Unakite

Metamorphic/Igneous (altered granite)

Unakite (altered granite composed of epidote, pink orthoclase feldspar, and quartz)

Also known as: Epidotized Granite

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Description

Unakite is a distinctive altered granite characterized by its unique color combination of pistachio green, pink, and often colorless to grey. It is composed primarily of epidote (green), pink orthoclase feldspar, and quartz. The green color is due to epidote, a calcium aluminum iron sorosilicate mineral. The pink is attributed to orthoclase feldspar, a potassium aluminum silicate. Quartz, a silicon dioxide mineral, appears as colorless to grey grains. The texture is typically granular, reflecting its granitic origin, and it can range from fine-grained to coarse-grained. It is a tough and durable rock, making it suitable for various ornamental uses.

How to Identify

Color: Distinctive mottled or speckled pattern of pistachio green (epidote), pink (orthoclase feldspar), and colorless to grey (quartz).
Luster: Vitreous to sub-vitreous for quartz and feldspar, resinous to vitreous for epidote.
Texture: Granular, interlocking crystalline texture, characteristic of an altered igneous rock. Grain size can vary from fine to coarse.
Crystal Form: Individual mineral grains (epidote, orthoclase, quartz) are anhedral to subhedral within the rock matrix.
Cleavage: Orthoclase exhibits two distinct cleavages at nearly 90 degrees. Epidote has one perfect cleavage. Quartz has no cleavage.
Geological Environment: Formed in areas where granitic rocks have undergone hydrothermal alteration or low-grade regional metamorphism, often associated with fault zones or areas of fluid flow.

Key Facts

Hardness: 6 to 7 on the Mohs scale (due to quartz and feldspar, epidote is 6-7).
Specific Gravity: 2.85 to 3.20 (variable depending on mineral proportions, epidote is denser).
Crystal System: Monoclinic (epidote, orthoclase), Trigonal (quartz). Unakite as a rock does not 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 orthoclase (two directions at ~90°), perfect in epidote (one direction), absent in quartz.
Composition: Primarily epidote (Ca2(Al,Fe)3(SiO4)3(OH)), orthoclase feldspar (KAlSi3O8), and quartz (SiO2).

Quick Check

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

Physical Characteristics

Crystal Habit: Granular, massive, anhedral to subhedral crystals within a rock matrix.
Cleavage Type: Good in orthoclase (prismatic), perfect in epidote (basal), absent in quartz.
Fracture Type: Conchoidal (quartz), uneven to splintery (feldspar, epidote).
Tenacity: Brittle.
Luster Type: Vitreous to sub-vitreous.

Formation

Unakite forms from the hydrothermal alteration of granite. This process, known as epidotization, involves the replacement of plagioclase feldspar within the granite by epidote, often accompanied by the alteration of mafic minerals (if present) to chlorite. The characteristic pink orthoclase feldspar and colorless to grey quartz remain largely unaffected, contributing to the distinctive color palette. The alteration occurs under conditions of low-grade metamorphism or hydrothermal activity, where calcium-rich fluids interact with the granite.

Usage

Unakite is primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in decorative items. Its attractive colors make it popular in jewelry and as a material for polished spheres and tumbled stones. It is also used in architectural applications for facing stones and tiles.

Age Distribution

Typically Precambrian to Paleozoic, depending on the original granite and alteration event.

Where to Find

Unaka Mountains, North Carolina and Tennessee, USA

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

Virginia, USA

Significant deposits are found in the Blue Ridge Mountains of Virginia, particularly in the Shenandoah National Park area.

South Africa

Occurrences reported in various regions, often associated with granitic intrusions.

Brazil

Found in several states, typically in areas with Precambrian granitic basement rocks.

China

Deposits have been identified in various provinces.

Sierra Leone

Known for its unakite occurrences.

Finding Tips

Geological Maps

Consult geological maps of areas known for granitic intrusions and hydrothermal alteration zones. Look for regions with Precambrian or Paleozoic granites.

Riverbeds and Glacial Till

Unakite is durable and often found as pebbles and cobbles in riverbeds and glacial till, having been transported from its source rock.

Outcrops

Look for outcrops of altered granite in mountainous regions or areas with exposed basement rock. The distinctive green and pink coloration can be a good indicator.

Associated Minerals

While Unakite itself is a rock, the presence of epidote-rich veins or altered granites in an area can indicate potential Unakite occurrences.

Similar Rocks

Granite

Also known as: Granitic rock

Epidosite

Also known as: Epidote rock

Gneiss

Also known as: Banded metamorphic rock

Scientific Classification

Mineral Class: Silicate (for its constituent minerals)
Group: Rock (altered granite)
Crystal System: Polycrystalline (composed of minerals with different crystal systems)
Chemical Formula: Variable, as it is a rock composed of multiple minerals: (Ca2(Al,Fe)3(SiO4)3(OH) + KAlSi3O8 + SiO2)
Composition: Epidote, Orthoclase Feldspar, Quartz

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