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Aquamarine is a blue to blue-green variety of the mineral beryl. When found 'in matrix,' it means the aquamarine crystals are still embedded within their original host rock. This presentation provides a natural context for the gemstone, often showcasing the crystal habit and association with other minerals like feldspar, quartz, muscovite, or tourmaline. The color of aquamarine ranges from pale blue to deep sea-blue, with the most valuable specimens exhibiting a clear, intense blue. The crystals are typically hexagonal prisms, often with striations parallel to the c-axis.
How to Identify
- Color
- Pale blue to deep blue, sometimes with a greenish tint. The color is typically uniform within a single crystal, though zoning can occur. The matrix color will vary depending on the host rock (e.g., white/pink for feldspar, grey for quartz, silvery for muscovite).
- Luster
- Vitreous (glassy) on crystal faces and fracture surfaces for aquamarine. The matrix minerals will have their characteristic lusters (e.g., vitreous for quartz, pearly for muscovite, dull to vitreous for feldspar).
- Texture
- Aquamarine crystals are typically smooth on their prismatic faces, sometimes with fine striations. The matrix can be granular (pegmatite), massive, or foliated (schist), depending on the rock type.
- Crystal Form
- Aquamarine forms hexagonal prisms, often terminated by pinacoids or pyramidal faces. Crystals can be euhedral (well-formed) to anhedral (irregularly shaped) depending on growth conditions. When in matrix, well-formed crystals are highly sought after.
- Cleavage
- Aquamarine has imperfect basal cleavage {0001}, which is rarely observed. The matrix minerals will have their own distinct cleavage patterns (e.g., perfect cleavage in feldspar and muscovite, no cleavage in quartz).
- Geological Environment
- Primarily granitic pegmatites, but also hydrothermal veins and sometimes metamorphic rocks. Look for coarse-grained igneous rocks or veins cutting through other rock types.
Key Facts
- Hardness: 7.5 - 8 on the Mohs scale (for aquamarine)
- Specific Gravity: 2.68 - 2.80 g/cm³ (for aquamarine)
- Crystal System: Hexagonal
- Color: Blue, blue-green
- Luster: Vitreous
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Imperfect basal {0001}
- Composition: Beryllium aluminum silicate (Be3Al2Si6O18)
Quick Check
- Color: Blue to blue-green
- Luster: Vitreous (glassy)
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, typically hexagonal prisms, often striated parallel to the c-axis. Can be massive or granular.
- Cleavage Type: Imperfect basal {0001}, rarely observed.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous (glassy)
Formation
Aquamarine forms in granitic pegmatites, which are coarse-grained igneous rocks that crystallize from residual magma enriched in volatile components (like water, fluorine, and boron) and incompatible elements (like beryllium, lithium, and tantalum). It also occurs in hydrothermal veins and sometimes in metamorphic rocks. The characteristic blue color is due to trace amounts of iron (Fe2+ and Fe3+) substituting for aluminum in the beryl crystal structure. The 'matrix' refers to the surrounding host rock, which can be pegmatite, granite, schist, or other associated rock types.
Usage
Aquamarine is a highly prized gemstone, used extensively in jewelry. Specimens 'in matrix' are particularly valued by collectors for their aesthetic appeal and for demonstrating the natural geological environment of the crystal's formation. Beryl, the parent mineral, is also a primary ore of beryllium, a light and strong metal used in aerospace, nuclear, and electronics industries. However, aquamarine itself is rarely mined solely for beryllium due to its value as a gemstone.
Age Distribution
Typically found in granitic pegmatites and hydrothermal veins, which can range from Precambrian to Cenozoic in age, depending on the specific geological setting.
Where to Find
Minas Gerais, Brazil
World-renowned for producing large, high-quality aquamarine crystals, often found in pegmatite pockets. Famous localities include the Marambaia, Teófilo Otoni, and Governador Valadares regions.
Pakistan (Gilgit-Baltistan)
Known for producing fine aquamarine crystals, often with excellent clarity and color, from pegmatite deposits in high-altitude regions.
Nigeria
Significant producer of aquamarine, particularly from the Jos Plateau region, often found in pegmatites.
Madagascar
Produces aquamarine from pegmatite occurrences, often with a distinct blue-green hue.
United States (Colorado)
Mount Antero in Colorado is famous for producing aquamarine, often associated with phenakite and fluorite, from high-altitude granitic pegmatites.
Russia (Ural Mountains)
Historical source of aquamarine, particularly from the Mursinka-Alabashka region.
Afghanistan
Produces aquamarine from pegmatite deposits, often associated with other gem minerals.
Finding Tips
Target Pegmatite Outcrops
Focus your search on areas known for granitic pegmatites. These are often found in regions with a history of continental collision and mountain building.
Look for Associated Minerals
Aquamarine is commonly found with quartz, feldspar (orthoclase, albite), muscovite, biotite, and sometimes tourmaline, garnet, or topaz. Identifying these associated minerals can indicate the presence of a beryl-bearing pegmatite.
Examine Weathered Zones
Weathering can expose pegmatite veins and release crystals. Look for crystal fragments in stream beds or eroded slopes downstream from pegmatite outcrops.
Check for Hexagonal Forms
Keep an eye out for the characteristic hexagonal prismatic crystal habit of beryl. Even small fragments can show this form.
Safety Precautions
When collecting in pegmatite environments, be aware of unstable rock, potential for falling debris, and the need for appropriate safety gear (hard hat, gloves, eye protection). Some pegmatites can contain radioactive minerals, so use a Geiger counter if there's a possibility.
Similar Rocks
Topaz
Al2SiO4(F,OH)2
Also known as: Aluminum silicate fluoride hydroxide
Blue Tourmaline
Na(Li,Al)3Al6(BO3)3Si6O18(OH)4 (Elbaite variety)
Also known as: Indicolite
Euclase
BeAlSiO4(OH)
Also known as: Beryllium aluminum silicate hydroxide
Kyanite
Al2SiO5
Also known as: Aluminum silicate
Scientific Classification
- Mineral Class
- Silicates
- Group
- Cyclosilicates (Beryl Group)
- Crystal System
- Hexagonal
- Chemical Formula
- Be3Al2Si6O18
- Composition
- Beryllium aluminum silicate. The blue color in aquamarine is due to trace amounts of iron (Fe2+ and Fe3+) substituting for aluminum.
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