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Labradorite is a member of the plagioclase feldspar series, specifically an intermediate to calcic plagioclase with a composition ranging from approximately An50 to An70 (50-70% anorthite, 30-50% albite). Its most distinctive feature is labradorescence, a schiller effect or play of colors (typically blue, green, yellow, orange, red, and purple) caused by light interference within microscopic lamellar exsolution intergrowths of two feldspar phases (Bøggild intergrowth). These lamellae are typically 50 to 150 nanometers thick. The specific colors observed depend on the thickness of these lamellae and the angle of incident light. Without this optical phenomenon, labradorite appears dull gray to grayish-black.
How to Identify
- Color
- Typically gray, dark gray, greenish-gray, or black in ordinary light. Exhibits strong labradorescence (iridescence) with flashes of blue, green, yellow, orange, red, and purple when light strikes it at certain 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 intergrowths.
- Crystal Form
- Usually occurs as anhedral to subhedral grains in igneous rocks. When euhedral, it forms tabular to prismatic crystals, often twinned (polysynthetic twinning, visible as fine parallel lines on cleavage surfaces).
- Cleavage
- Perfect in two directions, intersecting at approximately 94 degrees (basal {001} and pinacoidal {010}). A third, less perfect cleavage may be present.
- Geological Environment
- Mafic igneous rocks (gabbro, basalt, norite, anorthosite), some metamorphic rocks, and rarely in detrital sediments.
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, black; exhibits labradorescence (iridescent play of colors)
- Luster: Vitreous to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions ({001} and {010}) at approximately 94 degrees
- Composition: Sodium calcium aluminum silicate (Na,Ca)(Al,Si)4O8, specifically an intermediate member of the plagioclase solid solution series (An50-An70)
Quick Check
- Color: Dull gray to black, 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 basal {001} and perfect pinacoidal {010}, intersecting at 94°12'.
- 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, norite, 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 occur in some metamorphic rocks, particularly those derived from mafic protoliths, and rarely in detrital sediments.
Usage
Labradorite is highly prized as an ornamental stone and gemstone due to its unique play of color (labradorescence). It is used in jewelry, carvings, and decorative objects. Large blocks are sometimes used as facing stone for buildings. Due to its hardness and durability, it can also be used as an aggregate in construction, though this is less common given its aesthetic value.
Age Distribution
Labradorite can be found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting of its formation.
Where to Find
Labrador, Canada
The original and type locality, where it was first discovered in 1770 by Moravian missionaries. Found in anorthosite intrusions.
Finland
Known for the high-quality variety called Spectrolite, which exhibits a full spectrum of colors. Found in anorthosite and rapakivi granite intrusions.
Norway
Occurs in various igneous and metamorphic rocks, often exhibiting strong labradorescence.
Madagascar
Significant source of gem-quality labradorite, often with vibrant blue and green flashes.
Russia
Found in anorthosite massifs, particularly in the Kola Peninsula.
United States (Oregon)
Oregon Sunstone, a variety of labradorite, is known for its aventurescence (schiller caused by copper inclusions) and can also exhibit labradorescence.
Australia
Occurrences in various igneous rocks.
Mexico
Sources of labradorite with good color play.
Finding Tips
Look for Mafic Igneous Rocks
Labradorite is most commonly found in dark-colored igneous rocks like gabbro, basalt, norite, and especially anorthosite. Focus your search in areas known for these rock types.
Observe the Play of Color
The most distinctive feature is labradorescence. Rotate suspected samples under natural light to observe flashes of blue, green, yellow, or other colors. This effect is often best seen on freshly broken surfaces or polished faces.
Check for Cleavage
Feldspars, including labradorite, exhibit good cleavage. Look for two distinct cleavage planes intersecting at nearly right angles (approximately 94 degrees). Polysynthetic twinning lines may also be visible on cleavage surfaces.
Consider Associated Minerals
In its typical geological environment, labradorite is often associated with other mafic minerals such as pyroxenes (e.g., augite) and olivine, and sometimes magnetite.
Examine Rock Outcrops and Glacial Till
Search in bedrock outcrops of anorthosite or gabbro. In glaciated regions, look for distinctive 'float' (loose rocks) in glacial till, as labradorite-bearing rocks can be transported long distances.
Similar Rocks
Andesine
Plagioclase Feldspar (Na,Ca)(Al,Si)4O8
Also known as: Red Andesine
Oligoclase
Plagioclase Feldspar (Na,Ca)(Al,Si)4O8
Also known as: Sunstone (a variety)
Bytownite
Plagioclase Feldspar (Na,Ca)(Al,Si)4O8
Also known as: None
Scientific Classification
- Mineral Class
- Silicates
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Na,Ca)(Al,Si)4O8
- Composition
- A solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with labradorite having a composition of 50-70% anorthite (An50-An70).
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