Strontium

Strontium is the chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to air. Strontium has physical and chemical properties similar to those of its two vertical neighbors in the periodic table, calcium and barium. It occurs naturally mainly in the minerals celestine and strontianite, and is mostly mined from these. While natural strontium is stable, the synthetic 90Sr isotope is radioactive and is one of the most dangerous components of nuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium, on the other hand, is not hazardous to health.

Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank; it was identified as a new element the next year from its crimson-red flame test color. Strontium was first isolated as a metal in 1808 by Humphry Davy using the then-newly discovered process of electrolysis. During the 19th century, strontium was mostly used in the production of sugar from sugar beet (see strontian process). At the peak of production of television cathode ray tubes, as much as 75 percent of strontium consumption in the United States was used for the faceplate glass.[6] With the replacement of cathode ray tubes with other display methods, consumption of strontium has dramatically declined.[6]

Strontium,  38Sr
Strontium destilled crystals
Strontium
Pronunciation/ˈstrɒnʃiəm, -tiəm/ (STRON-shee-əm, -⁠tee-əm)
Appearancesilvery white metallic; with a pale yellow tint[1]
Standard atomic weight Ar, std(Sr)87.62(1)[2]
Strontium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ca

Sr

Ba
rubidiumstrontiumyttrium
Atomic number (Z)38
Groupgroup 2 (alkaline earth metals)
Periodperiod 5
Blocks-block
Element category  alkaline earth metal
Electron configuration[Kr] 5s2
Electrons per shell
2, 8, 18, 8, 2
Physical properties
Phase at STPsolid
Melting point1050 K ​(777 °C, ​1431 °F)
Boiling point1650 K ​(1377 °C, ​2511 °F)
Density (near r.t.)2.64 g/cm3
when liquid (at m.p.)2.375 g/cm3
Heat of fusion7.43 kJ/mol
Heat of vaporization141 kJ/mol
Molar heat capacity26.4 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 796 882 990 1139 1345 1646
Atomic properties
Oxidation states+1,[3] +2 (a strongly basic oxide)
ElectronegativityPauling scale: 0.95
Ionization energies
  • 1st: 549.5 kJ/mol
  • 2nd: 1064.2 kJ/mol
  • 3rd: 4138 kJ/mol
Atomic radiusempirical: 215 pm
Covalent radius195±10 pm
Van der Waals radius249 pm
Color lines in a spectral range
Spectral lines of strontium
Other properties
Natural occurrenceprimordial
Crystal structureface-centered cubic (fcc)
Face-centered cubic crystal structure for strontium
Thermal expansion22.5 µm/(m·K) (at 25 °C)
Thermal conductivity35.4 W/(m·K)
Electrical resistivity132 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic
Magnetic susceptibility−92.0·10−6 cm3/mol (298 K)[4]
Young's modulus15.7 GPa
Shear modulus6.03 GPa
Poisson ratio0.28
Mohs hardness1.5
CAS Number7440-24-6
History
Namingafter the mineral strontianite, itself named after Strontian, Scotland
DiscoveryWilliam Cruickshank (1787)
First isolationHumphry Davy (1808)
Main isotopes of strontium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
82Sr syn 25.36 d ε 82Rb
83Sr syn 1.35 d ε 83Rb
β+ 83Rb
γ
84Sr 0.56% stable
85Sr syn 64.84 d ε 85Rb
γ
86Sr 9.86% stable
87Sr 7.00% stable
88Sr 82.58% stable
89Sr syn 50.52 d ε 89Rb
β 89Y
90Sr trace 28.90 y β 90Y

Characteristics

Strontium 1
Oxidized dendritic strontium

Strontium is a divalent silvery metal with a pale yellow tint whose properties are mostly intermediate between and similar to those of its group neighbors calcium and barium.[7] It is softer than calcium and harder than barium. Its melting (777 °C) and boiling (1655 °C) points are lower than those of calcium (842 °C and 1757 °C respectively); barium continues this downward trend in the melting point (727 °C), but not in the boiling point (2170 °C). The density of strontium (2.64 g/cm3) is similarly intermediate between those of calcium (1.54 g/cm3) and barium (3.594 g/cm3).[8] Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C.[9]

The standard electrode potential for the Sr2+/Sr couple is −2.89 V, approximately midway between those of the Ca2+/Ca (−2.84 V) and Ba2+/Ba (−2.92 V) couples, and close to those of the neighboring alkali metals.[10] Strontium is intermediate between calcium and barium in its reactivity toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. Strontium metal burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature, it forms only the oxide spontaneously.[8] Besides the simple oxide SrO, the peroxide SrO2 can be made by direct oxidation of strontium metal under a high pressure of oxygen, and there is some evidence for a yellow superoxide Sr(O2)2.[11] Strontium hydroxide, Sr(OH)2, is a strong base, though it is not as strong as the hydroxides of barium or the alkali metals.[12] All four dihalides of strontium are known.[13]

Due to the large size of the heavy s-block elements, including strontium, a vast range of coordination numbers is known, from 2, 3, or 4 all the way to 22 or 24 in SrCd11 and SrZn13. The Sr2+ ion is quite large, so that high coordination numbers are the rule.[14] The large size of strontium and barium plays a significant part in stabilising strontium complexes with polydentate macrocyclic ligands such as crown ethers: for example, while 18-crown-6 forms relatively weak complexes with calcium and the alkali metals, its strontium and barium complexes are much stronger.[15]

Organostrontium compounds contain one or more strontium–carbon bonds. They have been reported as intermediates in Barbier-type reactions.[16][17][18] Although strontium is in the same group as magnesium, and organomagnesium compounds are very commonly used throughout chemistry, organostrontium compounds are not similarly widespread because they are more difficult to make and more reactive. Organostrontium compounds tend to be more similar to organoeuropium or organosamarium compounds due to the similar ionic radii of these elements (Sr2+ 118 pm; Eu2+ 117 pm; Sm2+ 122 pm). Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, strontium dicyclopentadienyl, Sr(C5H5)2, must be made by directly reacting strontium metal with mercurocene or cyclopentadiene itself; replacing the C5H5 ligand with the bulkier C5(CH3)5 ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.[19]

Because of its extreme reactivity with oxygen and water, strontium occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestine. It is kept under a liquid hydrocarbon such as mineral oil or kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal is pyrophoric, meaning that it will ignite spontaneously in air at room temperature. Volatile strontium salts impart a bright red color to flames, and these salts are used in pyrotechnics and in the production of flares.[8] Like calcium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, strontium metal dissolves directly in liquid ammonia to give a dark blue solution.[7]

Isotopes

Natural strontium is a mixture of four stable isotopes: 84Sr, 86Sr, 87Sr, and 88Sr.[8] Their abundance increases with increasing mass number and the heaviest, 88Sr, makes up about 82.6% of all natural strontium, though the abundance varies due to the production of radiogenic 87Sr as the daughter of long-lived beta-decaying 87Rb.[20] Of the unstable isotopes, the primary decay mode of the isotopes lighter than 85Sr is electron capture or positron emission to isotopes of rubidium, and that of the isotopes heavier than 88Sr is electron emission to isotopes of yttrium. Of special note are 89Sr and 90Sr. The former has a half-life of 50.6 days and is used to treat bone cancer due to strontium's chemical similarity and hence ability to replace calcium.[21][22] While 90Sr (half-life 28.90 years) has been used similarly, it is also an isotope of concern in fallout from nuclear weapons and nuclear accidents due to its production as a fission product. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia.[23] The 1986 Chernobyl nuclear accident contaminated about 30,000 km2 with greater than 10 kBq/m2 with 90Sr, which accounts for 5% of the core inventory of 90Sr.[24]

History

FlammenfärbungSr
Flame test for strontium

Strontium is named after the Scottish village of Strontian (Gaelic Sròn an t-Sìthein), where it was discovered in the ores of the lead mines.[25] Thomas Charles Hope originally named the element strontianite, but the name was soon after shortened to strontium.[26]

In 1790, Adair Crawford, a physician engaged in the preparation of barium, and his colleague William Cruickshank, recognised that the Strontian ores exhibited properties that differed from those in other "heavy spars" sources.[27] This allowed Adair to conclude on page 355 "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined." The physician and mineral collector Friedrich Gabriel Sulzer analysed together with Johann Friedrich Blumenbach the mineral from Strontian and named it strontianite. He also came to the conclusion that it was distinct from the witherite and contained a new earth (neue Grunderde).[28] In 1793 Thomas Charles Hope, a professor of chemistry at the University of Glasgow proposed the name strontites.[29][30][31][32] He confirmed the earlier work of Crawford and recounted: "... Considering it a peculiar earth I thought it necessary to give it an name. I have called it Strontites, from the place it was found; a mode of derivation in my opinion, fully as proper as any quality it may possess, which is the present fashion." The element was eventually isolated by Sir Humphry Davy in 1808 by the electrolysis of a mixture containing strontium chloride and mercuric oxide, and announced by him in a lecture to the Royal Society on 30 June 1808.[33] In keeping with the naming of the other alkaline earths, he changed the name to strontium.[34][35][36][37][38]

The first large-scale application of strontium was in the production of sugar from sugar beet. Although a crystallisation process using strontium hydroxide was patented by Augustin-Pierre Dubrunfaut in 1849[39] the large scale introduction came with the improvement of the process in the early 1870s. The German sugar industry used the process well into the 20th century. Before World War I the beet sugar industry used 100,000 to 150,000 tons of strontium hydroxide for this process per year.[40] The strontium hydroxide was recycled in the process, but the demand to substitute losses during production was high enough to create a significant demand initiating mining of strontianite in the Münsterland. The mining of strontianite in Germany ended when mining of the celestine deposits in Gloucestershire started.[41] These mines supplied most of the world strontium supply from 1884 to 1941. Although the celestine deposits in the Granada basin were known for some time the large scale mining did not start before the 1950s.[42]

During atmospheric nuclear weapons testing, it was observed that strontium-90 is one of the nuclear fission products with a relative high yield. The similarity to calcium and the chance that the strontium-90 might become enriched in bones made research on the metabolism of strontium an important topic.[43][44]

Occurrence

Celestine Poland
The mineral celestine (SrSO4)

Strontium commonly occurs in nature, being the 15th most abundant element on Earth (its heavier congener barium being the 14th), estimated to average approximately 360 parts per million in the Earth's crust[45] and is found chiefly as the sulfate mineral celestine (SrSO4) and the carbonate strontianite (SrCO3). Of the two, celestine occurs much more frequently in deposits of sufficient size for mining. Because strontium is used most often in the carbonate form, strontianite would be the more useful of the two common minerals, but few deposits have been discovered that are suitable for development.[46]

In groundwater strontium behaves chemically much like calcium. At intermediate to acidic pH Sr2+ is the dominant strontium species. In the presence of calcium ions, strontium commonly forms coprecipitates with calcium minerals such as calcite and anhydrite at an increased pH. At intermediate to acidic pH, dissolved strontium is bound to soil particles by cation exchange.[47]

The mean strontium content of ocean water is 8 mg/l.[48][49] At a concentration between 82 and 90 µmol/l of strontium, the concentration is considerably lower than the calcium concentration, which is normally between 9.6 and 11.6 mmol/l.[50][51] It is nevertheless much higher than that of barium, 13 μg/l.[8]

Production

World Strontium Production 2014
Strontium producers in 2014[52]

The three major producers of strontium as celestine as of 2015 are China (150,000 t), Spain (90,000 t), and Mexico (70,000 t); Argentina (10,000 t) and Morocco (2,500 t) are smaller producers. Although strontium deposits occur widely in the United States, they have not been mined since 1959.[52]

A large proportion of mined celestine (SrSO4) is converted to the carbonate by two processes. Either the celestine is directly leached with sodium carbonate solution or the celestine is roasted with coal to form the sulfide. The second stage produces a dark-coloured material containing mostly strontium sulfide. This so-called "black ash" is dissolved in water and filtered. Strontium carbonate is precipitated from the strontium sulfide solution by introduction of carbon dioxide.[53] The sulfate is reduced to the sulfide by the carbothermic reduction:

SrSO4 + 2 C → SrS + 2 CO2

About 300,000 tons are processed in this way annually.[54]

The metal is produced commercially by reducing strontium oxide with aluminium. The strontium is distilled from the mixture.[54] Strontium metal can also be prepared on a small scale by electrolysis of a solution of strontium chloride in molten potassium chloride:[10]

Sr2+ + 2
e
→ Sr
2 Cl → Cl2 + 2
e

Applications

Monitor.arp
CRT computer monitor front panel made from strontium and barium oxide-containing glass. This application used to consume most of the world's production of strontium.

Consuming 75% of production, the primary use for strontium is in glass for colour television cathode ray tubes,[54] where it prevents X-ray emission.[55][56] This application for strontium is declining because CRTs are being replaced by other display methods. This decline has a significant influence on the mining and refining of strontium.[46] All parts of the CRT must absorb X-rays. In the neck and the funnel of the tube, lead glass is used for this purpose, but this type of glass shows a browning effect due to the interaction of the X-rays with the glass. Therefore, the front panel is made from a different glass mixture with strontium and barium to absorb the X-rays. The average values for the glass mixture determined for a recycling study in 2005 is 8.5% strontium oxide and 10% barium oxide.[57]

Because strontium is so similar to calcium, it is incorporated in the bone. All four stable isotopes are incorporated, in roughly the same proportions they are found in nature. However, the actual distribution of the isotopes tends to vary greatly from one geographical location to another. Thus, analyzing the bone of an individual can help determine the region it came from.[58][59] This approach helps to identify the ancient migration patterns and the origin of commingled human remains in battlefield burial sites.[60]

87Sr/86Sr ratios are commonly used to determine the likely provenance areas of sediment in natural systems, especially in marine and fluvial environments. Dasch (1969) showed that surface sediments of Atlantic displayed 87Sr/86Sr ratios that could be regarded as bulk averages of the 87Sr/86Sr ratios of geological terranes from adjacent landmasses.[61] A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system.[62] Due to the differing ages of the rocks that constitute the majority of the Blue and White Nile, catchment areas of the changing provenance of sediment reaching the River Nile delta and East Mediterranean Sea can be discerned through strontium isotopic studies. Such changes are climatically controlled in the Late Quaternary.[62]

More recently, 87Sr/86Sr ratios have also been used to determine the source of ancient archaeological materials such as timbers and corn in Chaco Canyon, New Mexico.[63][64] 87Sr/86Sr ratios in teeth may also be used to track animal migrations.[65][66]

Strontium aluminate is frequently used in glow in the dark toys, as it is chemically and biologically inert.

Ignis Brunensis 2010-05-22 (5)
Strontium salts are added to fireworks in order to create red colors

Strontium carbonate and other strontium salts are added to fireworks to give a deep red colour.[67] This same effect identifies strontium cations in the flame test. Fireworks consumes about 5% of the world's production.[54] Strontium carbonate is used in the manufacturing of hard ferrite magnets.[68][69]

Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight.[70] Small amounts are used in the refining of zinc to remove small amounts of lead impurities.[8] The metal itself has a limited use as a getter, to remove unwanted gases in vacuums by reacting with them, although barium may also be used for this purpose.[10]

The ultra-narrow optical transition between the [Kr]5s2 1S0 electronic ground state and the metastable [Kr]5s5p 3P0 excited state of 87Sr is one of the leading candidates for the future re-definition of the second in terms of an optical transition as opposed to the current definition derived from a microwave transition between different hyperfine ground states of 133Cs.[71] Current optical atomic clocks operating on this transition already surpass the precision and accuracy of the current definition of the second.

Radioactive strontium

89Sr is the active ingredient in Metastron,[72] a radiopharmaceutical used for bone pain secondary to metastatic bone cancer. The strontium is processed like calcium by the body, preferentially incorporating it into bone at sites of increased osteogenesis. This localization focuses the radiation exposure on the cancerous lesion.[22]

Soviet RTG
RTGs from Soviet era lighthouses

90Sr has been used as a power source for radioisotope thermoelectric generators (RTGs). 90Sr produces approximately 0.93 watts of heat per gram (it is lower for the form of 90Sr used in RTGs, which is strontium fluoride).[73] However, 90Sr has one third the lifetime and a lower density than 238Pu, another RTG fuel. The main advantage of 90Sr is that it is cheaper than 238Pu and is found in nuclear waste. The Soviet Union deployed nearly 1000 of these RTGs on its northern coast as a power source for lighthouses and meteorology stations.[74][75]

Biological role

Strontium
Hazards
GHS pictograms The flame pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
H261, H315
P223, P231+232, P370+378, P422[76]
NFPA 704
Flammability code 0: Will not burn. E.g., waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroformReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
0
2
2

Acantharea, a relatively large group of marine radiolarian protozoa, produce intricate mineral skeletons composed of strontium sulfate.[77] In biological systems, calcium is substituted in a small extent by strontium.[78] In the human body, most of the absorbed strontium is deposited in the bones. The ratio of strontium to calcium in human bones is between 1:1000 and 1:2000, roughly in the same range as in the blood serum.[79]

Effect on the human body

The human body absorbs strontium as if it were its lighter congener calcium. Because the elements are chemically very similar, stable strontium isotopes do not pose a significant health threat. The average human has an intake of about two milligrams of strontium a day.[80] In adults, strontium consumed tends to attach only to the surface of bones, but in children, strontium can replace calcium in the mineral of the growing bones and thus lead to bone growth problems.[81]

The biological half-life of strontium in humans has variously been reported as from 14 to 600 days,[82][83] 1000 days,[84] 18 years,[85] 30 years[86] and, at an upper limit, 49 years.[87] The wide-ranging published biological half-life figures are explained by strontium's complex metabolism within the body. However, by averaging all excretion paths, the overall biological half-life is estimated to be about 18 years.[88] The elimination rate of strontium is strongly affected by age and sex, due to differences in bone metabolism.[89]

The drug strontium ranelate aids bone growth, increases bone density, and lessens the incidence of vertebral, peripheral, and hip fractures.[90][91] However, strontium ranelate also increases the risk of venous thromboembolism, pulmonary embolism, and serious cardiovascular disorders, including myocardial infarction. Its use is therefore now restricted.[92] Its beneficial effects are also questionable, since the increased bone density is partially caused by the increased density of strontium over the calcium which it replaces. Strontium also bioaccumulates in the body.[93] Despite restrictions on strontium ranelate, strontium is still contained in some supplements.[94][95] There is not much scientific evidence on risks of strontium chloride when taken by mouth. Those with a personal or family history of blood clotting disorders are advised to avoid strontium.[94][95]

Strontium has been shown to inhibit sensory irritation when applied topically to the skin.[96][97] Topically applied, strontium has been shown to accelerate the recovery rate of the epidermal permeability barrier (skin barrier).[98]

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Bibliography

External links

Alkaline earth metal

The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The elements have very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.Structurally, they have in common an outer s- electron shell which is full;

that is, this orbital contains its full complement of two electrons, which these elements readily lose to form cations with charge +2, and an oxidation state of +2.All the discovered alkaline earth metals occur in nature, although radium occurs only through the decay chain of uranium and thorium and not as a primordial element. Experiments have been conducted to attempt the synthesis of element 120, the next potential member of the group, but they have all met with failure.

Celestine (mineral)

Celestine or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys.

Chemical depilatory

A chemical depilatory is a cosmetic preparation used to remove hair from the skin. Common active ingredients are salts of thioglycolic acid and thiolactic acids. These compounds break the disulfide bonds in keratin and also hydrolyze the hair so that it is easily removed. Formerly, sulfides such as strontium sulfide were used, but due to their unpleasant odor, they have been replaced by thiols.The main chemical reaction effected by the thioglycolate is:

2 HSCH2CO2H (thioglycolic acid) + R-S-S-R (cystine) → HO2CCH2-S-S-CH2CO2H (dithiodiglycolic acid) + 2 RSH (cysteine)Chemical depilatories contain 5–6% calcium thioglycolate in a cream base (to avoid runoff). Calcium hydroxide or strontium hydroxide maintain a pH of about 12. Hair destruction requires about 10 minutes. Depilation is followed by careful rinsing with water, and various conditioners are applied to restore the skin's pH to normal. Depilation does not destroy the dermal papilla, and the hair grows back.Chemical depilatories are available in gel, cream, lotion, aerosol, roll-on, and powder forms. Common brands include Nair, Magic Shave and Veet.

Depilatory ointments, or plasters, were known to Greek and Roman authors as psilothrum.

Ferrite (magnet)

A ferrite is a ceramic material made by mixing and firing large proportions iron(III) oxide (Fe2O3, rust) blended with small proportions of one or more additional metallic elements, such as barium, manganese, nickel, and zinc. They are both electrically non-conductive, meaning that they are insulators, and ferrimagnetic, meaning they can easily be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their resistance to being demagnetized (magnetic coercivity).

Hard ferrites have high coercivity, so are difficult to demagnetize. They are used to make permanent magnets for refrigerator magnets, loudspeakers, small electric motors, and so on.

Soft ferrites have low coercivity, so they easily change their magnetization, and act as conductors of magnetic fields. They are used in the electronics industry to make efficient magnetic cores called ferrite cores for high-frequency inductors and transformers, and in various microwave components.

Ferrite compounds have extremely low cost, being made of mostly rusted iron (iron oxide), and also have excellent corrosion resistance. They are very stable and difficult to demagnetize, and can be made with both high and low coercive forces. Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology synthesized the first ferrite compounds in 1930.

Hydroxyapatite

Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.

Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. Hydroxyapatite crystals are also found in the small calcifications, within the pineal gland and other structures, known as corpora arenacea or 'brain sand'.

Igneous petrology

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Isotopes of strontium

The alkaline earth metal strontium (38Sr) has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). Its standard atomic weight is 87.62(1).

Only 87Sr is radiogenic; it is produced by decay from the radioactive alkali metal 87Rb, which has a half-life of 4.88 × 1010 years (i.e. more than three times longer than the current age of the universe). Thus, there are two sources of 87Sr in any material: primordial, formed during nucleosynthesis along with 84Sr, 86Sr and 88Sr; and that formed by radioactive decay of 87Rb. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because strontium has an electron configuration similar to that of calcium, it readily substitutes for Ca in minerals.

In addition to the four stable isotopes, thirty-one unstable isotopes of strontium are known to exist (see Table, below): the longest-lived of these are 90Sr with a half-life of 28.9 years and 85Sr with a half-life of 64.853 days. Of importance are strontium-89 (89Sr) with a half-life of 50.57 days, and strontium-90 (90Sr). They decay by emitting an electron and an antineutrino () in beta decay (β decay) to become yttrium:

89Sr is an artificial radioisotope used in treatment of bone cancer. In circumstances where cancer patients have widespread and painful bony metastases, the administration of 89Sr results in the delivery of beta particles directly to the area of bony problem,[further explanation needed] where calcium turnover is greatest.

90Sr is a by-product of nuclear fission, present in nuclear fallout. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr. It causes health problems, as it substitutes for calcium in bone, preventing expulsion from the body. Because it is a long-lived high-energy beta emitter, it is used in SNAP (Systems for Nuclear Auxiliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is required.

The lightest isotope is 73Sr and the heaviest is 107Sr.

All other strontium isotopes have half-lives shorter than 55 days, most under 100 minutes.

Strontium-90

Strontium-90 (90Sr) is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.

Strontium Dog

Strontium Dog is a long-running British comics series featuring in the British science fiction weekly 2000 AD, starring Johnny Alpha, a mutant bounty hunter with an array of imaginative gadgets and weapons.

The series was created by writer John Wagner (under the pseudonym T. B. Grover) and artist Carlos Ezquerra for Starlord, a short-lived weekly science fiction comic, in 1978. When Starlord was cancelled, the series transferred to 2000 AD. In 1980, Wagner was joined by co-writer Alan Grant, although scripts were normally credited to Grant alone. Grant wrote the series solo from 1988 to 1990. After Ezquerra's death in October 2018 the series was put in indefinite hiatus with no plans for its continuation as of January 2019.

Strontium bromide

Strontium bromide is a chemical compound with a formula SrBr2. At room temperature it is a white, odorless, crystalline powder. Strontium bromide burns bright red in a flame test. It is used in flares and also has some pharmaceutical uses.

Strontium carbonate

Strontium carbonate (SrCO3) is the carbonate salt of strontium that has the appearance of a white or grey powder. It occurs in nature as the mineral strontianite.

Strontium chloride

Strontium chloride (SrCl2) is a salt of strontium and chloride. It is a typical salt, forming neutral aqueous solutions. Like all compounds of Sr, this salt emits a bright red colour in a flame; in fact it is used as a source of redness in fireworks. Its chemical properties are intermediate between those for barium chloride, which is more toxic, and calcium chloride.

Strontium fluoride

Strontium fluoride, SrF2, also called strontium difluoride and strontium(II) fluoride, is a fluoride of strontium. It is a stable brittle white crystalline solid with melting point of 1477 °C and boiling point 2460 °C. It appears as the mineral strontiofluorite.

Strontium hydroxide

Strontium hydroxide, Sr(OH)2, is a caustic alkali composed of one strontium ion and two hydroxide ions. It is synthesized by combining a strontium salt with a strong base. Sr(OH)2 exists in anhydrous, monohydrate, or octahydrate form.

Strontium nitrate

Strontium nitrate is an inorganic compound made of the elements strontium and nitrogen with the formula Sr(NO3)2. This colorless solid is used as a red colorant and oxidizer in pyrotechnics.

Strontium oxide

Strontium oxide or strontia, SrO, is formed when strontium reacts with oxygen. Burning strontium in air results in a mixture of strontium oxide and strontium nitride. It also forms from the decomposition of strontium carbonate SrCO3. It is a strongly basic oxide.

Strontium sulfate

Strontium sulfate (SrSO4) is the sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is more soluble in dilute HCl and nitric acid and appreciably soluble in alkali chloride solutions (e.g. sodium chloride).

Strontium sulfide

Strontium sulfide is the inorganic compound with the formula SrS. It is a white solid. The compound is an intermediate in the conversion of strontium sulfate, the main strontium ore called celestite, to other more useful compounds.

Strontium titanate

Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure. At low temperatures it approaches a ferroelectric phase transition with a very large dielectric constant ~104 but remains paraelectric down to the lowest temperatures measured as a result of quantum fluctuations, making it a quantum paraelectric. It was long thought to be a wholly artificial material, until 1982 when its natural counterpart—discovered in Siberia and named tausonite—was recognised by the IMA. Tausonite remains an extremely rare mineral in nature, occurring as very tiny crystals. Its most important application has been in its synthesized form wherein it is occasionally encountered as a diamond simulant, in precision optics, in varistors, and in advanced ceramics.

The name tausonite was given in honour of Lev Vladimirovich Tauson (1917–1989), a Russian geochemist. Disused trade names for the synthetic product include strontium mesotitanate, Fabulite, Diagem, and Marvelite. Other than its type locality of the Murun Massif in the Sakha Republic, natural tausonite is also found in Cerro Sarambi, Concepción department, Paraguay; and along the Kotaki River of Honshū, Japan.

Strontium compounds

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