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Labradorite is a tectosilicate mineral, a calcium-sodium feldspar, belonging to the plagioclase series. Its most striking feature is labradorescence, a schiller effect (iridescence) caused by light interference within submicroscopic lamellar exsolution intergrowths of albite and anorthite. These intergrowths are typically oriented parallel to the (010) plane. The colors displayed can range from blues, greens, yellows, and oranges to reds and purples, often appearing as a flash of color when the mineral is viewed from different angles. The specific composition of labradorite falls within the range of An50 to An70 (50-70% anorthite, 30-50% albite).
How to Identify
- Color
- Typically gray, dark gray, greenish-gray, or brownish-gray. The most distinctive feature is the iridescent play of colors (labradorescence) in blues, greens, yellows, oranges, reds, and purples.
- 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 submicroscopic lamellar twinning/exsolution.
- Crystal Form
- Usually occurs as anhedral to subhedral grains in igneous rocks, or as tabular to blocky crystals. Crystals are triclinic.
- Cleavage
- Perfect in two directions, intersecting at approximately 94 degrees (near 90 degrees), and good in a third direction. This produces characteristic step-like fractures.
- Geological Environment
- Mafic igneous rocks (gabbro, basalt, norite, anorthosite), and some high-grade metamorphic rocks. It is a primary constituent of anorthosite bodies.
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, with iridescent flashes of blue, green, yellow, orange, red, purple (labradorescence)
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions ({001} and {010}) intersecting at approximately 94 degrees, good in a third direction ({110})
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)4O8 with An50-An70 (50-70% anorthite, 30-50% albite)
Quick Check
- Color: Gray to dark gray with iridescent flashes (labradorescence)
- 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 basal ({001}) and perfect pinacoidal ({010}), intersecting at 93°12' to 94°10'. Good prismatic ({110}).
- 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. It crystallizes from magma rich in calcium and sodium, typically at high temperatures. It can also be found in some metamorphic rocks.
Usage
Primarily used as a gemstone, ornamental stone, and for decorative purposes due to its unique labradorescence. It is also used in architectural applications and as a component in some ceramics.
Age Distribution
Found in igneous and metamorphic rocks of various ages, particularly in anorthosite intrusions which can be Precambrian to Phanerozoic.
Where to Find
Labrador, Canada
The type locality, where it was first discovered in 1770 by Moravian missionaries. Known for producing high-quality material.
Finland
Produces a highly prized variety known as Spectrolite, which exhibits a full spectrum of colors due to a finer lamellar structure.
Norway
Significant deposits, particularly in the Larvik region, where it is found in a rock called Larvikite (a monzonite containing abundant labradorite).
Madagascar
A major source of gem-quality labradorite.
Russia
Deposits found in various regions.
United States
Occurrences in states like Oregon (often with aventurescence, sometimes called Oregon Sunstone), New York, and others.
Australia
Some occurrences reported.
Finding Tips
Look for Mafic Igneous Rocks
Labradorite is common in dark-colored igneous rocks like gabbro, basalt, and especially anorthosite. Focus your search in areas known for these rock types.
Observe the Schiller Effect
The most distinctive feature is labradorescence. Rotate suspected samples under a light source to observe flashes of iridescent color. This effect is often best seen on freshly broken or polished surfaces.
Check for Cleavage
Feldspars, including labradorite, exhibit good cleavage. Look for flat, reflective surfaces that break along distinct planes, often at nearly right angles.
Consider Associated Minerals
In anorthosite, labradorite is the dominant mineral. In gabbro or basalt, it will be associated with pyroxenes (e.g., augite) and olivine.
Examine Rock Outcrops
Large anorthosite intrusions can form significant rock masses. Look for these in geological maps of potential areas.
Similar Rocks
Moonstone
Orthoclase or Albite (with adularescence)
Also known as: Adularia, Peristerite
Sunstone
Oligoclase or Labradorite (with aventurescence)
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). Labradorite specifically has a composition of An50-An70 (50-70% anorthite, 30-50% albite).
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