Unakite

Metasomatized Granite

Granite (Epidote, Pink Feldspar, Quartz)

Also known as: Epidote Granite, Epidosite

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Description

Unakite is a distinctive metamorphic rock, specifically a metasomatized granite, characterized by its unique mottled appearance of pistachio-green epidote, pink orthoclase or K-feldspar, and colorless to grey quartz. The relative proportions of these minerals can vary, leading to different shades and patterns. It is generally medium-grained to coarse-grained, with an interlocking crystalline texture.

How to Identify

Color: Mottled green (epidote), pink (feldspar), and colorless/grey (quartz). The green can range from yellowish-green to dark pistachio-green, and the pink from salmon to reddish-pink.
Luster: Vitreous to sub-vitreous for quartz and feldspar, resinous to vitreous for epidote.
Texture: Granitic, medium to coarse-grained, interlocking crystalline texture.
Crystal Form: Anhedral to subhedral grains of quartz, feldspar, and epidote, typically massive.
Cleavage: Feldspar exhibits two distinct cleavages at or near 90 degrees. Epidote has one perfect cleavage. Quartz has no cleavage.
Geological Environment: Formed in areas where granitic rocks have undergone hydrothermal alteration, often associated with regional metamorphism or contact metamorphism near intrusive bodies, particularly along fault zones or shear zones where fluid flow is concentrated.

Key Facts

Hardness: 6-7 on Mohs scale (due to quartz and feldspar, epidote is 6-7).
Specific Gravity: 2.85 - 3.20 (varies with mineral proportions, epidote is denser).
Crystal System: Monoclinic (epidote, K-feldspar), Trigonal (quartz). Unakite itself is a rock, not a single mineral, so it doesn't have a single crystal system.
Color: Pistachio-green, pink, and colorless/grey.
Luster: Vitreous to sub-vitreous.
Transparency: Opaque to translucent.
Fracture: Conchoidal (quartz), uneven to splintery (feldspar, epidote).
Cleavage: Good in feldspar (2 directions), perfect in epidote (1 direction), none in quartz.
Composition: Primarily composed of epidote (Ca2(Al,Fe)3(SiO4)3(OH)), potassium feldspar (KAlSi3O8, typically orthoclase), and quartz (SiO2).

Quick Check

Color: Mottled green, pink, and grey/white.
Luster: Vitreous to sub-vitreous.
Streak: White (for quartz and feldspar components), greenish-white (for epidote component).

Physical Characteristics

Crystal Habit: Massive, granular aggregates of interlocking crystals.
Cleavage Type: Feldspar: two distinct cleavages at ~90 degrees. Epidote: one perfect basal cleavage {001}. Quartz: no cleavage.
Fracture Type: Conchoidal (quartz), uneven to splintery (feldspar, epidote).
Tenacity: Brittle.
Luster Type: Vitreous to sub-vitreous.

Formation

Unakite forms from the metasomatic alteration of granite. This process involves the replacement of plagioclase feldspar (typically albite or oligoclase) by epidote, often accompanied by the alteration of mafic minerals (like hornblende or biotite) to chlorite, and the preservation of quartz and potassium feldspar. The alteration is driven by hydrothermal fluids, often rich in calcium, iron, and aluminum, circulating through the granite, particularly along fault zones or shear zones.

Usage

Primarily used as an ornamental stone, for lapidary work (cabochons, beads, carvings), and in decorative architectural applications. It is also popular in metaphysical and New Age practices.

Age Distribution

Typically associated with Precambrian to Paleozoic granitic intrusions that have undergone hydrothermal alteration.

Where to Find

Unaka Mountains, North Carolina and Tennessee, USA

The type locality, where it was first discovered and named. Found in the Precambrian basement rocks.

Virginia, USA

Significant deposits are found in the Blue Ridge Mountains.

Brazil

Various occurrences, often associated with granitic intrusions.

South Africa

Found in certain granitic terrains.

China

Reported from several localities.

Sierra Leone

Known occurrences.

Finding Tips

Geological Maps

Consult geological maps for areas known to have granitic intrusions and evidence of hydrothermal alteration or regional metamorphism.

Outcrop Examination

Look for outcrops of altered granite, particularly in stream beds, road cuts, or areas with exposed bedrock. The distinctive green and pink coloration is usually quite visible.

Associated Minerals

Unakite is often found in association with other metamorphic minerals or altered igneous rocks.

Weathering

Unakite is relatively resistant to weathering, so it can be found as float in stream beds and glacial till.

Similar Rocks

Epidosite

Also known as: Epidote-rich rock

Granite

Also known as: Granitic rock

Gneiss

Also known as: Banded metamorphic rock

Scientific Classification

Mineral Class: Silicate (for all constituent minerals)
Group: Rock (metasomatized granite)
Crystal System: Not applicable for the rock as a whole; constituent minerals are Monoclinic (epidote, K-feldspar) and Trigonal (quartz).
Chemical Formula: Variable, as it is a rock composed of multiple minerals. Key components: Epidote (Ca2(Al,Fe)3(SiO4)3(OH)), K-feldspar (KAlSi3O8), Quartz (SiO2).
Composition: A rock composed predominantly of epidote, potassium feldspar (orthoclase), and quartz, with minor accessory minerals.

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