Hubbry Logo
ApatiteApatiteMain
Open search
Apatite
Community hub
Apatite
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Apatite
Apatite
Apatite
View on Wikipedia
from Wikipedia
Apatite group
Apatite (purple), on siderite, from Portugal
General
CategoryPhosphate mineral
FormulaCa5(PO4)3(F,Cl,OH)
IMA symbolAp[1]
Strunz classification8.BN.05
Crystal systemHexagonal
Crystal classDipyramidal (6/m)
(same H-M symbol)[2]
Space groupP63/m (no. 176)
Identification
ColorTransparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown.[3]
Crystal habitTabular, prismatic crystals, massive, compact or granular
Cleavage[0001] indistinct, [1010] indistinct[2]
FractureConchoidal to uneven[3]
Mohs scale hardness5[3] (defining mineral)
LusterVitreous[3] to subresinous
StreakWhite
DiaphaneityTransparent to translucent[2]
Specific gravity3.16–3.22[2]
Polish lusterVitreous[3]
Optical propertiesDouble refractive, uniaxial negative[3]
Refractive index1.634–1.638 (+0.012, −0.006)[3]
Birefringence0.002–0.008[3]
PleochroismBlue stones – strong, blue and yellow to colorless. Other colors are weak to very weak.[3]
Dispersion0.013[3]
Ultraviolet fluorescenceYellow stones – purplish-pink, which is stronger in long wave; blue stones – blue to light-blue in both long and short wave; green stones – greenish-yellow, which is stronger in long wave; violet stones – greenish-yellow in long wave, light-purple in short wave.[3]

Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ion, respectively, in the crystal. The formula of the admixture of the three most common endmembers is written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6F2 and Ca10(PO4)6Cl2.

The mineral was named apatite by the German geologist Abraham Gottlob Werner in 1786,[4] although the specific mineral he had described was reclassified as fluorapatite in 1860 by the German mineralogist Karl Friedrich August Rammelsberg. Apatite is often mistaken for other minerals. This tendency is reflected in the mineral's name, which is derived from the Greek word ἀπατάω (apatáō), which means to deceive.[5][6]

As hydroxyapatite, it forms a major part of the teeth and bones of vertebrate animals.

Geology

[edit]

Apatite is very common as an accessory mineral in igneous and metamorphic rocks, where it is the most common phosphate mineral. However, occurrences are usually as small grains which are often visible only in thin section. Coarsely crystalline apatite is usually restricted to pegmatites, gneiss derived from sediments rich in carbonate minerals, skarns, or marble. Apatite is also found in clastic sedimentary rock as grains eroded out of the source rock.[7][8] Phosphorite is a phosphate-rich sedimentary rock containing as much as 80% apatite,[9] which is present as cryptocrystalline masses referred to as collophane.[10] Economic quantities of apatite are also sometimes found in nepheline syenite or in carbonatites.[7]

Apatite is the defining mineral for 5 on the Mohs scale.[11] It can be distinguished in the field from beryl and tourmaline by its relative softness. It is often fluorescent under ultraviolet light.[12]

Apatite is one of a few minerals produced and used by biological micro-environmental systems.[7] Hydroxyapatite (IMA name: Hydroxylapatite), is the major component of tooth enamel and bone mineral. A relatively rare form of apatite in which most of the OH groups are absent and containing many carbonate and acid phosphate substitutions is a large component of bone material.[13]

Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite; in the mid-20th century, it was discovered that communities whose water supply naturally contained fluorine had lower rates of dental caries.[14] Fluoridated water allows exchange in the teeth of fluoride ions for hydroxyl groups in apatite. Similarly, toothpaste typically contains a source of fluoride anions (e.g. sodium fluoride, sodium monofluorophosphate). Too much fluoride results in dental fluorosis and/or skeletal fluorosis.[15]

Fission tracks in apatite are commonly used to determine the thermal histories of orogenic belts and of sediments in sedimentary basins.[16] (U-Th)/He dating of apatite is also well established from noble gas diffusion studies[17][18][19][20][21][22][23] for use in determining thermal histories[24][25] and other, less typical applications such as paleo-wildfire dating.[26]

Uses

[edit]

The primary use of apatite is as a source of phosphate in the manufacture of fertilizer and in other industrial uses. It is occasionally used as a gemstone.[27] Ground apatite was used as a pigment for the Terracotta Army of 3rd-century BCE China,[28] and in Qing dynasty enamel for metalware.[29]

During digestion of apatite with sulfuric acid to make phosphoric acid, hydrogen fluoride is produced as a byproduct from any fluorapatite content. This byproduct is a minor industrial source of hydrofluoric acid.[30] Apatite is also occasionally a source of uranium and vanadium, present as trace elements in the mineral.[27]

Fluoro-chloro apatite forms the basis of the now obsolete halophosphor fluorescent tube phosphor system. Dopant elements of manganese and antimony, at less than one mole-percent — in place of the calcium and phosphorus — impart the fluorescence, and adjustment of the fluorine-to-chlorine ratio alter the shade of white produced. This system has been almost entirely replaced by the tri-phosphor system.[31]

Apatites are also a proposed host material for storage of nuclear waste, along with other phosphates.[32][33][34]

Gemology

[edit]
Faceted blue apatite, Brazil

Apatite is infrequently used as a gemstone. Transparent stones of clean color have been faceted, and chatoyant specimens have been cabochon-cut.[3] Chatoyant stones are known as cat's-eye apatite,[3] transparent green stones are known as asparagus stone,[3] and blue stones have been called moroxite.[35] If crystals of rutile have grown in the crystal of apatite, in the right light the cut stone displays a cat's-eye effect. Major sources for gem apatite are[3] Brazil, Myanmar, and Mexico. Other sources include[3] Canada, Czech Republic, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.

Use as an ore mineral

[edit]
Apatite in photomicrographs of a thin section from the Siilinjärvi apatite mine. In cross-polarized light on left, plane-polarized light on right.
An apatite mine in Siilinjärvi, Finland

Apatite is occasionally found to contain significant amounts of rare-earth elements and can be used as an ore for those metals.[36] This is preferable to traditional rare-earth ores such as monazite,[37] as apatite is not very radioactive and does not pose an environmental hazard in mine tailings. However, apatite often contains uranium and its equally radioactive decay-chain nuclides.[38]

The town of Apatity in the Arctic North of Russia was named for its mining operations for these ores.

Apatite is an ore mineral at the Hoidas Lake rare-earth project.[39]

Thermodynamics

[edit]

The standard enthalpies of formation in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, have been determined by reaction-solution calorimetry. Speculations on the existence of a possible fifth member of the calcium apatites family, iodoapatite, have been drawn from energetic considerations.[40]

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X= OH, F, Cl, Br), have been investigated using an all-atom Born-Huggins-Mayer potential[41] by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of c. 4% for the haloapatites and 8% for hydroxyapatite. High-pressure simulation runs, in the range 0.5–75 kbar, were performed in order to estimate the isothermal compressibility coefficient of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state[42] with an accuracy better than 1%.[43]

The monoclinic solid phases Ca10(PO4)6(X)2 (X= OH, Cl) and the molten hydroxyapatite compound have also been studied by molecular dynamics.[44][45]

Lunar science

[edit]

Moon rocks collected by astronauts during the Apollo program contain traces of apatite.[46] Following new insights about the presence of water in the Moon,[47] re-analysis of these samples in 2010 revealed water trapped in the mineral as hydroxyl, leading to estimates of water on the lunar surface at a rate of at least 64 parts per billion – 100 times greater than previous estimates – and as high as 5 parts per million.[48] If the minimum amount of mineral-locked water was hypothetically converted to liquid, it would cover the Moon's surface in roughly one meter of water.[49]

Bio-leaching

[edit]

The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from apatite. Release of phosphate from apatite is one of the most important activities of mycorrhizal fungi,[50] which increase phosphorus uptake in plants.[51]

Apatite group and supergroup

[edit]

Apatite is the prototype of a class of chemically, stoichometrically or structurally similar minerals, biological materials, and synthetic chemicals.[52] Those most similar to apatite are also known as apatites, such as lead apatite (pyromorphite) and barium apatite (alforsite). More chemically dissimilar minerals of the apatite supergroup include belovites, britholites, ellestadites and hedyphanes.

Apatites have been investigated for their potential use as pigments (copper-doped alkaline earth apatites), as phosphors and for absorbing and immobilising toxic heavy metals.

In apatite minerals strontium, barium and lead can be substituted for calcium; arsenate and vanadate for phosphate; and the final balancing anion can be fluoride (fluorapatites), chloride (chlorapatites), hydroxide (hydroxyapatites) or oxide (oxyapatites). Synthetic apatites add hypomanganate, hypochromate, bromide (bromoapatites), iodide (iodoapatites), sulfide (sulfoapatites), and selenide (selenoapatites). Evidence for natural sulfide substitution has been found in lunar rock samples.[53]

Furthermore, compensating substitution of monovalent and trivalent cations for calcium, of dibasic and tetrabasic anions for phosphate, and of the balancing anion, can occur to a greater or lesser degree. For example, in biological apatites there is appreciable substitution of sodium for calcium and carbonate for phosphate, in belovite sodium and cerium or lanthanum substitute for a pair of divalent metal ions, in germanate-pyromorphite germanate replaces phosphate and chloride, and in ellestadites silicate and sulphate replace pairs of phosphate anions. Metals forming smaller divalent ions, such as magnesium and iron, cannot substitute extensively for the relatively large calcium ions but may be present in small quantities.[54]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Apatite

View on Grokipedia
from Grokipedia
Apatite is a group of minerals characterized by the general Ca₅(PO₄)₃(F,Cl,OH), where the anion can be , , or , making it a essential in geological and biological contexts. This group includes common end-members such as (dominant in most natural occurrences), chlorapatite, and hydroxylapatite (also known as ). Apatite forms hexagonal prismatic crystals and exhibits a vitreous to greasy luster, with colors ranging from green and yellow to blue, violet, and colorless, though green is most typical. It has a Mohs hardness of 5, serving as a standard index , and a specific of approximately 3.1 to 3.2. In , apatite occurs as an accessory in nearly all igneous and metamorphic rocks and as a primary constituent in sedimentary deposits, which are vital sources of . These deposits form in marine environments through biological and are economically significant for . Biologically, the hydroxylapatite variety constitutes about 70% of by weight and up to 97% of , providing structural rigidity and hardness to skeletons and . The primary industrial use of apatite is as a source of for fertilizers, supporting global by enhancing levels and yields. It is also utilized in the production of for various chemical applications, including detergents and food additives. Additionally, transparent varieties with attractive colors, such as blue or yellow apatite, are cut as gemstones for jewelry, though their softness limits durability. In scientific research, apatite's content aids in and paleoenvironmental studies, while synthetic forms mimic biological apatite for biomedical implants and dental materials.

Definition and Composition

Chemical Formula and Structure

Apatite is a group of phosphate minerals with the general \ceCa5(PO4)3(F,Cl,OH)\ce{Ca5(PO4)3(F, Cl, OH)}, where the anion site is occupied by (F), (Cl), or hydroxide (OH) ions, or a combination thereof. This formula represents the ideal end-member compositions: (\ceCa5(PO4)3F\ce{Ca5(PO4)3F}), which dominates in most natural occurrences due to its stability; chlorapatite (\ceCa5(PO4)3Cl\ce{Ca5(PO4)3Cl}), more common in certain metamorphic and igneous settings; and hydroxylapatite (\ceCa5(PO4)3(OH)\ce{Ca5(PO4)3(OH)}), prevalent in biological materials like and teeth. These end-members form a series, allowing variable proportions of F, Cl, and OH within the crystal lattice. The crystal structure of apatite exhibits hexagonal symmetry in the space group P63/mP6_3/m, with unit cell parameters approximately a=9.37a = 9.37 Å and c=6.88c = 6.88 Å. In this lattice, two distinct calcium sites are present: the Ca(1) site, coordinated by nine oxygen atoms from phosphate groups, and the Ca(2) site, which is coordinated by seven oxygen atoms and one anion (F, Cl, or OH) along the c-axis channel. Phosphate ions (\cePO43−\ce{PO4^3-}) form isolated tetrahedra that link the calcium polyhedra, creating a framework that accommodates the channel-like arrangement of the halides or hydroxide. Substitutions in the apatite structure are extensive, enabling compositional variability while maintaining the overall framework. Calcium at the M sites can be partially replaced by divalent ions such as strontium (Sr) or lead (Pb), or by trivalent rare earth elements (REE) like cerium or yttrium, often coupled with sodium (Na) for charge balance. The phosphate tetrahedra (\cePO43−\ce{PO4^3-}) may undergo substitution by arsenate (\ceAsO43−\ce{AsO4^3-}) or silicate (\ceSiO44−\ce{SiO4^4-}), with the latter typically balanced by oxygen vacancies or additional cations to preserve electroneutrality. These coupled substitutions influence the mineral's stability and trace element incorporation, making apatite a key recorder of geochemical conditions in rocks. The name "apatite" originates from the Greek word "apatao," meaning "to deceive," coined in 1788 by due to the 's frequent misidentification with other species like or beryl during early 18th-century examinations.

Mineral Varieties

Apatite encompasses several varieties distinguished primarily by the dominant anion in the X-site of the general Ca₅(PO₄)₃X and additional substitutions. , with X = F, is the most abundant natural variety, forming the majority of apatite occurrences in igneous, metamorphic, and sedimentary rocks. It typically exhibits a range of 3.10–3.25 g/cm³. Chlorapatite, where X = Cl, is significantly rarer and features a comparable but slightly elevated of 3.17–3.18 g/cm³ owing to the heavier chloride ion. Hydroxylapatite, with X = OH, occurs infrequently in natural settings due to its instability under typical geological conditions, though it plays a crucial role in biogenic mineralization. Among rarer varieties, carbonate-fluorapatite, commonly known as francolite, incorporates CO₃²⁻ substitutions for PO₄³⁻ groups, predominantly in sedimentary phosphorites. Manganoapatite refers to manganese-enriched , often manifesting in pink to purple colors from Mn²⁺ incorporation at Ca sites. Britholite subtypes, such as britholite-(Ce), are silica-rich and enriched in rare earth elements (REE), with Si⁴⁺ replacing P⁵⁺ and REE³⁺ substituting for Ca²⁺, marking them as distinct end-members in the apatite supergroup. Varieties are identified through differences in unit cell parameters, particularly the a-axis length, which increases with anion size: (smallest, ~9.37 Å in ) to (~9.42 Å) to (largest, ~9.60 Å in chlorapatite). Compositional analysis relies on techniques like electron microprobe microanalysis to quantify anion and cation substitutions. Laboratory-synthesized apatite varieties, including pure end-members like hydroxylapatite, are routinely produced via or methods for applications in advanced ceramics.

Physical and Optical Properties

Crystal Habit and Symmetry

Apatite crystals are characteristically formed in the , with P6₃/m and 6/m, displaying a high degree of that results in well-defined prismatic or tabular external morphology. The most common crystal habits include short to long prismatic forms elongated along the c-axis, often terminated by pyramids or basal pinacoids, as well as thick tabular crystals parallel to {0001} or massive granular aggregates. Dominant crystal forms are the {10\overline{1}0} and the basal pinacoid {0001}, which contribute to the mineral's typical elongated, stubby appearance in rock sections. Optically, apatite is uniaxial negative, exhibiting low that makes it challenging to distinguish in thin sections without higher . Refractive indices typically range from nω ≈ 1.63–1.67 to nε ≈ 1.62–1.64, with values of 0.002–0.008, depending on compositional variations such as content. Colored varieties, including blue or green , may show weak , with absorption strongest parallel to the c-axis, aiding in gemological identification. Twinning in apatite is rare, occurring occasionally on {11\overline{2}1} or {10\overline{1}3} planes, and is not a prominent feature for diagnostic purposes. Inclusions are common and significantly impact transparency, consisting of fluid-filled cavities, healed fractures, or enclosed minerals such as , , or , which can create or asterism in cut gems. Diagnostic tests for apatite leverage its luminescent properties: fluorapatite often fluoresces yellow-green under short-wave (SW UV) light due to rare-earth activators like , while long-wave UV may produce weaker responses. reveals zoning patterns in electron bombardment, typically blue hues from activation, useful for revealing growth sectors and distributions in geological studies.

Hardness, Density, and Cleavage

Apatite possesses a Mohs of 5, which defines that value on the scale and typically ranges from 4.5 to 5.5 depending on compositional variations such as substitutions in the apatite group. This moderate renders apatite susceptible to scratching by common minerals like ( 7), limiting its durability in applications requiring abrasion resistance. The specific gravity of apatite, a measure of its relative to , varies slightly among varieties due to differences in anionic content; fluorapatite exhibits values of 3.1 to 3.2, while chlorapatite reaches 3.17 to 3.18. These densities are calculated from the mineral's volume and the atomic masses of its components, with heavier anions like contributing to marginally higher values compared to . Apatite displays poor cleavage along the {0001} and {1010} planes, resulting in irregular breaks, and exhibits a conchoidal to uneven . It is brittle in tenacity, prone to shattering under stress without significant deformation. Thermally, apatite shows anisotropic expansion with a of α≈9×10−6\alpha \approx 9 \times 10^{-6} −1^{-1} parallel to the c-axis, and melts at approximately 1670°C.

Geological Occurrence and Formation

Igneous and Metamorphic Contexts

Apatite commonly occurs as an accessory in igneous rocks, particularly in varieties such as granites, syenites, and pegmatites, where it crystallizes early from melts during magmatic differentiation. In these settings, apatite forms at temperatures ranging from 800 to 1200°C under typical crustal pressures of 0.1 to 0.5 GPa (1 to 5 kbar), incorporating trace elements like (U) and (Th) into its structure due to its ability to accommodate rare earth elements and actinides. These trace elements enhance apatite's utility as a petrogenetic indicator, reflecting the conditions and melt evolution during crystallization. Typically, apatite constitutes 0.1 to 1% by volume in rocks, often appearing as prismatic or acicular crystals intergrown with major phases. In paragenesis, apatite is frequently associated with , feldspars (such as K-feldspar and ), and mafic minerals like or , forming part of the late-stage magmatic assemblage in these rocks. This association underscores its role in budgeting within the , where it sequesters from the evolving melt. In metamorphic contexts, apatite recrystallizes during regional in rocks like gneisses and schists, often retaining or adjusting its composition in response to fluid interactions. The halogen ratios, particularly F/Cl, in metamorphic apatite serve as proxies for fluid compositions, with higher Cl contents indicating saline or acidic metamorphic fluids derived from devolatilization processes. Apatite's geochronological applications are prominent in both igneous and metamorphic settings, enabling precise dating of rock formation and thermal evolution. U-Pb dating of igneous apatite yields crystallization ages spanning from (over 2.5 billion years) to recent () events, providing robust constraints on magmatic episodes when common Pb corrections are applied. Complementarily, fission track analysis in apatite records cooling histories below approximately 120°C, revealing exhumation rates and tectonic through the partial annealing of radiation-induced tracks accumulated from U fission. These methods, often integrated, offer a multi-scale view of geological processes, from initial cooling to post-metamorphic uplift.

Sedimentary Deposits and Major Localities

Apatite in sedimentary environments primarily forms as deposits through biogenic and authigenic processes, where minerals accumulate from the remains of marine organisms or precipitate directly from pore waters in sediments. These deposits are predominantly composed of carbonate-fluorapatite, a variety prevalent in such settings, and develop in regions of high biological productivity, such as coastal zones where nutrient-rich deep waters rise to the surface, promoting blooms and subsequent fixation into that mineralizes post-mortem. Authigenic formation occurs during early , with apatite crystals growing within sediments under low-oxygen conditions that favor concentration from organic decay. Sedimentary apatite deposits exhibit diverse types, including nodular and bedded , which consist of nodules or pellets formed through repeated cycles of deposition and reworking in marine shelves; bioclastic phosphorites derived from fragmented skeletal remains; and guano-derived accumulations from bird or excrement on islands, as exemplified by the phosphorite caps on Island in the Pacific. Additionally, secondary sedimentary deposits arise from the of primary igneous apatite sources, where is leached and redeposited in residual soils or fluvial systems, contributing to economic concentrations in tropical regions. The world's major sedimentary apatite deposits are concentrated in a few key regions, with hosting the largest reserves in the and basins, accounting for 50 billion metric tons and representing about 68% of global reserves of 74 billion metric tons as of . In the United States, the Platform features extensive bone-bed from marine transgressions, yielding high-grade deposits with up to 30% P₂O₅ content. China's sedimentary resources, particularly in the and basins, include vast Permian phosphorite layers formed in ancient upwelling systems, while Russia's contributions stem from and sedimentary beds in the region, though its is noted more for igneous occurrences. Globally, sedimentary phosphate rock reserves totaled approximately 74 billion metric tons as of , underscoring the dominance of these biogenic accumulations in meeting agricultural demands. Exploration for sedimentary apatite deposits relies on geophysical surveys, such as seismic reflection and , to delineate stratigraphic layers rich in phosphorites, often integrated with aerial magnetic and radiometric methods to detect anomalies. Geochemical further targets halos of elevated P₂O₅ and associated trace elements like and rare earths in or samples, enabling vectoring toward concealed ore bodies in covered terrains. These combined approaches have proven effective in expanding known reserves, particularly in tectonically stable platform settings.

Uses and Economic Importance

Gemological Applications

Apatite, particularly the fluorapatite variety, is prized as a collector's for its attractive transparency and vivid colors when suitable material is available. Transparent occurs in notable hues including neon blue from deposits in , golden yellow from iron mines in , and rare violet from . These gem-quality specimens are commonly cut as faceted stones to showcase their dispersion and sparkle, or as cabochons for pieces with inclusions that enhance or color depth. To improve marketability, apatite gems often undergo treatments for color and clarity enhancement. can intensify or shift colors, such as deepening yellow tones or stabilizing blue varieties, while surface oiling or impregnation addresses inclusions that reduce transparency in natural rough. These processes are disclosed in reputable trade settings to maintain buyer trust. The gem's Mohs hardness of 5 renders it prone to scratching and wear, restricting its use primarily to earrings, pendants, and brooches rather than daily-wear rings. This relative softness, combined with the of facetable material over one carat in saturated colors, elevates its status as a niche collectible. Prices for fine, untreated stones typically range from $10 to $100 per carat, depending on color vividness and size. Apatite is frequently confused with simulants like (softer and more cubic) or (harder with stronger ).

Phosphate Ore Extraction

Phosphate ore extraction primarily targets apatite-rich deposits to supply for fertilizers and industrial chemicals. Mining methods vary by deposit geology: large sedimentary beds, such as those in , are exploited via open-pit operations using dragline excavators to strip and recover a matrix of phosphate pebbles, quartz sand, and clay. Igneous carbonatite-hosted deposits, like the Phalaborwa complex in , are mined via open-pit methods to access apatite-bearing formations, though underground techniques such as block are used in adjacent areas for other commodities like . Ore processing begins with beneficiation to upgrade content and remove impurities. is the dominant method, involving crushing, scrubbing, and selective chemical to float apatite particles, concentrating P₂O₅ from 25-40% in the raw to 30-35% in the final product suitable for downstream production. This process also enables byproduct recovery, notably rare earth elements from apatite lattices in certain deposits, adding economic value to operations. Global phosphate rock production reached 240 million metric tons in 2024, up from 220 million metric tons in 2023, led by at approximately 46% of output (110 million metric tons) and at about 13% (30 million metric tons), reflecting their dominance in sedimentary and marine deposits. In 2024, production increased amid concerns, including export restrictions from major producers affecting global availability. Economically, apatite-derived is indispensable for , comprising up to 80-90% of phosphorus and enabling higher crop yields to support global food production for over 8 billion people. exceed 71 billion metric tons of phosphate rock, sufficient for more than 300 years at current extraction rates, ensuring sustained supply despite rising demand. These activities generate environmental challenges, including large volumes of and waste from processing, alongside high water usage—up to 2-3 tons per ton of —for transport and flotation. efforts focus on reclamation and water recycling to address risks from and radionuclides in effluents.

Biological and Environmental Roles

In Biomineralization (Bones and Teeth)

In biological systems, apatite primarily manifests as hydroxylapatite with the formula [Ca₅(PO₄)₃OH], forming the inorganic matrix of hard tissues. This constitutes approximately 70% of by weight, where it integrates with an organic framework, and up to 96% of , the hardest substance in the . Biological hydroxylapatite crystals are nanoscale, typically 20-100 nm in length, and exhibit substitutions such as ions replacing or hydroxyl groups, along with trace proteins that influence crystal orientation and stability. These modifications render the mineral less stoichiometric than synthetic forms, enhancing its adaptability to physiological stresses. Biomineralization of hydroxylapatite occurs through specialized cells that deposit the mineral in a controlled manner to form bone and teeth. In bone, osteoblasts secrete an extracellular matrix rich in , which serves as a template for and growth of hydroxylapatite crystals, resulting in a hierarchical structure that provides mechanical strength. For enamel, ameloblasts orchestrate the formation of highly oriented, prismatic crystals during tooth development, creating a non-vascularized, acellular layer resistant to wear. The process maintains calcium-to-phosphate (Ca/P) homeostasis, with a molar ratio near 1.67, regulated by hormones such as (PTH), which modulates serum calcium levels and influences activity to prevent excessive mineralization or resorption. Medically, disruptions in hydroxylapatite density underlie conditions like osteoporosis, where reduced mineral content leads to bone fragility and increased fracture risk, often assessed via bone mineral density measurements calibrated against hydroxyapatite standards. In dentistry, hydroxyapatite-based toothpastes promote enamel remineralization by delivering nano-sized particles that fill microscopic defects and restore mineral gradients, offering an alternative to fluoride for caries prevention without altering tooth color. Evolutionarily, hydroxylapatite emerged in early s during the period, enabling the development of robust skeletal elements that supported diverse body plans and ecological roles. This mineral's durability facilitates exceptional preservation, as apatite resists diagenetic alteration and retains isotopic signatures of ancient environments, providing insights into vertebrate phylogeny from lagerstätten onward.

Bio-leaching Processes

Bio-leaching processes utilize acidophilic microorganisms to extract bound in minerals, offering a sustainable alternative to conventional extraction methods. Acid-producing , such as Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, play a central role by oxidizing elemental or ions to generate , which protonates and solubilizes the components of apatite. This biogenic acidification facilitates the breakdown of the lattice through direct H⁺ attack on PO₄³⁻ groups, releasing soluble phosphorus species. The core reaction for dissolution is: \ceCa5(PO4)3F+5H2SO4−>3H3PO4+5CaSO4+HF\ce{Ca5(PO4)3F + 5 H2SO4 -> 3 H3PO4 + 5 CaSO4 + HF} This mechanism has been demonstrated in studies using sulfur-oxidizing consortia, where the produced acidity effectively mimics chemical leaching while minimizing external acid inputs. Applications of bio-leaching focus on recovering phosphorus from low-grade apatite ores and industrial wastes, such as iron mine tailings and phosphogypsum byproducts from fertilizer production. In laboratory settings, these processes have achieved phosphorus dissolution efficiencies of up to 97% from low-grade fluorapatite ores over 21 days, particularly when using adapted acidophilic bacteria under optimized conditions like low pulp density and continuous aeration. For phosphogypsum waste, bio-leaching targets residual apatite fractions, enabling phosphorus recovery alongside decontamination of associated impurities, with reported solubilization rates exceeding 50% in microbial systems. These approaches are especially valuable for processing uneconomical deposits, enhancing resource utilization in phosphorus-limited regions. Bio-leaching offers distinct advantages over chemical leaching, including lower —due to ambient operations—and greater environmental compatibility, as it reduces the need for harsh and generates less acidic effluent. Field trials in since the 2010s, such as large-scale column experiments (up to 2 tons) on apatite-rich low-grade ores, have validated and co-recovery with efficiencies around 69% over 60 days, demonstrating scalability for applications. In , pilot studies on slimes have similarly explored bio-leaching to upgrade low-grade materials, integrating it with flotation for enhanced yields. Key challenges include the inherently slow kinetics of microbial processes, which can span weeks versus hours for chemical methods, and the need for precise management (typically 1.5–2.5) to sustain bacterial activity without inhibiting solubilization. Employing microbial consortia, combining - and iron-oxidizers, can mitigate rate limitations by improving acid production and tolerance to metal ions, though optimization remains essential for industrial viability. Ongoing research emphasizes designs to address these issues, aiming for broader adoption in sustainable extraction.

Advanced Scientific Aspects

Thermodynamic Properties

The solubility of fluorapatite, the most common variety of apatite, is governed by its solubility product constant (K_{sp}), which is approximately 10^{-60} at 25°C for the dissociation reaction \ce{Ca5(PO4)3F (s) <=> 5Ca^{2+} + 3PO4^{3-} + F^{-}}, reflecting its exceptional stability in neutral aqueous environments. This low K_{sp} value underscores fluorapatite's resistance to dissolution, with molar solubility on the order of 10^{-12} M under standard conditions. However, solubility exhibits strong pH dependence, increasing markedly below pH 6 due to protonation of phosphate species (e.g., formation of HPO4^{2-} and H2PO4^-), which shifts the equilibrium toward greater ion release; at pH 5, solubility can rise by several orders of magnitude compared to neutral pH. The standard of formation (\Delta G_f^\circ) for \ce{Ca5(PO4)3F} is -12983 kJ/mol at 25°C, signifying a highly exergonic formation process and thermodynamic favorability relative to its constituent oxides and elements. This value facilitates the construction of phase diagrams for apatite solid solutions, particularly those involving F^- - OH^- or Cl^- substitutions, where mixing is often nearly at elevated temperatures but shows non-ideal behavior at low temperatures due to short-range ordering in the anion channel. Such diagrams are essential for understanding stability fields in multi-component systems. Fluorapatite demonstrates robust thermal stability, remaining intact up to approximately 1400°C, beyond which it decomposes via the reaction \ce{2 Ca5(PO4)3F -> 3 beta-Ca3(PO4)2 + CaF2}, yielding β-tricalcium phosphate (whitlockite) and calcium fluoride. Calorimetric studies provide the standard entropy S^\circ = 776 J/mol·K and heat capacity data, approximated by the Shomate equation C_p = A + B T + C T^{-2} + D T^2 (with coefficients derived from low- to high-temperature measurements), enabling predictions of entropy changes (\Delta S) and enthalpy contributions (\Delta H) across a wide temperature range from 15 K to 1600 K. These properties underpin geochemical modeling of natural processes, including apatite in surficial environments, where acidic conditions accelerate release for bioavailable , and hydrothermal alteration in igneous and metamorphic settings, where elevated temperatures and fluid compositions drive recrystallization or substitution in solid solutions to form ore-associated phases.

Role in Lunar and Extraterrestrial

Apatite has been identified in lunar samples returned by the Apollo missions, particularly within mare basalts and KREEP-rich rocks, where it serves as a key indicator of volatile elements in the lunar interior. In these samples, such as basalts 15058 and 15555, apatite grains exhibit fluorine-dominant compositions with variable contents, and their OH/F ratios provide insights into the degassing history and primordial water budget of the . Secondary ion mass spectrometry (SIMS) analyses of apatite in sample 12039 reveal water contents ranging from approximately 3,000 to 6,000 ppm, suggesting the presence of indigenous in the lunar mantle rather than solely solar wind implantation. These findings imply that the retained volatile signatures from its formation, potentially from a water-bearing impactor during the . In meteoritic materials, apatite occurrences further illuminate aqueous processes on parent bodies. Enstatite chondrites contain Cl-rich apatite varieties, characterized by elevated Cl/F ratios compared to solar values, which record low-temperature aqueous alteration events on their highly reduced parent bodies. Similarly, Martian meteorites such as Nakhla and host Cl- and OH-bearing apatite, often associated with late-stage magmatic evolution and fluid interactions that imply localized hydration on Mars. SIMS techniques applied to these extraterrestrial apatites enable precise measurement of halogen isotopes, such as δ³⁷Cl, revealing fractionation due to volatilization or alteration and providing constraints on the conditions and in planetary environments. For instance, chlorine isotope compositions in lunar and meteoritic apatite highlight a heavy Cl reservoir in the , distinct from Earth's, with implications for differentiation processes. The presence of apatite in these materials also bears on astrobiological considerations, particularly through its role in phosphorus cycling. As a primary phosphate mineral, apatite represents a bioavailable source of phosphorus, an essential element for life, in extraterrestrial settings where solubility under aqueous conditions could facilitate prebiotic chemistry or microbial habitability. Recent analyses of Chang'e-5 mission samples from the lunar near-side, returned in 2020, confirm apatite in basaltic clasts and regolith, with halogen systematics indicating magmatic processes and water contents consistent with a heterogeneous lunar mantle reservoir. Analyses of Chang'e-6 samples from the lunar farside, returned in 2024, reveal apatite grains in basalts with water abundances ranging from 345 to 3,529 ppm (average 1,511 ppm), further supporting volatile heterogeneity in the lunar mantle. These discoveries, including chlorine isotope fractionation in Chang'e-5 apatite, extend our understanding of volatile delivery and retention across the inner Solar System.

Apatite Supergroup Classification

The Apatite Supergroup represents a comprehensive classification framework for a diverse array of , , , , and minerals sharing a characteristic hexagonal or pseudohexagonal , as established by the International Mineralogical Association's Commission on New Minerals, Nomenclature and (IMA-CNMNC) in 2010. This revision expanded the prior apatite group into a supergroup to better accommodate structural and compositional variations, particularly in cation and anion substitutions along channel-like motifs parallel to the c-axis. The general formula for supergroup minerals is IXM21VIIIM32(TO4)3XIXM^1_2 VIIIM^2_3 (TO_4)_3 X, where variability occurs at the IX (large cation), M sites (medium cations), T (tetrahedral), and X (anion) positions, enabling accommodation of elements like Ca, Pb, REE, P, As, , Si, and S alongside halides or hydroxyl groups. The supergroup is subdivided into five groups based on dominant cations at the M1 and M2 sites, tetrahedral occupants, and anion types, with the Apatite group comprising the core phosphate-dominant species such as fluorapatite [Ca₅(PO₄)₃F], chlorapatite [Ca₅(PO₄)₃Cl], and hydroxylapatite [Ca₅(PO₄)₃(OH)], which feature Ca-dominant M sites and a unified M1 site. Other groups include the Hedyphane group (Pb- and Ca-bearing phosphates/arsenates/sulfates with ordered Ca/Pb distribution), Belovite group (REE- and Sr-bearing phosphates with split M1 site into M1 and M1' subsites), Britholite group (REE- and Ca-bearing silicates/phosphates), and Ellestadite group (Ca-bearing sulfates/silicates/phosphates). As of 2025, the supergroup encompasses 54 IMA-approved species, reflecting ongoing discoveries and refinements in structural criteria like anion variability in the X-site channels that influence stability and substitution limits. Key related minerals highlight compositional analogs within the supergroup; for instance, johnbaumite [Ca₅(AsO₄)₃(OH)] serves as the arsenate analog to hydroxylapatite in the Apatite group, occurring in metamorphosed deposits and demonstrating As-for-P substitution at the tetrahedral site. Similarly, hedyphane [Ca₂Pb₃(AsO₄)₃Cl] exemplifies a Pb-dominant analog in the Hedyphane group, with partial ordering of Ca and Pb across M sites, often found in oxidized Pb-As deposits. These analogs underscore the supergroup's flexibility, where structural integrity is maintained despite significant chemical diversity, as verified through single-crystal diffraction studies. The 2010 IMA revision not only formalized the supergroup status but also addressed inconsistencies, such as renaming certain REE-bearing and invalidating others lacking distinct end-member compositions, thereby standardizing classification around dominant-valence and site-occupancy rules. This update has facilitated precise identification in complex assemblages, emphasizing the supergroup's role in advancing mineral beyond simple categorization.

References

  1. https://geology.com/minerals/apatite.shtml
  2. https://www.mindat.org/min-29229.html
  3. https://webmineral.com/data/Apatite.shtml
  4. https://www.ncbi.nlm.nih.gov/books/NBK513314/
  5. https://www.sciencelearn.org.nz/resources/1796-bone-and-tooth-minerals
  6. https://www.gemsociety.org/article/apatite-jewelry-and-gemstone-information/
  7. https://pubs.geoscienceworld.org/msa/rimg/article/48/1/13/110611/Compositions-of-the-Apatite-Group-Minerals
  8. https://pubs.geoscienceworld.org/ejm/eurjmin/article/22/2/163/69447/Nomenclature-of-the-apatite-supergroup-minerals
  9. https://www.mdpi.com/2075-163X/12/5/527
  10. https://www.researchgate.net/publication/250130595_Compositions_of_the_Apatite-Group_Minerals_Substitution_Mechanisms_and_Controlling_Factors
  11. https://www.mindat.org/min-1572.html
  12. https://webmineral.com/data/Apatite-%28CaF%29.shtml
  13. https://www.mindat.org/min-1013.html
  14. https://pubs.geoscienceworld.org/msa/elements/article/11/3/195/137650/A-Technological-Gem-Materials-Medical-and
  15. https://www.mindat.org/min-895.html
  16. https://www.mindat.org/min-2562.html
  17. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/britholite
  18. http://www.geology.wisc.edu/~johnf/g777/AmMin/Goldoff-2012.pdf
  19. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/hydroxylapatite
  20. https://www.science.smith.edu/geosciences/petrology/petrography/apatite/apatite.html
  21. https://www.gemsociety.org/article/table-refractive-index-double-refraction-gems/
  22. https://www.mindat.org/mesg-564945.html
  23. https://www.gia.edu/gems-gemology/spring-2022-microworld-rare-blue-apatite-inclusion-smoky-quartz
  24. https://www.fluomin.org/uk/fiche.php?id=349
  25. https://timslab.princeton.edu/sites/g/files/toruqf2276/files/schoene_bgb_apatite_gca07.pdf
  26. https://www.handbookofmineralogy.org/pdfs/fluorapatite.pdf
  27. https://www.handbookofmineralogy.org/pdfs/chlorapatite.pdf
  28. https://pubs.aip.org/aip/jap/article-pdf/42/1/272/18356450/272_1_online.pdf
  29. https://surfacenet.de/files/kristall_daten.php?crystal=22
  30. https://link.springer.com/article/10.1007/BF00376966
  31. https://pubs.geoscienceworld.org/segweb/economicgeology/article/111/5/1187/152473/Apatite-Trace-Element-Compositions-A-Robust-New
  32. https://www.researchgate.net/publication/250130607_Apatite_in_Igneous_Systems
  33. https://www.researchgate.net/publication/249849363_Apatite_Composition_and_the_Fugacities_of_HF_and_HCl_in_Metamorphic_Fluids
  34. https://www.mdpi.com/2075-163X/11/10/1095
  35. https://royalsocietypublishing.org/doi/10.1098/rsta.1990.0124
  36. https://pubs.geoscienceworld.org/segweb/economicgeology/article/112/3/483/172162/Sedimentary-and-Igneous-Phosphate-Deposits
  37. https://www.intechopen.com/chapters/49984
  38. https://www.sciencedirect.com/science/article/pii/001670379390035U
  39. https://www.sciencedirect.com/topics/engineering/phosphate-deposits
  40. https://pubs.usgs.gov/periodicals/mcs2025/mcs2025-phosphate.pdf
  41. https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-phosphate.pdf
  42. https://www.mdpi.com/2075-163X/10/10/842
  43. https://pubs.geoscienceworld.org/geea/article-lookup?doi=10.1144/1467-7873/09-234
  44. http://nevada-outback-gems.com/Encyclopedia_pages/Apatite.htm
  45. https://www.gemselect.com/other-info/about-apatite.php
  46. https://floridadep.gov/water/mining-mitigation/content/phosphate
  47. https://www.saimm.co.za/Journal/v072n01p012.pdf
  48. https://www.mindat.org/loc-3091.html
  49. https://www.911metallurgist.com/blog/phosphate-beneficiation-process/
  50. https://www.miamiherald.com/news/local/environment/article276607541.html
  51. https://www.csiro.au/en/news/all/articles/2019/june/phosphorous
  52. https://medcraveonline.com/IJH/environmental-impact-of-phosphate-mining-and-beneficiation-review.html
  53. https://pmc.ncbi.nlm.nih.gov/articles/PMC12142779/
  54. https://pmc.ncbi.nlm.nih.gov/articles/PMC5804748/
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC8152500/
  56. https://pmc.ncbi.nlm.nih.gov/articles/PMC4431766/
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC8930857/
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC5188419/
  59. https://pmc.ncbi.nlm.nih.gov/articles/PMC3830883/
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC11190287/
  61. https://etd.auburn.edu/bitstream/handle/10415/911/BINGHAM_PATRICK_34.pdf?isAllowed=y&sequence=1
  62. https://www.sciencedirect.com/science/article/abs/pii/S0892687505003729
  63. https://www.sciencedirect.com/science/article/abs/pii/S0304386X1400173X
  64. https://www.researchgate.net/publication/269387336_Bioleaching_of_Phosphorus_from_Low_Grade_Ores_and_Concentrates_with_Acidophilic_Iron-_and_Sulphur-Oxidizing_Bacteria
  65. https://ui.adsabs.harvard.edu/abs/2025JSusM.tmp...99L/abstract
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC10974981/
  67. https://www.sciencedirect.com/science/article/pii/S187661021101530X
  68. https://www.cetem.gov.br/antigo/images/periodicos/2017/CAN0028-00-17.pdf
  69. https://ami-journals.onlinelibrary.wiley.com/doi/full/10.1111/lam.13745
  70. https://www.sciencedirect.com/science/article/abs/pii/S2213343723011521
  71. https://link.springer.com/article/10.1007/s10311-023-01611-4
  72. https://pubs.geoscienceworld.org/msa/ammin/article/99/5-6/890/46127/A-novel-technique-for-fluorapatite-synthesis-and
  73. https://www.mdpi.com/1996-1944/16/4/1502
  74. https://pubs.acs.org/doi/10.1021/ja01156a022
  75. https://www.sciencedirect.com/science/article/abs/pii/S002449372300097X
  76. https://pmc.ncbi.nlm.nih.gov/articles/PMC4128269/
  77. https://www.sciencedirect.com/science/article/abs/pii/S0016703711003450
  78. https://www.sciencedirect.com/science/article/abs/pii/S0012821X11002433
  79. https://royalsocietypublishing.org/doi/10.1098/rsta.2013.0254
  80. https://pubs.geoscienceworld.org/msa/ammin/article/108/7/1185/624296/On-the-origin-of-fluorine-poor-apatite-in
  81. https://www.hou.usra.edu/meetings/lpsc2025/pdf/1509.pdf
  82. https://www.pnas.org/doi/10.1073/pnas.2014503117
  83. https://www.nature.com/articles/s41598-019-42224-8
  84. https://www.mdpi.com/2075-163X/14/6/591
  85. https://www.sciencedirect.com/science/article/abs/pii/S0016703724004381
  86. https://www.nature.com/articles/s41586-025-08870-x
  87. https://the-innovation.org/article/doi/10.59717/j.xinn-geo.2023.100014
  88. https://fmm.ru/images/5/53/Pasero2010.pdf
  89. https://www.mindat.org/min-32275.html
  90. https://www.mindat.org/min-274.html
  91. https://pubs.geoscienceworld.org/msa/ammin/article/65/11-12/1143/104694/Johnbaumite-a-new-member-of-the-apatite-group-from
  92. https://www.mindat.org/min-1844.html
Add your contribution
Related Hubs
User Avatar
No comments yet.