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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), pyrite (metallic yellow), and other minerals such as sodalite, hauyne, and diopside. Its characteristic intense blue color is due to the presence of lazurite, a tectosilicate mineral belonging to the sodalite group. The presence of golden pyrite inclusions is often considered a hallmark of high-quality Lapis Lazuli, while excessive white calcite can reduce its value. It typically exhibits a granular to massive texture.
How to Identify
- Color
- Intense azure blue to violet-blue, often with golden flecks (pyrite) and white streaks or patches (calcite). The blue color is typically uniform but can vary in intensity.
- Luster
- Dull to greasy, sometimes vitreous on freshly broken surfaces.
- Texture
- Fine-grained to medium-grained, granular, massive. Can feel slightly rough due to the presence of different minerals.
- Crystal Form
- Lapis Lazuli is a rock, not a single mineral, so it does not have a distinct crystal form. The constituent mineral lazurite typically forms dodecahedral crystals, but these are rarely visible in the massive rock. Pyrite may appear as small, disseminated euhedral to anhedral grains.
- Cleavage
- Lapis Lazuli, as a rock, does not exhibit cleavage. Its constituent mineral lazurite has imperfect dodecahedral cleavage, but this is not typically observed in the rock mass. Calcite has perfect rhombohedral cleavage.
- Geological Environment
- Typically found in contact metamorphic zones, particularly in metamorphosed limestones or marbles, where metasomatic alteration has occurred due to the intrusion of igneous bodies (e.g., granites). These environments provide the necessary chemical components (sulfur, sodium, aluminum, silicon) and thermal conditions for lazurite formation.
Key Facts
- Hardness: 5 to 6 (Mohs scale)
- Specific Gravity: 2.7 to 2.9
- Crystal System: Isometric (for lazurite, the primary blue mineral component)
- Color: Intense azure blue to violet-blue
- Luster: Dull 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)], with calcite (CaCO3), pyrite (FeS2), and often sodalite, hauyne, and diopside.
Quick Check
- Color: Deep blue with golden pyrite flecks and white calcite patches.
- Luster: Dull to greasy.
- Streak: Light blue (due to lazurite) to white (due to calcite).
Physical Characteristics
- Crystal Habit: Massive, granular (for the rock); dodecahedral (for individual lazurite crystals, rarely visible)
- Cleavage Type: None (as a rock); imperfect dodecahedral (for lazurite)
- Fracture Type: Uneven to conchoidal
- Tenacity: Brittle
- Luster Type: Dull to greasy
Formation
Lapis Lazuli forms through contact metamorphism of calcareous rocks (limestone or marble) that have been metasomatically altered. This process involves the introduction of sulfur-rich fluids and often occurs in association with granitic intrusions. The primary blue mineral, lazurite, is a feldspathoid, and its formation requires specific conditions of temperature (around 300-600 °C) and pressure, along with the presence of sulfur, sodium, aluminum, and silicon. The characteristic golden flecks are pyrite, and the white veins are calcite, both common accessory minerals in its formation environment.
Usage
Historically, Lapis Lazuli has been highly prized as a gemstone for jewelry, carvings, and ornamental objects. It was also ground into a pigment known as 'ultramarine' for paints, especially during the Renaissance, due to its intense blue color and lightfastness. Today, it is still used extensively in jewelry, decorative arts, and as a collector's specimen.
Age Distribution
Typically associated with contact metamorphic zones, often Precambrian to Paleozoic in age, but can be younger depending on the specific geological setting.
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, Siberia, Russia
Another important source, found in marble deposits in the Tultui and Malo-Bystrinskoye areas.
Ovalle, Coquimbo Region, Chile
Significant deposits are found in the Andes Mountains, often associated with copper mines.
Baffin Island, Nunavut, Canada
Smaller deposits have been found in metamorphic rocks.
Colorado, USA
Minor occurrences have been reported in the San Juan Mountains.
Finding Tips
Look for Metamorphic Zones
Focus your search in areas known for contact metamorphism, especially where limestones or marbles have been intruded by granitic or other igneous rocks.
Identify Associated Minerals
Lapis Lazuli is often found alongside calcite (white), pyrite (golden metallic), diopside, and other skarn minerals. The presence of these can indicate a favorable environment.
Observe Color and Texture
The distinctive deep blue color, often with golden pyrite flecks and white calcite veins, is the primary identifier. Look for a granular to massive texture.
Check for Hardness
Lapis Lazuli has a Mohs hardness of 5 to 6, meaning it can be scratched by a steel file but will scratch glass. This helps differentiate it from softer or harder blue minerals.
Acid Test (Caution Advised)
A small drop of dilute hydrochloric acid (HCl) will cause effervescence (fizzing) on the calcite components of Lapis Lazuli. This test should be performed with caution and proper safety equipment, and only on a small, inconspicuous area as it can damage the rock surface.
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)
- Group
- Sodalite Group (for lazurite)
- Crystal System
- Isometric (for lazurite)
- Chemical Formula
- A rock, not a single mineral. Primary blue component is Lazurite: (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]
- Composition
- A metamorphic rock composed of lazurite, calcite, pyrite, and other silicates.
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