Epidote-Feldspar Rock

Metamorphic Rock (or Igneous with alteration)

Epidote-Feldspar rock

Also known as: Epidosite (when epidote is dominant), Unakite (a specific variety with pink feldspar and epidote)

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Description

Epidote in feldspar refers to a rock or mineral assemblage where the mineral epidote is intimately associated with, or has replaced, feldspar minerals. This often results in a distinctive green coloration (from epidote) contrasting with the lighter colors of feldspar (white, pink, grey). The texture can range from fine-grained to coarse-grained, depending on the original rock and the metamorphic or alteration intensity. It is a common product of low-to-medium grade metamorphism and hydrothermal alteration.

How to Identify

Color: Typically green (epidote) intergrown with white, pink, or grey (feldspar). The green can range from yellowish-green to pistachio green to dark green. Feldspar colors depend on its type (e.g., orthoclase is often pink, plagioclase can be white to grey).
Luster: Epidote typically has a vitreous to resinous luster. Feldspar has a vitreous to pearly luster.
Texture: Granular, often interlocking crystals. Can be massive, fine-grained, or coarse-grained. The epidote may appear as disseminated grains, veinlets, or larger crystals within the feldspar matrix.
Crystal Form: Epidote often forms prismatic crystals, sometimes acicular, or granular aggregates. Feldspar forms blocky, tabular, or prismatic crystals.
Cleavage: Epidote has perfect cleavage on {001} and good cleavage on {100}. Feldspar has two distinct cleavages at or near 90 degrees.
Geological Environment: Common in regionally metamorphosed mafic to intermediate igneous rocks (e.g., metabasalts, metagabbros, metadiorites) and calc-silicate rocks. Also found in hydrothermally altered igneous rocks and skarns.

Key Facts

Hardness: Epidote: 6-7; Feldspar: 6-6.5 (Mohs scale)
Specific Gravity: Epidote: 3.3-3.5; Feldspar: 2.5-2.8
Crystal System: Epidote: Monoclinic; Feldspar: Monoclinic (Orthoclase) or Triclinic (Plagioclase)
Color: Epidote: Pistachio green, yellowish-green, dark green; Feldspar: White, grey, pink, red, green (amazonite)
Luster: Vitreous to resinous (epidote), vitreous to pearly (feldspar)
Transparency: Epidote: Transparent to translucent; Feldspar: Transparent to translucent
Fracture: Epidote: Uneven to conchoidal; Feldspar: Uneven to conchoidal
Cleavage: Epidote: Perfect on {001}, good on {100}; Feldspar: Two distinct cleavages at or near 90 degrees
Composition: Epidote: Ca2(Al,Fe)3(SiO4)3(OH); Feldspar: KAlSi3O8 (Orthoclase) or NaAlSi3O8-CaAl2Si2O8 solid solution (Plagioclase)

Quick Check

Color: Green (epidote) and white/pink/grey (feldspar)
Luster: Vitreous to resinous (epidote), vitreous to pearly (feldspar)
Streak: White to grayish white (for both epidote and feldspar)

Physical Characteristics

Crystal Habit: Epidote: Prismatic, acicular, granular, massive; Feldspar: Tabular, blocky, prismatic, massive
Cleavage Type: Epidote: Perfect basal, good prismatic; Feldspar: Perfect basal and good pinacoidal
Fracture Type: Uneven to conchoidal for both
Tenacity: Brittle for both
Luster Type: Vitreous to resinous (epidote), Vitreous to pearly (feldspar)

Formation

Epidote in feldspar typically forms through metasomatism or regional metamorphism of igneous rocks (like granite, granodiorite, or diorite) or certain sedimentary rocks. During these processes, plagioclase feldspar is often altered to epidote, zoisite, and albite in a process called saussuritization, particularly under greenschist to amphibolite facies conditions. Hydrothermal alteration can also lead to the formation of epidote within feldspar-rich rocks.

Usage

Primarily used as an ornamental stone, especially the Unakite variety, for carvings, cabochons, and decorative objects. Less commonly, it can be a minor source of epidote for collectors. It also serves as an indicator of metamorphic conditions and hydrothermal alteration in geological studies.

Age Distribution

Varies widely depending on the geological event that formed it, from Precambrian to Cenozoic.

Where to Find

Unaka Mountains, USA

Type locality for Unakite, a variety of epidote-feldspar rock, found in the Appalachian Mountains of North Carolina and Tennessee.

Norway

Various localities for epidote-rich rocks, often associated with metamorphic terrains.

Austria

Alpine regions are known for well-formed epidote crystals, often in association with feldspar in metamorphic rocks.

Pakistan

Skardu region is known for high-quality epidote crystals, sometimes found in matrix with feldspar.

Many Orogenic Belts Worldwide

Due to its formation through metamorphism and hydrothermal alteration, epidote-feldspar assemblages are common in ancient and active mountain ranges and areas of significant crustal deformation and fluid flow.

Finding Tips

Look for Green and Pink/White Combinations

The most striking examples, like Unakite, will have a clear contrast between the pistachio-green epidote and pink orthoclase feldspar, or white/grey plagioclase.

Check Metamorphic Terrains

Focus on areas known for regional metamorphism of igneous rocks, especially those that were originally mafic to intermediate in composition.

Examine Hydrothermal Alteration Zones

Epidote is a common alteration mineral, so look for it in areas where hot fluids have interacted with existing rocks, such as around ore deposits or fault zones.

Observe Crystal Habits

Epidote often forms distinctive prismatic crystals or granular aggregates. Feldspar will show its characteristic blocky or tabular forms.

Similar Rocks

Unakite

Also known as: Epidote-Feldspar-Quartz rock

Saussurite

Also known as: Epidote-zoisite-albite assemblage

Granite

Also known as: Granitic rock

Scientific Classification

Mineral Class: Silicates
Group: Epidote Group (Epidote), Feldspar Group (Feldspar)
Crystal System: Monoclinic (Epidote, Orthoclase), Triclinic (Plagioclase)
Chemical Formula: Epidote: Ca2(Al,Fe3+)3(SiO4)3(OH); Feldspar: KAlSi3O8 (Orthoclase) or (Na,Ca)(Al,Si)AlSi2O8 (Plagioclase)
Composition: Epidote is a calcium aluminum iron sorosilicate. Feldspars are tectosilicates of aluminum and alkali/alkaline earth metals.

Explore Epidote-Feldspar Rock

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