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Apatite is a group of phosphate minerals, typically referring to Fluorapatite (Ca5(PO4)3F), Chlorapatite (Ca5(PO4)3Cl), and Hydroxylapatite (Ca5(PO4)3(OH)). Blue apatite is a color variety of these minerals, with its blue hue often attributed to the presence of rare earth elements, manganese, or structural defects. It crystallizes in the hexagonal system and can form well-developed prismatic crystals, often with a vitreous to sub-resinous luster. It is relatively soft for a mineral, making it susceptible to scratching.
How to Identify
- Color
- Typically sky blue to deep blue, sometimes with greenish or violet undertones. Can also be colorless, yellow, green, brown, or pink, but 'Blue Apatite' specifically refers to the blue varieties.
- Luster
- Vitreous (glassy) to sub-resinous.
- Texture
- Smooth to slightly uneven on crystal faces.
- Crystal Form
- Hexagonal prismatic crystals, often with pyramidal terminations. Can also occur as massive, granular, or botryoidal aggregates.
- Cleavage
- Poor basal cleavage {0001}, often indistinct.
- Geological Environment
- Found in igneous rocks (especially pegmatites, carbonatites, and alkaline igneous complexes), metamorphic rocks (marbles, gneisses, schists), and as detrital grains in sedimentary rocks. Gem-quality blue apatite is often found in pegmatites and hydrothermal veins.
Key Facts
- Hardness: 5 on the Mohs scale
- Specific Gravity: 3.16 - 3.22 (Fluorapatite), 3.17 - 3.23 (Chlorapatite), 3.15 - 3.20 (Hydroxylapatite)
- Crystal System: Hexagonal
- Color: Blue (sky blue, deep blue, greenish-blue, violet-blue)
- Luster: Vitreous to sub-resinous
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Poor basal cleavage {0001}
- Composition: Calcium phosphate with varying amounts of fluoride, chloride, or hydroxyl ions.
Quick Check
- Color: Sky blue to deep blue
- Luster: Vitreous to sub-resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, tabular, massive, granular, botryoidal
- Cleavage Type: Poor, basal {0001}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to sub-resinous
Formation
Apatite forms in a wide range of geological environments. It is a common accessory mineral in igneous rocks (e.g., granites, syenites, carbonatites, pegmatites), metamorphic rocks (e.g., gneisses, schists, marbles), and sedimentary rocks (e.g., phosphorites, limestones). Blue apatite specifically often forms in pegmatites, hydrothermal veins, and contact metamorphic zones.
Usage
The primary industrial use of apatite is as the main source of phosphate for fertilizers and phosphoric acid production. Gem-quality blue apatite is used in jewelry, though its relative softness (Mohs 5) makes it less durable for everyday wear. It is also collected by mineral enthusiasts. Synthetic apatite is used in biomedical applications due to its biocompatibility, particularly in bone and dental implants.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.
Where to Find
Brazil
Minas Gerais is a significant source of gem-quality blue apatite, often found in pegmatites.
Madagascar
Known for producing vibrant blue apatite crystals, particularly from the region around Fort Dauphin (Tôlanaro).
Mexico
Various localities, including Cerro de Mercado, Durango, have produced fine apatite specimens, some of which are blue.
Canada
Ontario and Quebec have localities with apatite, including some blue varieties, often associated with metamorphic and igneous rocks.
Myanmar (Burma)
Produces gem-quality apatite, including blue specimens.
United States
Localities in Maine, California, and other states have yielded apatite, with some blue occurrences.
Finding Tips
Geological Context
Look for blue apatite in areas known for pegmatites, hydrothermal veins, or contact metamorphic zones, especially where other phosphate minerals or rare earth element mineralization is present.
Associated Minerals
Apatite can be found alongside quartz, feldspar, mica, tourmaline, beryl, and various sulfide minerals.
Hardness Test
Due to its relatively low hardness (Mohs 5), apatite can be scratched by a steel file or quartz. This helps differentiate it from harder blue minerals like topaz or aquamarine.
Crystal Habit
Search for characteristic hexagonal prismatic crystals, which can be well-formed in vugs or pockets within host rock.
Fluorescence
Some apatite specimens, including blue varieties, can exhibit fluorescence under ultraviolet light, often appearing yellow or greenish-yellow. This can be a useful identification aid.
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
Na(Mg,Fe,Li,Al)3Al6(BO3)3Si6O18(OH)4
Also known as: Indicolite
Scientific Classification
- Mineral Class
- Phosphates
- Group
- Apatite Group
- Crystal System
- Hexagonal
- Chemical Formula
- Ca5(PO4)3(F,Cl,OH)
- Composition
- Calcium phosphate with fluoride, chloride, or hydroxyl. Specifically, Fluorapatite: Ca5(PO4)3F; Chlorapatite: Ca5(PO4)3Cl; Hydroxylapatite: Ca5(PO4)3(OH).
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