APATITE(GEM)
Listing description
Apatite is a group of phosphate minerals, usually referring to hydroxyapatite,
fluorapatite,
chlorapatite
and bromapatite,
named for high concentrations of OH−,
F−,
Cl−
or Br− ions,
respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca10(PO4)6(OH,
F, Cl, Br)2, and the crystal unit cell formulae of the individual minerals
are written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2,
Ca10(PO4)6(Cl)2 and Ca10(PO4)6(Br)2.
Detailed description
Apatite is a group of phosphate
minerals, usually
referring to hydroxyapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal. The formula of the admixture of the
four most common endmembers is written as Ca10(PO4)6(OH, F, Cl, Br)2,
and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2,
Ca10(PO4)6(F)2, Ca10(PO4)6(Cl)2
and Ca10(PO4)6(Br)2.
Apatite is one of
a few minerals that are produced and used by biological micro-environmental
systems. Apatite is the defining mineral for 5 on Mohs Scale hardness. Hydroxyapatite, also known as
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.
Fluorapatite (or
fluoroapatite) is more resistant to acid attack than is hydroxyapatite. For
this reason, toothpaste typically contains a source of fluoride anions (e.g. sodium fluoride, sodium
monofluorophosphate).
Similarly, fluoridated
water allows exchange
in the teeth of fluoride ions for hydroxyl groups in apatite. Too much fluoride results
in dental
fluorosis and/or skeletal
fluorosis.
Fission
tracks in apatite are
commonly used to determine the thermal history of orogenic (mountain) belts and
of sediments in sedimentary
basins. (U-Th)/He dating of apatite is also well-established
for use in determining thermal histories and other, less typical applications
such as paleo-wildfire dating.
Phosphorite is a phosphate-rich sedimentary rock, that contains between 18% and 40% P2O5.
The apatite in phosphorite is present as cryptocrystalline masses referred to as collophane.
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Uses
The primary use
of apatite is in the manufacture of fertilizer - it is a source of phosphorus.
It is occasionally used as a gemstone.
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 adjusts the shade of white
produced. Now almost entirely replaced by the Tri-Phosphor system.[3]
In the United States, apatite derived fertilizers are used
to supplement the nutrition of many agricultural crops by providing a valuable
source of phosphate.
Gemology
Apatite is
infrequently used as a gemstone. Transparent stones of clean color have been
faceted, and chatoyant specimens have been cabochon cut.[1] Chatoyant stones are known as cat's-eye
apatite,[1] transparent green stones are known as
asparagus stone,[1] and blue stones have been called moroxite.[4] Crystals of rutile may have grown in
the crystal of apatite so when in the right light, the cut stone displays a
cat's eye effect. Major sources for gem apatite are[1]
Brazil, Burma, and Mexico. Other sources include[1] Canada, Czechoslovakia, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.
Use as an ore mineral
Apatite is
occasionally found to contain significant amounts of rare
earth elements
and can be used as an ore for those metals [5]. This is preferable to traditional
rare earth ores, as Apatite is non-radioactive [6] and does not pose an environmental
hazard in mine
tailings.
Thermodynamics
The standard (p =
0.1 MPa) molar enthalpies of formation in the crystalline state of
hydroxyapatite, chlorapatite and a preliminary value for bromapatite, at T =
298.15 K, have already 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.[8]
Lunar science
Moon rocks collected by astronauts during the Apollo program contain traces of apatite.[9] 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.[10] 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.[11]
A
phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic
chemistry, a phosphate, or organophosphate,
is an ester
of phosphoric acid. Organic phosphates are important in biochemistry
and biogeochemistry or ecology.
Inorganic phosphates are mined to obtain phosphorus for use in agriculture and
industry.[1][2][3]
At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates.|
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Chemical properties
This is the structural
formula of the phosphoric acid functional group as found in a weakly acidic aqueous solution. In more basic aqueous solutions, the group donates the two hydrogen
atoms
and ionizes
as a phosphate group with a negative charge of 2. [4]
The
phosphate ion is a polyatomic ion with the empirical
formula PO3−4 and a molar
mass of 94.973 g/mol. It consists of one central phosphorus
atom surrounded by four oxygen atoms in a tetrahedral
arrangement. The phosphate ion carries a negative three formal charge and is
the conjugate base of the hydrogen phosphate ion, HPO2−4, which is the conjugate base of H2PO−4,
the dihydrogen phosphate ion, which in turn is the conjugate base of H3PO4, phosphoric
acid. It is a hypervalent molecule (the phosphorus atom has 10
electrons in its valence shell). Phosphate is also an organophosphorus
compound with the formula OP(OR)3. A phosphate salt forms when a
positively-charged ion attaches to the negatively-charged oxygen atoms of the
ion, forming an ionic compound.
Many phosphates are not soluble in water at standard temperature and pressure.
The sodium, potassium, rubidium, caesium and ammonium phosphates are all water
soluble. Most other phosphates are only slightly soluble or are insoluble in
water. As a rule, the hydrogen and dihydrogen phosphates are slightly more
soluble than the corresponding phosphates. The pyrophosphates
are mostly water soluble.
Biochemistry of phosphates
In
biological systems, phosphorus is found as a free
phosphate ion in solution and is called inorganic phosphate, to
distinguish it from phosphates bound in various phosphate esters. Inorganic
phosphate is generally denoted Pi and at physiological
(neutral) pH
primarily consists of a mixture of HPO2−4 and H2PO−4 ions.Inorganic phosphate can be created by the hydrolysis of pyrophosphate, which is denoted PPi:
However, phosphates are most commonly found in the form of adenosine phosphates, (AMP, ADP and ATP) and in DNA and RNA and can be released by the hydrolysis of ATP or ADP. Similar reactions exist for the other nucleoside diphosphates and triphosphates. Phosphoanhydride bonds in ADP and ATP, or other nucleoside diphosphates and triphosphates, contain high amounts of energy which give them their vital role in all living organisms. They are generally referred to as high energy phosphate, as are the phosphagens in muscle tissue. Compounds such as substituted phosphines have uses in organic chemistry but do not seem to have any natural counterparts.
The addition and removal of phosphate from proteins in all cells is a pivotal strategy in the regulation of metabolic processes.
Geochemistry of phosphates
Phosphates
are the naturally occurring form of the element phosphorus,
found in many phosphate minerals. In mineralogy and geology,
phosphate refers to a rock or ore containing phosphate ions. Inorganic
phosphates are mined to obtain phosphorus for use in agriculture and industry.[1][2][3]The largest phosphorite or rock phosphate deposits in North America lie in the Bone Valley region of central Florida, United States, the Soda Springs region of Idaho, and the coast of North Carolina. Smaller deposits are located in Montana, Tennessee, Georgia and South Carolina near Charleston along Ashley Phosphate road. The small island nation of Nauru and its neighbor Banaba Island, which used to have massive phosphate deposits of the best quality, have been mined excessively. Rock phosphate can also be found in Egypt, Israel, Morocco, Navassa Island, Tunisia, Togo and Jordan, countries that have large phosphate mining industries.
Ecology of phosphates
In
ecological terms, because of its important role in biological systems,
phosphate is a highly sought after resource. Once used, it is often a limiting
nutrient in environments,
and its availability may govern the rate of growth of organisms. This is
generally true of freshwater environments, whereas nitrogen is more often the
limiting nutrient in marine (seawater) environments. Addition of high levels of
phosphate to environments and to micro-environments in which it is typically
rare can have significant ecological consequences. For example, blooms in the
populations of some organisms at the expense of others, and the collapse of populations
deprived of resources such as oxygen (see eutrophication)
can occur. In the context of pollution, phosphates are one component of total dissolved solids,
a major indicator of water quality.Calcium hydroxyapatite and calcite precipitates can be found around bacteria in alluvial topsoil.[8] As clay minerals promote biomineralization, the presence of bacteria and clay minerals resulted in calcium hydroxyapatite and calcite precipitates.[8]
Phosphate deposits can contain significant amounts of naturally occurring heavy metals. Mining operations processing phosphate rock can leave tailings piles containing elevated levels of cadmium, lead, nickel, copper, chromium, and uranium. Unless carefully managed, these waste products can leach heavy metals into groundwater or nearby estuaries. Uptake of these substances by plants and marine life can lead to concentration of toxic heavy metals in food products.
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