Unakite

Metamorphic Rock (specifically a granitoid altered by epidotization)

Unakite (Epidote, Feldspar, Quartz)

Also known as: Epidote Granite, Epidosite

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Description

Unakite is a distinctive metamorphic rock characterized by its variegated appearance, typically displaying shades of pistache green (epidote), pink to reddish-orange (potassium feldspar), and often colorless to greyish (quartz). It is a granitoid rock that has undergone a specific type of hydrothermal alteration called epidotization. This process transforms the original plagioclase feldspar into epidote, giving the rock its characteristic green hue. The texture is typically granular, and the individual mineral components are usually visible to the naked eye, giving it a speckled or mottled appearance. It is not a true mineral but rather a rock composed of multiple minerals.

How to Identify

Color: Mottled green (epidote), pink to reddish-orange (potassium feldspar), and colorless to greyish (quartz). The green is typically the most dominant and characteristic color.
Luster: Vitreous to sub-vitreous for quartz and feldspar, resinous to vitreous for epidote.
Texture: Granular, phaneritic (coarse-grained) to medium-grained, with interlocking crystals. The texture is typically massive, lacking foliation.
Crystal Form: Individual mineral grains (epidote, feldspar, quartz) are anhedral to subhedral, forming an interlocking mosaic. Epidote may sometimes show prismatic habits within the rock.
Cleavage: Potassium feldspar exhibits two distinct cleavages at approximately 90 degrees. Epidote has one perfect cleavage. Quartz has no cleavage but exhibits conchoidal fracture.
Geological Environment: Formed from the hydrothermal alteration (epidotization) of granitic rocks. This typically occurs in areas of low-grade regional metamorphism, contact metamorphism, or along fault zones where hydrothermal fluids have circulated through pre-existing granites. It is often found in association with other metamorphic rocks and altered igneous intrusions.

Key Facts

Hardness: 6-7 on the Mohs scale (due to quartz and epidote components).
Specific Gravity: 2.8-3.2 (variable depending on mineral proportions, epidote is denser).
Crystal System: Monoclinic (epidote), Triclinic (plagioclase feldspar, if any remains), Monoclinic (orthoclase), Triclinic (microcline), Trigonal (quartz). As a rock, it does not have a single crystal system.
Color: Green (epidote), pink/reddish-orange (potassium feldspar), colorless/grey (quartz).
Luster: Vitreous to sub-vitreous.
Transparency: Opaque to translucent.
Fracture: Conchoidal (quartz), uneven to splintery (feldspar and epidote).
Cleavage: Good in epidote (one direction), good in feldspar (two directions at ~90 degrees), absent in quartz.
Composition: Primarily composed of epidote, potassium feldspar (orthoclase or microcline), and quartz. Minor accessory minerals may include plagioclase feldspar (unaltered remnants), chlorite, magnetite, and zircon.

Quick Check

Color: Mottled green, pink/reddish-orange, and grey/white.
Luster: Vitreous to sub-vitreous.
Streak: White (for all component minerals).

Physical Characteristics

Crystal Habit: Granular aggregate of anhedral to subhedral crystals. Epidote may show prismatic forms.
Cleavage Type: Perfect in epidote (one direction), good in feldspar (two directions).
Fracture Type: Conchoidal (quartz), uneven to splintery (epidote, feldspar).
Tenacity: Brittle.
Luster Type: Vitreous to sub-vitreous.

Formation

Unakite is a granitoid rock that has undergone epidotization, a type of hydrothermal alteration. This process involves the metasomatic replacement of plagioclase feldspar 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 typically occurs when granitic rocks are subjected to low-grade metamorphism or hydrothermal fluid circulation, often in fault zones or areas of significant fluid flow. The characteristic green color comes from epidote, while the pink to reddish-orange is from potassium feldspar (orthoclase or microcline), and the colorless to greyish component is quartz.

Usage

Unakite is primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in decorative items. Its attractive mottled green and pink appearance makes it popular in jewelry and as a decorative building material (e.g., tiles, countertops) where its hardness and aesthetic qualities are valued. It is also collected by mineral enthusiasts.

Age Distribution

Typically associated with Precambrian to Paleozoic granitic intrusions, but can form in any geological period where the specific alteration conditions are met.

Where to Find

Unaka Mountains, North Carolina, USA

The type locality for Unakite, where it was first discovered. Found in the Unaka Range of the Appalachian Mountains.

Virginia, USA

Commonly found in the Blue Ridge Mountains of Virginia, particularly in the Shenandoah National Park area.

South Africa

Significant deposits are found in various regions of South Africa.

Brazil

Occurrences reported in several states, often associated with granitic intrusions.

China

Deposits are found in certain granitic terrains.

Sierra Leone

Known for its unakite occurrences.

Norway

Found in some areas with altered granitic rocks.

Finding Tips

Look for Granitic Terrains

Unakite forms from altered granite, so search in areas known for granitic intrusions, especially those that have undergone low-grade metamorphism or hydrothermal alteration.

Identify Key Colors

The distinctive combination of pistache green (epidote) and pink/reddish-orange (feldspar) is the primary visual identifier. Quartz will appear colorless or greyish.

Check for Granular Texture

Unakite typically has a coarse to medium-grained granular texture, where individual mineral components are visible.

Examine Outcrops and Riverbeds

Unakite can be found in outcrops, as float in stream beds, or in glacial till, particularly in regions where it is known to occur.

Consider Hardness

The presence of quartz (Mohs 7) makes Unakite relatively hard, which can be a useful distinguishing feature from softer altered rocks.

Similar Rocks

Epidosite

Also known as: Epidote-Quartz Rock

Granite

Also known as: Granitic Rock

Gneiss

Also known as: Banded Metamorphic Rock

Scientific Classification

Mineral Class: Silicate (Epidote, Feldspar, Quartz are all silicates)
Group: Rock (specifically a granitoid altered by epidotization)
Crystal System: Not applicable to the rock as a whole; individual minerals have their own systems (e.g., Monoclinic for epidote, Trigonal for quartz).
Chemical Formula: Not applicable to the rock as a whole; individual minerals have their own formulas (e.g., Epidote: Ca2(Al,Fe)3(SiO4)3(OH); Quartz: SiO2; Potassium Feldspar: KAlSi3O8).
Composition: A mixture of calcium aluminum iron sorosilicate (epidote), potassium aluminum silicate (potassium feldspar), and silicon dioxide (quartz).

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