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Blue Apatite refers to the blue varieties of the apatite mineral group, which are calcium phosphate minerals. The group includes fluorapatite (Ca5(PO4)3F), chlorapatite (Ca5(PO4)3Cl), and hydroxylapatite (Ca5(PO4)3OH), with the specific blue color often attributed to trace impurities or structural defects. It typically forms hexagonal crystals, often prismatic, and can be found in massive or granular habits. Its relatively low hardness makes it susceptible to scratching, but its vibrant blue color makes it a popular choice for collectors and some jewelry applications.
How to Identify
- Color
- Varies from light blue to deep, vibrant blue, sometimes with a greenish tint. The color can be unevenly distributed.
- Luster
- Vitreous (glassy) to sub-resinous.
- Texture
- Typically smooth on crystal faces, granular in massive forms.
- Crystal Form
- Hexagonal prismatic crystals are common, often with pyramidal terminations. Can also be massive, granular, or botryoidal.
- Cleavage
- Poor to indistinct on {0001} and {1010}. Often exhibits conchoidal fracture rather than distinct cleavage.
- Geological Environment
- Found in igneous rocks (e.g., pegmatites, carbonatites, alkaline igneous rocks), metamorphic rocks (e.g., marbles, schists, gneisses), and sedimentary rocks (e.g., phosphorite deposits). Also occurs in hydrothermal veins.
Key Facts
- Hardness: 5 on the Mohs scale
- Specific Gravity: 3.1-3.2
- Crystal System: Hexagonal
- Color: Blue, green, yellow, brown, violet, pink, colorless (blue is specific to Blue Apatite)
- Luster: Vitreous to sub-resinous
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct on {0001} and {1010}
- Composition: Calcium phosphate with varying amounts of fluoride, chloride, and hydroxyl ions
Quick Check
- Color: Blue (light to deep, sometimes greenish)
- Luster: Vitreous to sub-resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Hexagonal prismatic crystals, often with pyramidal terminations; also massive, granular, columnar, or botryoidal.
- Cleavage Type: Poor to indistinct basal {0001} and prismatic {1010}. Often fractures rather than cleaves cleanly.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to sub-resinous.
Formation
Apatite forms in various geological environments. In igneous rocks, it crystallizes from magmas, often as an early-forming accessory mineral in felsic to mafic intrusions and extrusions. In metamorphic rocks, it can form during regional or contact metamorphism from pre-existing phosphate-bearing minerals or by metasomatic processes. In sedimentary rocks, it is a primary constituent of phosphorite deposits, forming through biogenic precipitation (e.g., bone, teeth, guano) or direct chemical precipitation from seawater. Hydrothermal veins can also host apatite.
Usage
Blue Apatite is primarily used as a gemstone, though its relative softness (Mohs 5) limits its use in jewelry to pieces not subjected to hard wear. It is also a collector's mineral. Industrially, apatite (regardless of color) is the primary source of phosphorus for fertilizers, phosphoric acid, and various chemical products. It is also used in some metallurgical processes and as a pigment.
Age Distribution
Apatite is a common accessory mineral found in a wide range of igneous, metamorphic, and sedimentary rocks, thus its age distribution is as varied as the rocks it occurs in, spanning billions of years from Precambrian to Cenozoic.
Where to Find
Brazil
Minas Gerais is a significant source of gem-quality blue apatite, often found in pegmatites.
Madagascar
Known for producing fine blue apatite crystals, particularly from the region of Ambatofinandrahana.
Myanmar (Burma)
Produces blue apatite, sometimes associated with other gemstones.
Canada
Ontario and Quebec have localities yielding apatite, including blue varieties, often in metamorphic or igneous settings.
Mexico
Some localities produce blue apatite, often in association with other minerals.
United States
Various states, including Maine and California, have produced blue apatite, typically from pegmatite occurrences.
Finding Tips
Geological Context
Look for blue apatite in areas known for pegmatites, carbonatites, or metamorphic terrains, as these are common host rocks. Sedimentary phosphorite deposits are also a source, though gem-quality material is less common there.
Associated Minerals
Apatite can be found alongside minerals such as quartz, feldspar, mica, tourmaline, beryl, and calcite, depending on the geological environment.
Crystal Habit
Search for hexagonal prismatic crystals, which are characteristic of apatite. These can range from microscopic to several centimeters in length.
Hardness Test (Caution)
Due to its relatively low hardness (Mohs 5), apatite can be scratched by a steel knife or even a piece of glass. This can help distinguish it from harder blue minerals like topaz or aquamarine, but be cautious not to damage a valuable specimen.
Specific Gravity
Apatite has a specific gravity of 3.1-3.2, which is moderately heavy. This can be a useful diagnostic property when comparing it to lighter or heavier minerals of similar appearance.
Similar Rocks
Topaz
Al2SiO4(F,OH)2
Also known as: Blue Topaz
Aquamarine
Be3Al2Si6O18
Also known as: Blue Beryl
Kyanite
Al2SiO5
Also known as: Disthene
Blue Tourmaline
Complex borosilicate
Also known as: Indicolite
Scientific Classification
- Mineral Class
- Phosphate
- Group
- Apatite Group
- Crystal System
- Hexagonal
- Chemical Formula
- Ca5(PO4)3(F,Cl,OH)
- Composition
- Calcium phosphate with fluoride, chloride, or hydroxyl as the dominant anion. Blue color is often due to trace rare earth elements, manganese, or structural defects.
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