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Labradorite is a tectosilicate mineral, a member of the plagioclase feldspar series, which is a solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Labradorite specifically falls within the compositional range of An50 to An70 (50-70% anorthite). Its most distinctive feature is labradorescence, a schiller effect or iridescence caused by light interference within submicroscopic lamellar intergrowths of exsolved plagioclase phases. This optical phenomenon produces a spectacular play of colors, typically blue, green, yellow, and red, which shifts with the angle of observation.
How to Identify
- Color
- Typically gray, dark gray, black, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) when light strikes it at certain angles. The colors can include blue, green, yellow, orange, red, and purple.
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Often massive, granular, or in tabular crystals within host rocks. The internal structure responsible for labradorescence is submicroscopic.
- Crystal Form
- Usually occurs as anhedral to subhedral grains in igneous rocks. When euhedral, it forms tabular to prismatic crystals, typically triclinic.
- Cleavage
- Perfect in two directions, nearly at right angles (approximately 86 degrees and 94 degrees), characteristic of plagioclase feldspars. This can be observed as flat, reflective surfaces.
- Geological Environment
- Found in mafic igneous rocks (gabbro, basalt, anorthosite), some intermediate igneous rocks (diorite), and certain high-grade metamorphic rocks. It is a primary constituent of anorthosite intrusions.
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, black, or greenish-gray, with characteristic iridescent play of colors (labradorescence)
- Luster: Vitreous to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal), nearly at right angles (86° and 94°)
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically An50-An70 (50-70% anorthite, 30-50% albite)
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 prismatic crystals. Twinning (e.g., Albite twinning) is common but often microscopic.
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
- Fracture Type: Uneven to conchoidal, particularly in massive specimens.
- 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. It can also be found in some metamorphic rocks, particularly those derived from mafic protoliths under high-grade conditions.
Usage
Primarily used as a gemstone and ornamental stone due to its unique play of colors (labradorescence). It is fashioned into cabochons, beads, carvings, and used in jewelry. Larger pieces are used for decorative architectural elements and countertops.
Age Distribution
Found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic.
Where to Find
Labrador, Canada
The original and type locality, where it was first discovered in 1770 by Moravian missionaries. Produces high-quality material.
Finland (Ylämaa)
Known for producing 'Spectrolite,' a variety of labradorite with an exceptionally broad and intense spectrum of colors, including all colors of the rainbow.
Madagascar
A significant source of commercial-grade labradorite, often exhibiting strong blue and green flashes.
Norway
Found in various localities, often associated with anorthosite complexes.
Russia (Kola Peninsula)
Another source of labradorite, particularly in anorthosite massifs.
United States (Oregon, New York)
Oregon produces a variety of plagioclase feldspar known as Oregon Sunstone, which can sometimes exhibit labradorescence in addition to aventurescence. New York has anorthosite occurrences with labradorite.
Australia
Found in some igneous and metamorphic terrains.
Finding Tips
Look for Mafic Igneous Rocks
Focus your search on areas known for gabbro, basalt flows, and especially anorthosite intrusions, as these are the primary host rocks for labradorite.
Observe for Labradorescence
The most distinctive feature is the play of colors. Rotate potential samples under natural light to observe the iridescent flashes. Even dull-looking gray rocks can reveal spectacular colors when properly oriented.
Check Cleavage Planes
Labradorite, like other feldspars, exhibits good cleavage. Look for flat, reflective surfaces that meet at nearly right angles (approximately 86° and 94°). These planes can also help reveal the labradorescence.
Examine Rock Outcrops
In anorthosite bodies, labradorite can be a major rock-forming mineral, appearing as large, often dark, crystals within the rock mass. Look for areas where the rock has been fractured or weathered to expose fresh surfaces.
Consider Associated Minerals
In mafic rocks, labradorite is often associated with pyroxenes (like augite) and sometimes olivine or magnetite. Identifying these associated minerals can help confirm the geological environment.
Similar Rocks
Moonstone
Orthoclase, Albite, or Oligoclase with lamellar intergrowths
Also known as: Adularia, Peristerite
Sunstone
Oligoclase or Labradorite with hematite or goethite inclusions
Also known as: Heliolite
Andesine
Andesine (An30-An50)
Also known as: Red Andesine
Scientific Classification
- Mineral Class
- Tectosilicate (Feldspar Group)
- Group
- Plagioclase Feldspar Series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution of albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with labradorite specifically having a composition of An50-An70 (50-70% anorthite component).
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