Epidote itself has perfect basal cleavage {001} and good prismatic cleavage {100}, but in a massive rock, this may not be readily apparent. The rock as a whole does not exhibit distinct cleavage planes.
- Geological Environment
- Commonly found in areas of regional metamorphism, particularly in oceanic crust that has undergone hydrothermal alteration (e.g., mid-ocean ridges, ophiolite complexes) and subsequent subduction or obduction. Also found in contact metamorphic aureoles around intrusions into mafic rocks.
Key Facts
- Hardness: 6-7 on Mohs scale (for epidote mineral)
- Specific Gravity: 3.3-3.5 (for epidote mineral, rock density will vary slightly)
- Crystal System: Monoclinic (for epidote mineral)
- Color: Pistachio-green, yellowish-green, olive-green
- Luster: Vitreous to resinous
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal (for epidote mineral)
- Cleavage: Perfect basal cleavage {001} and good prismatic cleavage {100} (for epidote mineral), not typically observed in the rock mass.
- Composition: Primarily epidote (Ca2(Al,Fe)3(SiO4)3(OH)), often with quartz (SiO2) and minor amounts of other metamorphic minerals.
Quick Check
- Color: Pistachio-green to yellowish-green
- Luster: Vitreous to resinous
- Streak: White to grayish-white
Physical Characteristics
- Crystal Habit: Epidote within the rock can be anhedral to subhedral, prismatic, or granular. The rock itself is typically massive or granular.
- Cleavage Type: Not applicable to the rock as a whole; individual epidote crystals exhibit perfect basal and good prismatic cleavage.
- Fracture Type: Irregular to subconchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to resinous.
Formation
Epidosite forms through the metamorphism of mafic igneous rocks (like basalt or gabbro) or certain sedimentary rocks under greenschist to amphibolite facies conditions. It is a product of hydrothermal alteration or regional metamorphism, where calcium-rich plagioclase feldspar is altered to epidote, often with quartz. This process is known as epidotization.
Usage
Primarily of geological interest for understanding metamorphic processes and hydrothermal alteration. Occasionally used as an ornamental stone or for lapidary purposes due to its attractive green color and hardness. It can also be an indicator mineral for certain types of ore deposits.
Age Distribution
Variable, depending on the age of the protolith and the metamorphic event. Can range from Precambrian to Cenozoic.
Where to Find
Ophiolite Complexes Worldwide
Epidosite is a common component of altered oceanic crust found in ophiolite sequences, which are remnants of ancient oceanic lithosphere obducted onto continental margins. Examples include Cyprus, Oman, and parts of the Appalachian Mountains.
Alpine Orogeny Regions
Regions that have undergone significant Alpine-type metamorphism, such as the European Alps, often contain epidote-rich rocks derived from metabasalts and metagabbros.
Subduction Zones and Accretionary Wedges
Areas where oceanic crust is being subducted and metamorphosed, or where fragments of oceanic crust are incorporated into accretionary wedges, can host epidotized rocks.
Hydrothermal Alteration Zones
Associated with hydrothermal systems in various geological settings, including geothermal fields and areas of ore mineralization, where hot fluids have altered mafic rocks.
Finding Tips
Look for Green Rocks in Metamorphic Terranes
Epidosite's distinctive green color is a key indicator. Focus your search in areas mapped as metamorphic belts, especially those with a history of mafic igneous protoliths.
Examine Outcrops Near Ophiolites
If you are in a region known for ophiolite complexes, search for outcrops that show evidence of altered oceanic crust, such as metabasalts or metagabbros, which are common protoliths for epidosite.
Check for Associated Minerals
Epidosite often occurs with quartz, chlorite, actinolite, and albite. The presence of these minerals can help confirm the identification and geological context.
Consider Hydrothermal Veins
Epidote can also form in hydrothermal veins. Look for green, crystalline veins cutting through other rock types, particularly in areas of past volcanic or intrusive activity.
Similar Rocks
Greenschist
Greenschist
Also known as: Chlorite-epidote schist
Amphibolite
Amphibolite
Also known as: Hornblende-plagioclase rock
Serpentinite
Serpentinite
Also known as: Serpentine rock
Scientific Classification
- Mineral Class
- Silicate (Sorosilicate)
- Group
- Epidote Group
- Crystal System
- Monoclinic
- Chemical Formula
- Ca2(Al,Fe3+)3(SiO4)3(OH) (for epidote, the dominant mineral)
- Composition
- A metamorphic rock composed predominantly of epidote, typically greater than 50%, often with significant quartz, and minor amounts of chlorite, actinolite, albite, and other accessory minerals.
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