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Labradorite is a tectosilicate mineral, a calcium-rich member of the plagioclase feldspar solid solution series. It is renowned for its schiller effect, known as labradorescence, which displays a brilliant iridescent play of colors, typically blues, greens, yellows, and reds, when light strikes the mineral at certain angles. This optical phenomenon is caused by internal lamellar twinning and exsolution structures within the crystal lattice. Its chemical composition ranges between An50 to An70 (50-70% anorthite, 30-50% albite).
How to Identify
- Color
- Typically gray, dark gray, black, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) including blue, green, yellow, orange, red, and purple when light hits it at specific angles.
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Often massive, granular, or in tabular crystals within host rocks. The internal texture responsible for labradorescence is microscopic lamellar twinning.
- Crystal Form
- Usually occurs as anhedral to subhedral grains in igneous rocks, or as tabular, prismatic crystals. Twinning (polysynthetic albite twinning) is common and often visible under magnification.
- Cleavage
- Perfect in two directions, intersecting at approximately 94 degrees (near 90 degrees), characteristic of plagioclase feldspars.
- Geological Environment
- Found in mafic igneous rocks (gabbro, basalt, anorthosite, norite, diorite) and some high-grade metamorphic rocks. It is a primary constituent of anorthosite massifs.
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, greenish-gray, with strong iridescent play of colors (labradorescence)
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal), intersecting at approximately 94 degrees
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.5-0.3)Al(Si1.5-1.3)O8
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. Often twinned (polysynthetic twinning).
- Cleavage Type: Perfect on {001} and {010}, intersecting at 93°20' to 94°10'.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Labradorite forms primarily in mafic igneous rocks such as basalt, gabbro, and anorthosite, where it crystallizes from magma. It can also occur in some metamorphic rocks, particularly those derived from mafic protoliths, under conditions of regional metamorphism. The characteristic iridescence (labradorescence) is due to light interference within exsolution lamellae of different plagioclase compositions (albite-rich and anorthite-rich layers) within the crystal structure.
Usage
Primarily used as an ornamental stone, in jewelry, and for decorative carvings due to its striking labradorescence. It is also used in architectural applications for facing stones and countertops. Historically, it has been used in some cultures for its perceived mystical properties.
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 discovered in 1770. Produces high-quality material.
Finland
Known for producing 'Spectrolite,' a variety of labradorite with a particularly broad and intense spectrum of colors.
Norway
Significant deposits, often associated with anorthosite intrusions.
Madagascar
Produces large quantities of gem-quality labradorite.
Russia
Found in various regions, including the Kola Peninsula.
United States
Occurrences in states like Oregon (often as sunstone, a variety of labradorite/oligoclase), New York, and Arizona.
Australia
Deposits found in various igneous rock formations.
Finding Tips
Look in Mafic Igneous Rocks
Search for labradorite in outcrops of gabbro, basalt, anorthosite, and norite. These dark-colored rocks are its primary hosts.
Observe for Labradorescence
Rotate potential samples under natural light. The characteristic iridescent flash of colors is the most definitive identifier. This effect is often best seen on freshly broken surfaces or polished faces.
Check Cleavage
Examine broken surfaces for two distinct cleavage planes intersecting at nearly 90 degrees. This is typical for plagioclase feldspars.
Consider Associated Minerals
Labradorite often occurs with pyroxenes (augite), olivine, and magnetite in mafic rocks.
Similar Rocks
Moonstone
Orthoclase, Albite, or Oligoclase (feldspar group)
Also known as: Adularia, Peristerite
Sunstone
Oligoclase or Labradorite (feldspar group)
Also known as: Heliolite
Andesine
Andesine (a plagioclase feldspar)
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 series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with labradorite specifically having a composition of An50-An70 (50-70% anorthite, 30-50% albite).
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