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Labradorite is a tectosilicate mineral, a calcium-sodium feldspar, belonging to the plagioclase series. It is renowned for its striking optical phenomenon known as labradorescence, a schiller effect that produces a brilliant iridescent play of colors, typically blues, greens, yellows, and reds, when light strikes the mineral at certain angles. This effect is due to submicroscopic lamellar intergrowths within the crystal structure. The base color of the mineral is typically dark gray, black, or greenish-gray, providing a stark contrast to the vibrant flashes of color.
How to Identify
- Color
- Typically dark gray, black, or greenish-gray, with iridescent flashes of blue, green, yellow, orange, and red (labradorescence).
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Often massive, granular, or in tabular crystals within host rocks.
- Crystal Form
- Usually anhedral to subhedral grains in igneous rocks; can form tabular, prismatic crystals in pegmatites or vugs.
- Cleavage
- Two distinct cleavages at nearly 90 degrees (86 degrees and 94 degrees), characteristic of feldspars.
- Geological Environment
- Common in mafic igneous rocks such as gabbro, basalt, anorthosite, and norite. Also found in some high-grade metamorphic rocks and as detrital grains in sediments.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.68 to 2.72
- Crystal System: Triclinic
- Color: Dark gray, black, or greenish-gray, with iridescent blue, green, yellow, orange, red flashes (labradorescence)
- Luster: Vitreous to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal) at nearly 90 degrees (86° and 94°)
- Composition: Calcium-sodium aluminum silicate (Ca,Na)(Al,Si)AlSi2O8, with an anorthite content typically between An50 and An70 (50-70% anorthite, 30-50% albite).
Quick Check
- Color: Dark gray to black with iridescent flashes (labradorescence)
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Often anhedral to subhedral.
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Labradorite is a member of the plagioclase feldspar solid solution series, specifically an intermediate to calcic plagioclase. It forms primarily during the crystallization of mafic igneous rocks (e.g., gabbro, basalt, anorthosite) and also occurs in some metamorphic rocks. Its distinctive iridescence (labradorescence) is caused by light interference within microscopic lamellar exsolution intergrowths of two different feldspar compositions (typically albite-rich and anorthite-rich layers) that have unmixed during slow cooling.
Usage
Primarily used as a gemstone and ornamental stone due to its unique play of color. It is also used in architectural applications (countertops, tiles) and as a collector's mineral specimen. Historically, it has been used in jewelry for centuries.
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 by Moravian missionaries. Large deposits of anorthosite containing labradorite are found here.
Finland
Known for producing the highest quality labradorite, often called 'Spectrolite,' which exhibits a full spectrum of colors.
Norway
Significant deposits, particularly in the Larvik area, where a variety known as 'Larvikite' (a monzonite rock rich in labradorite) is quarried.
Madagascar
Produces large quantities of gem-quality labradorite.
Russia
Deposits found in various regions.
United States
Occurrences in states like Oregon (as Sunstone, a variety of oligoclase-andesine with aventurescence, sometimes confused with labradorite), and New York (in anorthosite).
Australia
Found in some igneous complexes.
Mexico
Known for some occurrences.
Finding Tips
Look in Mafic Igneous Rocks
Search for labradorite in outcrops of gabbro, basalt, and especially anorthosite, where it can be a major rock-forming mineral.
Observe for Labradorescence
Rotate potential specimens under natural light to observe the characteristic iridescent flashes of color. This is the most definitive visual identifier.
Check Cleavage
Examine broken surfaces for the two distinct cleavage planes intersecting at nearly 90 degrees, typical of feldspars.
Consider Associated Minerals
Labradorite often occurs with pyroxenes (e.g., augite), olivine, and magnetite in mafic rocks.
Field Identification
While labradorescence is key, its dark gray to black base color and vitreous luster on fresh surfaces can help distinguish it from other common minerals in the field.
Similar Rocks
Andesine
Andesine (Na,Ca)(Al,Si)AlSi2O8
Also known as: Red Labradorite (misnomer)
Oligoclase
Oligoclase (Na,Ca)(Al,Si)AlSi2O8
Also known as: Sunstone (a variety)
Anorthite
Anorthite CaAl2Si2O8
Also known as: Calcium Feldspar
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution mineral with an anorthite (CaAl2Si2O8) content ranging from 50% to 70% and an albite (NaAlSi3O8) content ranging from 30% to 50%.
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