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Labradorite is a calcium-rich member of the plagioclase feldspar series, characterized by its distinctive iridescent optical effect known as labradorescence. This phenomenon displays a brilliant play of spectral colors, typically blues, greens, yellows, and reds, as light interacts with microscopic exsolution lamellae within the mineral structure. The color display varies with the angle of observation. It is an important rock-forming mineral in many igneous and some metamorphic rocks.
How to Identify
- Color
- Typically gray, dark gray, greenish-gray, or brownish-gray, often with a dull appearance until light strikes it at a specific angle, revealing the iridescent labradorescence (blues, greens, yellows, reds, purples).
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Typically massive, granular, or in tabular crystals within host rocks. The internal texture responsible for labradorescence is microscopic exsolution lamellae.
- Crystal Form
- Usually anhedral to subhedral grains in igneous rocks; can form tabular to blocky crystals, often twinned (polysynthetic twinning visible on cleavage surfaces as fine parallel lines).
- Cleavage
- Perfect in two directions at nearly 90 degrees (86 degrees and 94 degrees), characteristic of feldspars.
- Geological Environment
- Mafic igneous rocks (gabbro, basalt, anorthosite), some intermediate igneous rocks (diorite), and certain metamorphic rocks derived from these protoliths. Also found in meteorites.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.68 to 2.72
- Crystal System: Triclinic
- Color: Gray, dark gray, greenish-gray, brownish-gray; exhibits labradorescence (iridescent play of colors)
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal) at 86° and 94°
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)AlSi2O8 with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8) end-members.
Quick Check
- Color: Gray to dark gray, with iridescent flashes of blue, green, yellow, etc.
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to blocky crystals; often twinned (polysynthetic twinning).
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous on fresh surfaces, pearly on cleavage planes.
Formation
Labradorite forms primarily in mafic igneous rocks such as basalt, gabbro, and anorthosite. It crystallizes from magma rich in calcium and sodium, typically at high temperatures and pressures deep within the Earth's crust. It can also be found in some metamorphic rocks, particularly those derived from mafic protoliths, and occasionally in detrital sediments.
Usage
Labradorite is highly prized as a gemstone due to its unique play of colors (labradorescence). It is used in jewelry, ornamental carvings, and as a decorative stone in architecture. High-quality specimens are sought after by collectors. Due to its hardness and durability, it can also be used as a facing stone.
Age Distribution
Found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic.
Where to Find
Labrador, Canada
The type locality, where it was first identified in 1770. Found in anorthosite intrusions.
Finland
Known for the high-quality, intensely iridescent variety called Spectrolite, found in anorthosite.
Madagascar
Significant deposits producing large quantities of gem-quality labradorite.
Norway
Found in various igneous and metamorphic complexes.
Russia
Occurrences in anorthosite bodies.
United States (Oregon, New York)
Oregon is known for Oregon Sunstone, a variety of labradorite/oligoclase with aventurescence. New York has anorthosite occurrences.
Australia
Found in some igneous rock formations.
Finding Tips
Look in Mafic Igneous Rocks
Target areas with known occurrences of gabbro, basalt, or especially anorthosite, as these are the primary host rocks for labradorite.
Observe for Labradorescence
Rotate samples under direct light (sunlight is best) to observe the characteristic iridescent flash of colors. This is the most definitive visual identifier.
Check for Cleavage
Examine broken surfaces for two distinct cleavage planes intersecting at nearly right angles. This is a key characteristic of all feldspars.
Consider Associated Minerals
Labradorite often occurs with pyroxenes (augite), olivine, and magnetite in mafic rocks.
Safety Precautions
Labradorite itself is not hazardous. However, when collecting in the field, always wear appropriate safety gear (gloves, eye protection) and be aware of your surroundings, especially in quarries or rocky terrain. No specific warnings for hazardous materials are associated with labradorite.
Similar Rocks
Andesine
Andesine (Plagioclase Feldspar)
Also known as: Red Andesine
Oligoclase
Oligoclase (Plagioclase Feldspar)
Also known as: Sunstone (a variety)
Bytownite
Bytownite (Plagioclase Feldspar)
Also known as: None commonly
Scientific Classification
- Mineral Class
- Silicate
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with labradorite specifically defined as having 50-70% anorthite component.
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