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Larimar is a captivating blue to blue-green variety of the mineral pectolite, characterized by its often mottled or streaky appearance, resembling tropical ocean waters. It is a relatively soft mineral, making it suitable for carving and polishing. Its unique color and limited geographic occurrence contribute to its desirability as a gemstone.
How to Identify
- Color
- Ranges from light blue to sky blue, often with white, gray, or reddish-brown streaks and patterns. The blue color can be intense, resembling the Caribbean Sea.
- Luster
- Vitreous to silky when polished, dull to sub-vitreous in rough form.
- Texture
- Typically massive, with a fine-grained, fibrous texture that is often visible on broken surfaces. Can appear somewhat translucent to opaque.
- Crystal Form
- Massive, compact, and fibrous aggregates. Individual crystals are acicular (needle-like) but rarely visible to the naked eye in Larimar.
- Cleavage
- Perfect on {100} and {001}, but rarely observed in massive Larimar specimens.
- Geological Environment
- Found exclusively in basaltic volcanic rocks, specifically in amygdaloidal cavities and fractures within highly altered basaltic andesite and basalt. It is a product of hydrothermal alteration.
Key Facts
- Hardness: 4.5-5 on the Mohs scale
- Specific Gravity: 2.74-2.88
- Crystal System: Triclinic
- Color: Light blue to sky blue, blue-green, often with white, gray, or reddish-brown patterns.
- Luster: Vitreous to silky
- Transparency: Translucent to opaque
- Fracture: Uneven to splintery
- Cleavage: Perfect on {100} and {001}
- Composition: Hydrous sodium calcium silicate
Quick Check
- Color: Sky blue to blue-green, often with white streaks.
- Luster: Vitreous to silky.
- Streak: White.
Physical Characteristics
- Crystal Habit: Massive, compact, fibrous aggregates; individual crystals are acicular (needle-like) but microscopic.
- Cleavage Type: Perfect in two directions, {100} and {001}, at nearly 90 degrees.
- Fracture Type: Uneven to splintery, due to its fibrous nature.
- Tenacity: Brittle
- Luster Type: Vitreous to silky, especially on polished surfaces.
Formation
Larimar is a rare blue variety of the mineral pectolite. It forms in the vugs and cavities within basaltic lavas, specifically within highly altered basaltic andesite and basaltic rocks. The distinctive blue color is attributed to the substitution of copper for calcium within the pectolite structure. Hydrothermal fluids carrying dissolved minerals, including copper, percolate through the volcanic rock, depositing pectolite in these open spaces as the fluids cool and pressure decreases. The fibrous, radiating crystal habit of pectolite is characteristic, and in Larimar, these fibers are often intergrown, creating a solid mass.
Usage
Primarily used as a gemstone for jewelry due to its attractive blue to blue-green color and unique patterns. It is typically cut into cabochons, beads, and polished freeform shapes. It is also collected as a mineral specimen.
Age Distribution
Formed during the Miocene epoch, approximately 1.5 to 5 million years ago.
Where to Find
Barahona Province, Dominican Republic
The only known commercial source of Larimar in the world. It is found in the Sierra de Bahoruco mountain range, specifically in the Los Chupaderos area, near the city of Barahona. The deposits are located in ancient volcanic flows.
Finding Tips
Geological Context
Larimar is found in specific geological settings: within the amygdaloidal cavities of altered basaltic volcanic rocks. Prospecting would involve identifying such formations.
Color and Pattern Recognition
Look for the characteristic blue to blue-green color with white, gray, or reddish-brown mottling. The patterns often resemble water or clouds.
Associated Minerals
Larimar can be found alongside other minerals common in volcanic vugs, such as calcite, hematite, and various zeolites, though these are not always present.
Hardness Test
Due to its relatively low hardness (4.5-5 on the Mohs scale), it can be scratched by harder materials like quartz. This can help differentiate it from harder blue minerals.
Similar Rocks
Turquoise
CuAl6(PO4)4(OH)8·4H2O
Also known as: Callais
Chrysocolla
(Cu,Al)2H2Si2O5(OH)4·nH2O
Also known as: Gem Silica
Blue Chalcedony
SiO2
Also known as: Blue Lace Agate (a banded variety)
Scientific Classification
- Mineral Class
- Inosilicates (chain silicates)
- Group
- Pectolite Group
- Crystal System
- Triclinic
- Chemical Formula
- NaCa2Si3O8(OH)
- Composition
- Sodium calcium silicate hydroxide. The blue color is due to trace amounts of copper substituting for calcium.
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