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Unakite is a distinctive igneous rock characterized by its variegated coloration, typically comprising pistachio-green epidote, pink orthoclase feldspar, and colorless to grey quartz. It is a type of altered granite, where the original plagioclase feldspar has been replaced by epidote through metasomatism. The texture is typically granular, reflecting its granitic origin, with interlocking crystals of its constituent minerals. The proportions of these minerals can vary, leading to differences in the dominant color and overall appearance of the rock.
How to Identify
- Color
- Distinctive mottled or speckled appearance with shades of pistachio-green (epidote), pink to reddish-orange (orthoclase feldspar), and colorless to grey (quartz).
- Luster
- Vitreous to sub-vitreous for quartz and feldspar; epidote can be vitreous to resinous.
- Texture
- Granular, phaneritic (coarse-grained), interlocking crystalline texture, characteristic of a granitic rock.
- Crystal Form
- Individual mineral grains are anhedral to subhedral, forming an interlocking mosaic. Epidote may show prismatic habit within the rock.
- Cleavage
- Orthoclase feldspar exhibits two distinct cleavages at approximately 90 degrees. Epidote has one perfect cleavage. Quartz has no cleavage.
- Geological Environment
- Found in areas where granitic intrusions have undergone hydrothermal alteration, often associated with regional metamorphism or fault 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: 2.8-3.2 (varies with mineral proportions, epidote is denser)
- Crystal System: Monoclinic (epidote), Triclinic (orthoclase), Hexagonal (quartz) - as a rock, it's an aggregate of these systems
- Color: Green (epidote), pink/red (orthoclase), colorless/grey (quartz)
- Luster: Vitreous to sub-vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal (quartz), uneven to splintery (feldspar, epidote)
- Cleavage: Good in feldspar (2 directions at ~90°), perfect in epidote (1 direction), none in quartz
- Composition: Primarily epidote, orthoclase feldspar, and quartz. Minor accessory minerals may be present.
Quick Check
- Color: Mottled green, pink, and grey/white
- Luster: Vitreous to sub-vitreous
- Streak: White (for quartz and feldspar components); epidote streak is typically colorless to grayish-white
Physical Characteristics
- Crystal Habit: Granular, massive aggregate of anhedral to subhedral crystals.
- Cleavage Type: Feldspar: two distinct cleavages at nearly 90 degrees. Epidote: one perfect basal cleavage. Quartz: no cleavage.
- Fracture Type: Conchoidal (quartz), uneven to splintery (feldspar, epidote).
- Tenacity: Brittle.
- Luster Type: Vitreous to sub-vitreous.
Formation
Unakite is a metasomatized granitic rock, meaning it is a granite that has undergone significant chemical alteration. Specifically, it forms when plagioclase feldspar within a granite is replaced by epidote, typically through hydrothermal alteration. This process, known as epidotization, occurs when hot, chemically active fluids (hydrothermal fluids) rich in calcium, iron, and aluminum interact with the granite. The pink orthoclase feldspar and colorless quartz components of the original granite remain largely unaltered, contributing to the characteristic appearance of unakite. The alteration often occurs in regional metamorphic settings or near hydrothermal vein systems.
Usage
Unakite is primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in decorative architectural applications. Its attractive color combination makes it popular in jewelry and as a decorative material.
Age Distribution
Typically associated with Precambrian to Paleozoic granitic intrusions, but can form in any age where appropriate alteration conditions exist.
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 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 localities for Unakite.
Brazil
Deposits found in certain granitic terrains.
China
Reported occurrences in several provinces.
Finding Tips
Geological Maps
Consult geological maps of areas known for granitic intrusions and hydrothermal alteration. Look for regions with reported epidotization.
Outcrop Examination
Examine outcrops in granitic terrains, particularly near fault zones, shear zones, or areas with evidence of hydrothermal activity (e.g., quartz veins, alteration halos).
Riverbeds and Glacial Till
Unakite is quite durable, so it can be found as cobbles and pebbles in riverbeds and glacial till downstream from its source.
Color Recognition
Train your eye to recognize the characteristic green, pink, and grey/white color combination. The green epidote is often the most distinctive component.
Similar Rocks
Epidosite
Epidosite
Also known as: Epidote-rich rock
Granite
Granite
Also known as: Granitic rock
Gneiss
Gneiss
Also known as: Banded metamorphic rock
Scientific Classification
- Mineral Class
- Silicate (for all constituent minerals)
- Group
- Igneous rock, metasomatized
- Crystal System
- Polycrystalline aggregate (individual minerals are monoclinic, triclinic, hexagonal)
- Chemical Formula
- Variable, as it's a rock. Constituent minerals: Epidote (Ca2(Al,Fe)3(SiO4)3(OH)), Orthoclase (KAlSi3O8), Quartz (SiO2).
- Composition
- A rock composed predominantly of epidote, orthoclase feldspar, and quartz, formed by the hydrothermal alteration of granite.
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