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Lapis Lazuli is a deep-blue metamorphic rock composed primarily of lazurite (25-40%), along with varying amounts of calcite (white streaks), pyrite (metallic gold flecks), and other minerals such as sodalite, hauyne, wollastonite, and diopside. Its characteristic intense blue color is due to the lazurite component. The presence of white calcite veins and golden pyrite inclusions are common and often considered desirable features, though excessive amounts can reduce its value. It is typically opaque with a vitreous to greasy luster.
How to Identify
- Color
- Intense azure blue to violet-blue, often with white calcite veins and golden pyrite flecks. The blue color is uniform in higher quality specimens.
- Luster
- Vitreous to greasy, sometimes dull on rough surfaces.
- Texture
- Fine-grained to medium-grained, granular, often with visible inclusions of calcite and pyrite.
- Crystal Form
- Lapis Lazuli is a rock, not a single mineral, so it does not have a distinct crystal form. Lazurite, its primary component, forms dodecahedral or cubic crystals, but these are rarely visible within the rock matrix.
- Cleavage
- No distinct cleavage as a rock. Lazurite itself exhibits imperfect dodecahedral cleavage.
- Geological Environment
- Typically found in contact metamorphic zones, particularly in metamorphosed limestones or marbles, often associated with skarn deposits. It forms where sulfur-rich fluids interact with carbonate rocks at elevated temperatures and pressures.
Key Facts
- Hardness: 5-6 (Mohs scale)
- Specific Gravity: 2.7-2.9 g/cm
- Crystal System: Cubic (for lazurite component, but Lapis Lazuli is a rock)
- Color: Intense azure blue to violet-blue
- Luster: Vitreous to greasy
- Transparency: Opaque
- Fracture: Uneven to conchoidal
- Cleavage: None (as a rock); imperfect dodecahedral (for lazurite)
- Composition: A rock composed primarily of lazurite (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)], calcite (CaCO3), and pyrite (FeS2), with minor amounts of sodalite, hauyne, and other silicates.
Quick Check
- Color: Deep blue with white and gold flecks
- Luster: Vitreous to greasy
- Streak: Light blue (for lazurite component)
Physical Characteristics
- Crystal Habit: Massive, granular (as a rock); lazurite can form dodecahedral crystals but rarely visible.
- Cleavage Type: None (as a rock); imperfect dodecahedral (for lazurite).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Lapis Lazuli forms through contact metamorphism of calcareous rocks (limestone or marble) in the presence of sulfur-rich fluids. This process typically occurs at temperatures between 300-600 C and pressures of 1-5 kbar. The blue color is primarily due to the presence of lazurite, a tectosilicate mineral of the sodalite group, which incorporates sulfur radicals (S3-) into its structure. The associated minerals like calcite and pyrite are integral to its formation, with calcite providing the host rock and pyrite forming under reducing conditions.
Usage
Historically, Lapis Lazuli has been highly prized as a gemstone, ornamental stone, and pigment (ultramarine). It is used in jewelry, carvings, mosaics, and architectural decoration. Its use as a pigment dates back millennia, valued for its intense blue color and lightfastness, though synthetic ultramarine has largely replaced it in modern art.
Age Distribution
Precambrian to Cenozoic, depending on the specific geological setting and associated metamorphic events.
Where to Find
Sar-e-Sang, Badakhshan Province, Afghanistan
The most historically significant and primary source of high-quality Lapis Lazuli for over 6,000 years. Occurs in crystalline marble within the Kokcha River valley.
Lake Baikal region, Siberia, Russia
Significant deposits found in the Malo-Bystrinskoye deposit, occurring in metamorphosed carbonate rocks.
Ovalle, Coquimbo Region, Chile
Deposits found in the Andes Mountains, often associated with copper mining operations. Chilean Lapis Lazuli tends to have more white calcite than Afghan material.
Baffin Island, Nunavut, Canada
More recent discoveries of Lapis Lazuli have been made in this Arctic region.
Colorado, USA
Minor occurrences have been reported in the United States, particularly in the Italian Mountain area.
Finding Tips
Geological Context
Focus on areas known for contact metamorphism of calcareous rocks, especially where sulfur-rich fluids were present. Look for marble or limestone outcrops near intrusive igneous bodies.
Associated Minerals
Search for areas with visible pyrite, calcite, and other skarn minerals, as these are often found in proximity to Lapis Lazuli deposits.
Color and Inclusions
Identify the characteristic deep blue color, often with white calcite veins and golden pyrite flecks. Be aware that excessive calcite or pyrite can indicate lower quality material.
Hardness Test
Lapis Lazuli has a Mohs hardness of 5-6. It can be scratched by a steel file but will scratch glass. This helps differentiate it from softer minerals.
Acid Test (Caution)
A small drop of dilute hydrochloric acid (HCl) will cause a slight effervescence on the calcite inclusions, releasing hydrogen sulfide (rotten egg smell) from the lazurite component due to the sulfur content. This test should be performed with extreme caution and proper ventilation due to the release of H2S gas, which is toxic. It is generally not recommended for collectors without proper safety equipment and knowledge.
Similar Rocks
Sodalite
Sodalite
Also known as: Blue Sodalite
Dumortierite
Dumortierite
Also known as: Blue Quartz
Azurite
Azurite
Also known as: Chessylite
Scientific Classification
- Mineral Class
- Tectosilicate (for lazurite component)
- Group
- Sodalite Group (for lazurite component)
- Crystal System
- Cubic (for lazurite component)
- Chemical Formula
- Rock, not a single mineral. Primary component lazurite: (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]
- Composition
- A metamorphic rock consisting of lazurite, calcite, pyrite, and other silicates.
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