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Lapis Lazuli is a vibrant blue metamorphic rock composed primarily of the mineral lazurite (a tectosilicate mineral of the sodalite group), along with varying amounts of calcite (white streaks), and pyrite (golden metallic flecks). The intensity of the blue color is directly related to the lazurite content. The presence of calcite can lighten the overall color, while pyrite inclusions add a distinctive golden sparkle. It is a relatively soft rock, making it suitable for carving and polishing.
How to Identify
- Color
- Intense azure to violet-blue, often with white calcite veins and golden pyrite specks. The blue color is due to the lazurite component.
- Luster
- Dull to greasy, sometimes vitreous on freshly broken surfaces.
- Texture
- Fine-grained to medium-grained, often granular. Can appear massive.
- Crystal Form
- Typically massive, compact, or granular. Individual lazurite crystals are rare and usually dodecahedral.
- Cleavage
- Lazurite exhibits imperfect dodecahedral cleavage, but in the rock, it is generally not observed due to its massive nature. Calcite has perfect rhombohedral cleavage.
- Geological Environment
- Formed in contact metamorphic zones, specifically in metamorphosed limestones or marbles that have been metasomatically altered by silica-poor, sulfur-rich fluids from nearby igneous intrusions.
Key Facts
- Hardness: 5-5.5 (Mohs scale)
- Specific Gravity: 2.7-2.9 g/cm³
- Crystal System: Cubic (for lazurite component)
- Color: Intense azure to violet-blue, often with white calcite and golden pyrite inclusions
- Luster: Dull to greasy, sometimes vitreous
- Transparency: Opaque
- Fracture: Uneven to conchoidal
- Cleavage: Imperfect dodecahedral (for lazurite), perfect rhombohedral (for calcite)
- Composition: A rock composed primarily of lazurite (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)], calcite (CaCO3), and pyrite (FeS2). Minor constituents can include sodalite, hauyne, diopside, and mica.
Quick Check
- Color: Deep blue with white and golden flecks
- Luster: Dull to greasy
- Streak: Light blue to white
Physical Characteristics
- Crystal Habit: Massive, granular, compact. Rarely forms distinct dodecahedral crystals.
- Cleavage Type: Imperfect dodecahedral for lazurite; perfect rhombohedral for calcite.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Dull to greasy, sometimes vitreous.
Formation
Lapis Lazuli forms through contact metamorphism of calcareous rocks (limestone or marble) with silica-poor igneous intrusions. The presence of sulfur-rich conditions is crucial for the formation of lazurite. The process involves metasomatism, where chemical alteration occurs due to the introduction of fluids from the igneous body. The primary minerals involved are lazurite, calcite, and pyrite, with minor amounts of diopside, sodalite, hauyne, and other silicates.
Usage
Historically, Lapis Lazuli has been highly prized as a gemstone, ornamental stone, and for carving. It was famously used to produce the pigment ultramarine, which was highly valued by artists for its intense blue color. Today, it is still used in jewelry, decorative objects, and as an inlay material. Its historical significance is immense, being found in ancient Egyptian tombs and used by various ancient civilizations.
Age Distribution
Typically found in geological formations ranging from Precambrian to Cenozoic, with significant deposits in the Paleozoic era.
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
Significant deposits found in the Malo-Bystrinskoye deposit, occurring in marbles and skarns.
Ovalle, Coquimbo Region, Chile
Deposits found in the Andes Mountains, often associated with copper deposits, occurring in metamorphosed limestones.
Baffin Island, Nunavut, Canada
More recently discovered deposits, also associated with metamorphosed carbonate rocks.
Colorado, USA
Minor occurrences, notably in the Italian Mountain area, associated with contact metamorphic zones.
Finding Tips
Geological Context
Focus on areas with known contact metamorphic zones, particularly where limestone or marble has been intruded by silica-poor igneous rocks. Look for skarn-like environments.
Associated Minerals
Lapis Lazuli is often found with other metamorphic minerals such as diopside, forsterite, mica, and sometimes graphite. The presence of white calcite and golden pyrite is a key indicator.
Color and Luster
Look for intense blue rocks with a dull to greasy luster. The characteristic white streaks of calcite and metallic golden flecks of pyrite are crucial for identification.
Hardness Test
Lapis Lazuli is relatively soft (Mohs 5-5.5). It can be scratched by a steel file but will scratch glass. This helps differentiate it from harder blue minerals.
Acid Test (Caution)
A small drop of dilute hydrochloric acid (HCl) will cause a slight effervescence on the calcite component, and a distinct rotten egg smell (H2S) may be produced from the lazurite due to its sulfur content. *Perform this test with extreme caution and proper ventilation, and only on a small, inconspicuous area.*
Similar Rocks
Sodalite
Na4Al3Si3O12Cl
Also known as: Blue Sodalite
Azurite
Cu3(CO3)2(OH)2
Also known as: Chessylite
Dumortierite
Al7(BO3)(SiO4)3O3
Also known as: Blue Quartz
Scientific Classification
- Mineral Class
- Tectosilicate (for lazurite), Carbonate (for calcite), Sulfide (for pyrite)
- Group
- Sodalite Group (for lazurite)
- Crystal System
- Cubic (for lazurite and pyrite), Trigonal (for calcite)
- Chemical Formula
- A rock, not a single mineral. Main components: Lazurite: (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)], Calcite: CaCO3, Pyrite: FeS2.
- Composition
- A metamorphic rock consisting of 25-40% lazurite, with significant amounts of calcite and pyrite, and minor amounts of other minerals like sodalite, hauyne, and diopside.
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