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Apatite is a group of phosphate minerals, typically referring to fluorapatite, chlorapatite, and hydroxylapatite, which are end-members of a solid solution series. It is a common and widely distributed mineral, often occurring as small, hexagonal crystals in various rock types. While typically green, blue, or yellow, it can exhibit a wide range of colors, including colorless, pink, purple, brown, and white. Its relatively low hardness (5 on the Mohs scale) distinguishes it from many other gem minerals.
How to Identify
- Color
- Highly variable, commonly green, blue, yellow, brown, pink, purple, or colorless. Often pastel shades.
- Luster
- Vitreous (glassy) to sub-resinous.
- Texture
- Typically smooth crystal faces, sometimes striated parallel to the c-axis.
- Crystal Form
- Hexagonal prisms, often terminated by pyramids or basal pinacoids. Can also be massive, granular, or botryoidal.
- Cleavage
- Poor to indistinct on {0001} and {1010}. Often appears to have no cleavage or a conchoidal fracture.
- Geological Environment
- Igneous rocks (granites, pegmatites, carbonatites), metamorphic rocks (marbles, gneisses), sedimentary rocks (phosphorites), hydrothermal veins.
Key Facts
- Hardness: 5 (Mohs scale)
- Specific Gravity: 3.16 - 3.22 g/cm³
- Crystal System: Hexagonal
- Color: Highly variable: green, blue, yellow, brown, pink, purple, colorless, white
- 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: Variable (green, blue, yellow, brown, pink, purple, colorless)
- Luster: Vitreous to sub-resinous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, tabular, massive, granular, botryoidal
- Cleavage Type: Poor/indistinct basal and prismatic
- 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., marbles, gneisses, schists), and sedimentary rocks (e.g., phosphorites, limestones, sandstones). It can also form through hydrothermal processes and as a primary precipitate in some marine environments.
Usage
Apatite is the primary source of phosphorus for fertilizers and phosphoric acid production. Gem-quality apatite is used in jewelry, though its relative softness limits its durability. It is also used in the production of certain ceramics and as a flux in metallurgy. Biologically, hydroxylapatite is the main mineral component of bone and tooth enamel.
Age Distribution
Found in rocks of all ages, from Precambrian to Cenozoic, reflecting its widespread formation in various geological settings.
Where to Find
Brazil
Minas Gerais is a significant source of gem-quality apatite, particularly blue and green varieties.
Mexico
Durango is known for producing fine, often yellow to green, apatite crystals.
Canada
Ontario and Quebec have localities yielding large, often green or brown, apatite crystals, particularly in pegmatites and metamorphic rocks.
Madagascar
Produces a variety of colors, including blue, green, and yellow apatite, often of gem quality.
Myanmar (Burma)
Known for producing fine blue and green apatite, sometimes resembling tourmaline or topaz.
United States
Various localities, including Maine, California, and New York, produce apatite, often as accessory minerals in igneous and metamorphic rocks.
Finding Tips
Check for Hexagonal Crystals
Apatite commonly forms distinctive hexagonal prismatic crystals, which can be a key identifier in the field.
Assess Hardness
Its Mohs hardness of 5 is a crucial diagnostic. It can be scratched by a steel file but will scratch glass. This helps differentiate it from harder minerals like beryl or topaz.
Look for Associated Minerals
In pegmatites, apatite is often found with quartz, feldspar, mica, and tourmaline. In metamorphic rocks, it can be associated with calcite, dolomite, and various silicates.
Consider Color and Luster
While variable, the vitreous luster and often pastel hues can be indicative. Blue and green are common colors.
Similar Rocks
Beryl
Be3Al2Si6O18
Also known as: Emerald, Aquamarine, Heliodor, Morganite
Tourmaline
Na(Li,Al)3Al6(BO3)3Si6O18(OH)4
Also known as: Schorl, Elbaite, Dravite
Topaz
Al2SiO4(F,OH)2
Also known as: Imperial Topaz, Sherry Topaz
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 as the dominant anion. The three main end-members are Fluorapatite (Ca5(PO4)3F), Chlorapatite (Ca5(PO4)3Cl), and Hydroxylapatite (Ca5(PO4)3OH).
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