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Lapis Lazuli is a deep blue metamorphic rock composed primarily of lazurite (a feldspathoid mineral), along with varying amounts of calcite (white streaks), pyrite (golden flecks), and other minerals such as sodalite, hauyne, and diopside. Its characteristic intense blue color is due to the presence of lazurite. The quality of Lapis Lazuli is often judged by the intensity and uniformity of its blue color, with less calcite and pyrite generally indicating higher quality, though some pyrite is often desired for aesthetic appeal.
How to Identify
- Color
- Intense azure blue to violet-blue, often with white streaks (calcite) and golden flecks (pyrite). The blue color is typically uniform but can show variations.
- Luster
- Dull to greasy, sometimes vitreous on fractured surfaces.
- Texture
- Fine-grained to medium-grained, massive. Can appear granular due to the intergrowth of constituent minerals.
- Crystal Form
- Lapis Lazuli is a rock, so it does not have a single crystal form. Lazurite, its primary component, typically forms dodecahedral crystals, but in Lapis Lazuli, it is usually massive and anhedral (lacking well-formed crystal faces).
- Cleavage
- Lapis Lazuli, as a rock, does not exhibit cleavage. Its constituent minerals have cleavage: lazurite has imperfect cleavage on {110}, calcite has perfect rhombohedral cleavage, and pyrite has no cleavage.
- Geological Environment
- Typically found in contact metamorphic zones, particularly in metamorphosed limestones or marbles, often associated with granitic intrusions or other igneous bodies that provide the necessary heat and metasomatic fluids.
Key Facts
- Hardness: 5 to 5.5 (Mohs scale, due to the mixture of minerals)
- Specific Gravity: 2.7 to 2.9 (variable depending on mineral composition)
- Crystal System: Lapis Lazuli is a rock; lazurite (its primary component) is isometric (cubic).
- Color: Intense azure blue to violet-blue, often with white (calcite) and golden (pyrite) inclusions.
- Luster: Dull to greasy, sometimes vitreous.
- Transparency: Opaque
- Fracture: Uneven to conchoidal
- Cleavage: None (as a rock); lazurite has imperfect cleavage, calcite has perfect rhombohedral cleavage.
- Composition: A metamorphic rock composed primarily of lazurite (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)], calcite (CaCO3), and pyrite (FeS2). May also contain sodalite, hauyne, diopside, and other silicates.
Quick Check
- Color: Deep blue with white and golden inclusions
- Luster: Dull to greasy
- Streak: Light blue (due to lazurite component)
Physical Characteristics
- Crystal Habit: Massive, granular (for the rock); lazurite can form dodecahedral crystals but is usually anhedral within the rock.
- Cleavage Type: Not applicable for the rock; constituent minerals vary.
- Fracture Type: Uneven to conchoidal
- Tenacity: Brittle
- Luster Type: Dull to greasy
Formation
Lapis Lazuli forms through contact metamorphism or regional metamorphism of calcareous rocks (limestone or marble) that have been metasomatically altered. This process involves the introduction of sulfur-rich fluids and other elements, leading to the formation of lazurite, the primary blue mineral component. The presence of associated minerals like calcite, pyrite, and diopside is characteristic of this formation environment.
Usage
Historically and currently used as a gemstone, ornamental stone, and pigment (ultramarine). Its intense blue color made it highly prized for jewelry, carvings, and in ancient times, for painting and cosmetics. Modern uses include cabochons, beads, small sculptures, and inlay work.
Age Distribution
Typically associated with regional metamorphism, ages vary widely depending on the geological setting of the parent rocks. Formations can range from Precambrian to Cenozoic.
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 metamorphosed Precambrian marbles.
Lake Baikal, Siberia, Russia
Significant deposits found in the Malo-Bystrinskoye deposit, occurring in Precambrian marbles and skarns.
Ovalle, Coquimbo Region, Chile
Deposits found in the Andes Mountains, associated with contact metamorphosed limestones.
Baffin Island, Nunavut, Canada
Smaller deposits found in metamorphic rocks.
Colorado, USA
Minor occurrences, such as in the Italian Mountain area, associated with contact metamorphic zones.
Finding Tips
Geological Context
Focus on areas with known occurrences of contact metamorphosed calcareous rocks (limestone, marble) near igneous intrusions. Skarn environments are particularly favorable.
Associated Minerals
Look for the presence of white calcite, golden pyrite, and sometimes diopside or other silicate minerals, which are common associates of Lapis Lazuli.
Color and Luster
Identify the characteristic intense blue color and the dull to greasy luster. The presence of white streaks and golden flecks is a strong indicator.
Hardness Test
While not a definitive test for the rock, the overall hardness of Lapis Lazuli (5-5.5 on Mohs scale) can help distinguish it from softer minerals like azurite (3.5-4) or harder ones.
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
- Not a single mineral, but a rock. Its primary blue component, lazurite, is a tectosilicate (feldspathoid).
- Group
- Metamorphic Rock (specifically, a contact or regionally metamorphosed calcareous rock)
- Crystal System
- Not applicable for the rock; lazurite is isometric.
- Chemical Formula
- Variable, as it is a rock. The main blue component, lazurite, is (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)].
- Composition
- A complex mixture of minerals, predominantly lazurite, calcite, and pyrite, with minor amounts of sodalite, hauyne, and other silicates.
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