Apatite
A phosphate mineral group essential to biology and industry.
Apatite is a group of phosphate minerals whose three most common endmembers are hydroxyapatite, fluorapatite, and chlorapatite. These are distinguished by high concentrations of hydroxide (OH−), fluoride (F−), and chloride (Cl−) ions, respectively, within the crystal structure. The general formula for the mixture of these endmembers is written as Ca10(PO4)6(OH,F,Cl)2, while the individual unit cell formulae are Ca10(PO4)6(OH)2, Ca10(PO4)6F2, and Ca10(PO4)6Cl2. The mineral was named in 1786 by German geologist Abraham Gottlob Werner, though the specific mineral he described was later reclassified as fluorapatite in 1860 by Karl Friedrich August Rammelsberg. The name derives from a Greek word meaning "to deceive," reflecting how easily apatite is mistaken for other minerals. As hydroxyapatite, it is a major component of vertebrate teeth and bones. In geology, apatite is very common as an accessory mineral in igneous and metamorphic rocks, where it is the most abundant phosphate mineral, though it usually occurs as small grains visible only in thin section. Coarsely crystalline apatite is largely restricted to pegmatites, certain gneisses, skarns, or marble. It also appears in clastic sedimentary rocks as eroded grains, and can form from animal skeleton breakdown or precipitation from seawater. Phosphorite is a sedimentary rock containing up to 80% apatite as cryptocrystalline masses called collophane. Economic deposits also occur in nepheline syenite or carbonatites. Apatite defines hardness 5 on the Mohs scale and can be distinguished from beryl and tourmaline by its relative softness; it is often fluorescent under ultraviolet light.
- Named by
- Abraham Gottlob Werner
- Mohs hardness
- 5
- Primary use
- Source of phosphate in fertilizer
- Major component of
- Teeth and bones of vertebrate animals
- Common endmembers
- Hydroxyapatite, fluorapatite, chlorapatite
Quick Facts
- Category
- Phosphate mineral
- Formula
- Ca / 5 / (PO / 4 / ) / 3 / (F,Cl,OH)
- Imasymbol
- Ap
- Strunz
- 8.BN.05
- Habit
- Tabular, prismatic crystals, massive, compact or granular
- System
- Hexagonal
- Class
- Dipyramidal (6/m) · (same H-M symbol)
- Symmetry
- P6 / 3 · m (no. 176)
- Cleavage
- [0001] indistinct, [1010] indistinct
- Fracture
- Conchoidal to uneven
- Mohs
- 5 (defining mineral)
- Luster
- Vitreous to subresinous
Facts from the source article.
Lore & Background
The name derives from the Greek word ἀπατάω (apatáō), meaning 'to deceive', reflecting its frequent misidentification for other minerals. Apatite is very common as an accessory mineral in igneous and metamorphic rocks, usually as small grains visible only in thin section, though coarsely crystalline forms occur in pegmatites, gneiss, skarns, and marble. It is also found in clastic sedimentary rocks and in phosphorite, a phosphate-rich sedimentary rock containing up to 80% apatite as cryptocrystalline masses called collophane. The mineral defines hardness 5 on the Mohs scale and can be distinguished in the field from beryl and tourmaline by its softness. It often fluoresces under ultraviolet light. Apatite is a group of phosphate minerals, primarily hydroxyapatite, fluorapatite, and chlorapatite, with the general formula Ca₁₀(PO₄)₆(OH,F,Cl)₂. Hydroxyapatite is a major component of vertebrate teeth and bones, while fluorapatite is more resistant to acid attack; fluoridated water and toothpaste exploit fluoride exchange for hydroxyl groups in tooth apatite. Fission tracks in apatite are used to determine thermal histories of orogenic belts and sedimentary basins, and (U-Th)/He dating is also employed. The primary use is as a phosphate source for fertilizer; it is occasionally faceted as a gemstone, with chatoyant specimens known as cat's-eye apatite, green stones as asparagus stone, and blue stones as moroxite. Ground apatite was used as pigment for the Terracotta Army and in Qing dynasty enamel. It can also be an ore for rare-earth elements, as it is less radioactive than traditional ores like monazite, though it often contains uranium and its decay products.
Reader's Guide
Apatite holds broad significance across geology, biology, and industry. As the defining mineral for hardness 5 on the Mohs scale, it aids in field identification of rocks. Hydroxyapatite is a major component of tooth enamel and bone mineral, while fluorapatite's resistance to acid attack led to the discovery that fluoridated water reduces dental caries, a finding applied in water fluoridation and toothpaste. Apatite is the primary source of phosphate for fertilizer and other industrial uses, and it has been used as a gemstone, as a pigment for the Terracotta Army, and in Qing dynasty enamel. Fission tracks and (U-Th)/He dating of apatite are established methods for determining thermal histories of orogenic belts and sedimentary basins. Apatite also serves as an ore for rare-earth elements, though it often contains uranium and its decay-chain nuclides. The mineral group is also studied for potential nuclear waste storage and for immobilizing toxic heavy metals.
Did You Know?
- Apatite is named from the Greek word ἀπατάω (apatáō), meaning 'to deceive', because it is often mistaken for other minerals.
- Fluorapatite is more resistant to acid attack than hydroxyapatite, leading to the use of fluoridated water to reduce dental caries.
- Apatite is the defining mineral for hardness 5 on the Mohs scale.
Frequently Asked Questions
What is Apatite's Mohs hardness and why does it matter?
Apatite sits at exactly 5 on the Mohs scale and serves as the defining reference mineral for that level. Geologists use it as the standard benchmark to gauge whether an unknown specimen is harder or softer than the mid-range.
What role does Apatite play in living organisms?
Apatite forms the principal mineral component of vertebrate teeth and bones, providing the structural rigidity those tissues need. Without this phosphate-rich mineral, skeletal and dental tissue in animals simply would not develop properly.
Why is Apatite important to modern industry and geology?
It is the primary natural source of phosphate harvested for agricultural fertilizer production. In geology, apatite is also used in thermochronology to reconstruct the thermal history of rock formations over deep time.
What are Apatite's main varieties?
The three principal endmembers are hydroxyapatite (dominated by OH⁻), fluorapatite (dominated by F⁻), and chlorapatite (dominated by Cl⁻). They share the same crystal framework but differ in which anion occupies the key structural site.
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