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Epidote in Epidosite refers to the mineral epidote as a primary constituent within the metamorphic rock known as epidosite. Epidosite is a dense, fine-grained to medium-grained metamorphic rock characterized by a high proportion of epidote (typically 50% or more) and quartz, with minor amounts of other minerals such as chlorite, actinolite, and plagioclase feldspar. The rock often exhibits a distinctive pistachio-green color due to the iron content in epidote. Epidote itself is a sorosilicate mineral, part of the epidote group, known for its characteristic green to yellowish-green color and often prismatic crystal habit.
How to Identify
- Color
- Epidosite is typically pistachio-green to yellowish-green, often with a mottled or speckled appearance due to the epidote and quartz. Epidote crystals within the rock are usually green, yellowish-green, or brownish-green.
- Luster
- Epidote exhibits a vitreous (glassy) to resinous luster. Epidosite as a rock will have an overall duller, more earthy luster on fresh surfaces, but individual epidote grains will show vitreous luster.
- Texture
- Epidosite is typically fine-grained to medium-grained, often granoblastic (equigranular, interlocking grains) or nematoblastic (with elongated minerals like epidote showing preferred orientation). Epidote crystals can be prismatic, acicular, or granular.
- Crystal Form
- Epidote commonly forms prismatic crystals, often elongated and striated parallel to the length. It can also occur as granular aggregates or massive forms within epidosite.
- Cleavage
- Epidote has one perfect cleavage parallel to {001}. This cleavage may be visible in larger epidote crystals within the epidosite.
- Geological Environment
- Epidosite is found in metamorphic terrains, particularly in areas that have undergone greenschist to lower amphibolite facies metamorphism. It is commonly associated with altered mafic igneous rocks (basalts, gabbros, diabases) and sometimes with altered calcareous sediments. It can be found in subduction zones, mid-ocean ridges (hydrothermal alteration), and regional metamorphic belts.
Key Facts
- Hardness: 6 - 7 (Mohs scale for epidote)
- Specific Gravity: 3.3 - 3.5 (for epidote)
- Crystal System: Monoclinic (for epidote)
- Color: Pistachio-green, yellowish-green, brownish-green (for epidote and epidosite)
- Luster: Vitreous to resinous (for epidote), dull to earthy (for epidosite)
- Transparency: Transparent to translucent to opaque (for epidote)
- Fracture: Uneven to conchoidal (for epidote)
- Cleavage: Perfect on {001} (for epidote)
- Composition: Epidosite: Primarily epidote (Ca2(Al,Fe)3(SiO4)3(OH)) and quartz (SiO2), with minor chlorite, actinolite, plagioclase. Epidote: Calcium aluminum iron sorosilicate hydroxide.
Quick Check
- Color: Pistachio-green to yellowish-green
- Luster: Vitreous to resinous (for epidote crystals), duller for the rock
- Streak: White to grayish-white
Physical Characteristics
- Crystal Habit: Prismatic, acicular, granular, massive (for epidote)
- Cleavage Type: Perfect on one direction ({001}) (for epidote)
- Fracture Type: Uneven to conchoidal (for epidote)
- Tenacity: Brittle (for epidote)
- Luster Type: Vitreous to resinous (for epidote)
Formation
Epidosite forms through the metamorphism of mafic igneous rocks (like basalt or gabbro) or calcareous sediments under greenschist to amphibolite facies conditions. This process, often termed epidotization, involves hydrothermal alteration where calcium, aluminum, iron, and silica are mobilized and recrystallized. Epidote itself is a common product of regional and contact metamorphism, particularly in rocks rich in calcium and aluminum, and is also found in hydrothermally altered zones.
Usage
Epidosite itself has limited direct industrial use, though it can be a component of crushed stone for construction aggregate. Epidote, when found in large, attractive crystals, is sometimes used as a gemstone or ornamental stone. Its presence in altered rocks can be an indicator for certain types of ore deposits (e.g., porphyry copper deposits).
Age Distribution
Varies widely depending on the protolith and metamorphic event, ranging from Precambrian to Cenozoic.
Where to Find
Alps (Europe)
Well-known for occurrences of epidote and epidote-rich rocks due to extensive Alpine metamorphism.
Sierra Nevada (California, USA)
Associated with metamorphosed mafic volcanic and intrusive rocks.
Appalachian Mountains (Eastern USA)
Found in various metamorphic belts.
Ophiolite Complexes Worldwide
Commonly found in altered oceanic crust sections, where mafic rocks have undergone hydrothermal alteration and metamorphism.
Finding Tips
Look for Green Rocks
Epidosite's distinctive pistachio-green color is a key indicator. Search in areas known for metamorphic rocks, especially those derived from mafic protoliths.
Check for Hardness
Epidote is relatively hard (6-7 on Mohs scale), so the rock will resist scratching with a steel knife. Quartz (7) also contributes to the hardness.
Examine Crystal Habit
Look for prismatic or granular green crystals within the rock matrix, which are characteristic of epidote.
Consider Geological Context
Epidosite is typically found in metamorphic terranes, often associated with greenschist facies rocks, amphibolites, and altered mafic igneous rocks.
Similar Rocks
Greenschist
Greenschist facies metamorphic rock
Also known as: Chlorite schist
Amphibolite
Amphibolite facies metamorphic rock
Also known as: Hornblende schist
Serpentinite
Serpentinite
Also known as: Serpentine rock
Scientific Classification
- Mineral Class
- Sorosilicate (for epidote)
- Group
- Epidote Group (for epidote)
- Crystal System
- Monoclinic (for epidote)
- Chemical Formula
- Ca2(Al,Fe)3(SiO4)3(OH) (for epidote)
- Composition
- Epidote: Calcium aluminum iron sorosilicate hydroxide. Epidosite: Metamorphic rock composed predominantly of epidote and quartz.
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