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Labradorite is a calcium-sodium feldspar mineral, a member of the plagioclase series. Its most distinctive feature is labradorescence, a schiller effect (a strong play of iridescent colors) that appears as a metallic luster in various shades of blue, green, yellow, orange, red, and purple when light strikes the mineral at certain angles. This optical phenomenon is caused by light diffraction within submicroscopic lamellar intergrowths of two exsolved feldspar phases. The base color of the mineral is typically dark gray, grayish-white, or greenish-gray.
How to Identify
- Color
- Typically dark gray, grayish-white, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) including blue, green, yellow, orange, red, and purple.
- Luster
- Vitreous to pearly on cleavage surfaces, with a distinctive metallic schiller (labradorescence) when oriented correctly.
- Texture
- Typically massive, granular, or in tabular crystals within igneous rocks.
- Crystal Form
- Usually occurs as anhedral to subhedral grains, or as tabular to blocky crystals. Twinning (polysynthetic twinning) is common and often visible as fine parallel lines on cleavage surfaces.
- Cleavage
- Perfect in two directions, nearly at right angles (approximately 94 degrees), characteristic of plagioclase feldspars.
- Geological Environment
- Found in mafic igneous rocks such as gabbro, basalt, norite, and anorthosite. Also occurs in some metamorphic rocks and as detrital grains in sediments derived from these sources.
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, with iridescent play of colors (labradorescence)
- Luster: Vitreous to pearly, with metallic schiller
- Transparency: Transparent to translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal), nearly at 90 degrees (94 degrees)
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.3-0.5)Al1-1.5Si2.5-3O8, with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8) end-members.
Quick Check
- Color: Dark gray to greenish-gray with iridescent flashes of blue, green, yellow, etc.
- Luster: Vitreous to pearly, with metallic schiller (labradorescence).
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to blocky crystals. Polysynthetic twinning is common.
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 94 degrees.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces; exhibits labradorescence (schiller effect).
Formation
Labradorite forms primarily in mafic igneous rocks such as basalt, gabbro, norite, and anorthosite. It crystallizes from magma rich in calcium and sodium, typically under conditions of slow cooling, allowing for the development of its characteristic lamellar twinning and exsolution lamellae responsible for labradorescence. It can also be found in some metamorphic rocks, though less commonly.
Usage
Primarily used as a gemstone and ornamental stone due to its unique optical phenomenon, labradorescence. It is carved into cabochons, beads, and sculptures. High-quality specimens are highly prized in jewelry. Less commonly, it can be used as an architectural facing stone.
Age Distribution
Found in igneous rocks of various ages, from Precambrian to Cenozoic, depending on the specific geological setting.
Where to Find
Labrador, Canada
The type locality, where it was first discovered in 1770. Produces excellent quality material.
Finland (Ylämaa)
Source of 'Spectrolite,' a highly prized variety known for its full spectrum of colors and intense labradorescence.
Norway
Significant deposits, particularly in the Larvik region, where it is found in a rock type called Larvikite (a monzonite containing abundant labradorite).
Madagascar
Produces large quantities of good quality labradorite.
Russia
Found in various locations, including the Kola Peninsula.
United States (Oregon, New York)
Oregon is known for Sunstone, which is a variety of labradorite or oligoclase. Anorthosite bodies in the Adirondack Mountains of New York also contain labradorite.
Australia
Occurrences in various igneous rock formations.
Finding Tips
Look in Mafic Igneous Rocks
Focus your search on areas with known occurrences of gabbro, basalt, norite, and especially anorthosite, which can be almost entirely composed of plagioclase feldspar, including labradorite.
Examine for Labradorescence
When examining potential specimens, rotate them under a light source (preferably natural sunlight) to observe the characteristic iridescent flash of colors. This is the primary identification feature.
Check for Cleavage
Look for two distinct cleavage planes intersecting at nearly 90 degrees. Polysynthetic twinning, appearing as fine parallel lines, may also be visible on cleavage surfaces.
Consider Associated Minerals
Labradorite is often found alongside other mafic minerals like pyroxenes (augite), olivine, and magnetite in its host rocks.
Similar Rocks
Moonstone
Orthoclase or Albite-Oligoclase
Also known as: Adularia, Peristerite
Sunstone
Oligoclase or Labradorite
Also known as: Heliolite
Andesine
Andesine
Also known as: Red Andesine
Scientific Classification
- Mineral Class
- Silicates
- 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). Labradorite specifically contains 50-70% anorthite component.
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