Unakite

Metamorphic Rock (specifically, a granitoid altered by epidotization)

Unakite (Epidote-Feldspar rock)

Also known as: Epidote-Feldspar rock, Epidosite (though Unakite specifically implies the presence of pink feldspar)

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Description

Unakite is a distinctive metamorphic rock characterized by its variegated coloration, typically a mottled combination of pistachio-green epidote, pink orthoclase or microcline feldspar, and colorless to grey quartz. It is a granitoid that has undergone metasomatic alteration, specifically epidotization. The relative proportions of these minerals can vary, leading to different shades and patterns. It is generally a medium-grained to coarse-grained rock.

How to Identify

Color: Mottled green (epidote), pink (feldspar), and sometimes colorless/grey (quartz). The green is typically pistachio to yellowish-green, and the pink can range from salmon to reddish-pink.
Luster: Vitreous to sub-vitreous for quartz and feldspar; vitreous to resinous for epidote.
Texture: Granitic, medium to coarse-grained, interlocking crystalline texture.
Crystal Form: Individual mineral grains (epidote, feldspar, quartz) are anhedral to subhedral, forming an interlocking mosaic within the rock.
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 been subjected to hydrothermal alteration, often associated with regional metamorphism or contact metamorphism near intrusive bodies, particularly in ancient mountain belts.

Key Facts

Hardness: 6-7 (Mohs scale, due to constituent minerals: epidote 6-7, feldspar 6-6.5, quartz 7)
Specific Gravity: 2.85-3.20 (variable, depending on mineral proportions, epidote is denser)
Crystal System: Monoclinic (epidote, feldspar), Trigonal (quartz) - refers to the individual minerals, not the rock as a whole
Color: Mottled green, pink, and grey/colorless
Luster: Vitreous to sub-vitreous
Transparency: Opaque to translucent
Fracture: Uneven to conchoidal (quartz), irregular (feldspar, epidote)
Cleavage: Good in feldspar (2 directions), perfect in epidote (1 direction), none in quartz
Composition: Primarily epidote (Ca2(Al,Fe)3(SiO4)3(OH)), orthoclase/microcline feldspar (KAlSi3O8), and quartz (SiO2).

Quick Check

Color: Mottled green and pink, with minor grey/colorless
Luster: Vitreous to sub-vitreous
Streak: White

Physical Characteristics

Crystal Habit: Granular, massive, interlocking anhedral to subhedral crystals of constituent minerals.
Cleavage Type: Good in feldspar (two directions at ~90°), perfect in epidote (one direction), absent in quartz.
Fracture Type: Uneven to conchoidal (quartz), irregular (feldspar, epidote).
Tenacity: Brittle.
Luster Type: Vitreous to sub-vitreous.

Formation

Unakite forms from the hydrothermal alteration of granitic rocks. This process, known as epidotization, involves the replacement of plagioclase feldspar and mafic minerals (like hornblende or biotite) by epidote, while potassium feldspar (orthoclase or microcline) remains largely unaltered or is stained pink by iron oxides. The alteration occurs under conditions of moderate temperature and pressure, often in the presence of calcium-rich fluids.

Usage

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

Age Distribution

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

Where to Find

Unaka Mountains, North Carolina, USA

The type locality, where it was first discovered and named. Found in the Appalachian Mountains.

Virginia, USA

Significant deposits are found in the Blue Ridge Mountains of Virginia.

South Africa

Occurrences reported in various regions.

Sierra Leone

Known source for unakite.

Brazil

Deposits found in several states.

China

Various localities produce unakite.

Finding Tips

Geological Context

Look for areas with exposed granitic intrusions that show signs of hydrothermal alteration. Riverbeds and glacial tills can also contain weathered unakite boulders.

Color Recognition

The distinctive combination of green and pink is the primary visual identifier. Look for rocks with these two dominant colors, often with some grey or colorless areas.

Hardness Test

While not a single mineral, the overall hardness is around 6-7 on the Mohs scale due to its constituent minerals. This can help differentiate it from softer altered rocks.

Similar Rocks

Epidosite

Also known as: Epidote rock

Granite

Also known as: Granitic rock

Gneiss

Also known as: Banded metamorphic rock

Scientific Classification

Mineral Class: Silicate (Epidote, Feldspar, Quartz)
Group: Metamorphic Rock (specifically, an altered granitoid)
Crystal System: Polycrystalline aggregate of monoclinic (epidote, feldspar) and trigonal (quartz) minerals.
Chemical Formula: Variable, as it is a rock composed of multiple minerals. Key components: Ca2(Al,Fe)3(SiO4)3(OH) (epidote), KAlSi3O8 (feldspar), SiO2 (quartz).
Composition: A composite rock consisting mainly of epidote, potassium feldspar (orthoclase or microcline), and quartz. Minor accessory minerals may include magnetite, zircon, apatite, and chlorite.

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