strontium [from Strontian, a Scottish town], a metallic chemical element; symbol Sr; at. no. 38; at. wt. 87.62; m.p. 769°C;; b.p. 1,384°C;; sp. gr. 2.6 at 20°C;; valence +2. Strontium is a soft, silver-yellow metal with three allotropic crystalline forms (see allotropy). It is an alkaline-earth metal; in its physical and chemical properties it resembles calcium and barium, the elements above and below it in Group 2 of the periodic table. Since strontium reacts vigorously with water and quickly tarnishes in air, it must be stored out of contact with air and water. Among its compounds are the oxide strontia, SrO; peroxide, SrO₂ ; hydroxide, Sr(OH)₂ ; nitrate, Sr(NO₃)₂ ; the carbonate strontianite, SrCO₃ ; the sulfate celestite, SrSO₄ ; carbide, SrC₂ ; and halides, SrBr₂, SrCl₂, SrF₂, and SrI₂. Celestite and strontianite are the chief ores of strontium. The metal may be prepared by electrolysis of fused strontium chloride; small amounts of the metal are used in semiconductor devices. Although strontium has uses similar to those of calcium and barium, it is rarely employed because of its higher cost. Principal uses of strontium compounds are in pyrotechnics (chiefly the nitrate) and in greases (the hydroxide). In fireworks and signal flares strontium compounds add a bright red or crimson color to the flame. Naturally occurring strontium is a mixture of four stable isotopes. Twelve unstable isotopes exist; the most stable of these is the radioactive isotope strontium-90 (half-life 28.1 years), which is the chief immediate hazard in fallout. As a result of atmospheric nuclear tests, strontium-90 is dispersed in varying concentrations throughout the earth's atmosphere and soil. Because of its chemical similarity to calcium, it is readily taken up in the tissues of plants and animals; it may enter the human food supply, mainly in milk. It is particularly dangerous for growing children as it is easily deposited in the bones and is believed to induce bone cancer and leukemia. Strontium-90 also has some uses in luminous signs and in nuclear batteries. Strontium was first recognized as distinct from barium in 1790 by A. Crawford in a sample of its carbonate from a mine near Strontian, Scotland; his finding was later confirmed by T. C. Hope, M. H. Klaproth, and others. It was first isolated by electrolysis in 1808 by Humphry Davy.

Chemical element, one of the alkaline earth metals, chemical symbol Sr, atomic number 38. A soft metal, it has a silvery lustre when freshly cut but reacts rapidly with air. In both the metal and the compounds (in which it has valence 2), strontium resembles calcium and barium so closely that it has few uses that the other two elements cannot supply more cheaply. The nitrate and chlorate, very volatile, give off brilliant crimson flames and are used in flares, fireworks, and tracer bullets. The radioactive isotope strontium-90 (see radioactivity), produced in nuclear explosions, is the principal health hazard in fallout; it can replace some of the calcium in foods, concentrate in bones and teeth, and cause radiation injury.

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Method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium-87 formed by the decay of the unstable isotope rubidium-87 that was present in the rock at the time of its formation. The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 48.8 billion years. Seealso dating.

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Strontium (IPA for English) is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and strontianite. The ⁹⁰Sr isotope is present in radioactive fallout and has a half-life of 29.10 years. It is named for Strontian - a village in Scotland - as it was found nearby.

Characteristics

Due to its extreme reactivity to oxygen and water, this element occurs naturally only in compounds with other elements, as in the minerals strontianite and celestite.

Strontium is a bright, silvery metal that is softer than calcium and even more reactive in water, with which strontium reacts on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380°C it will only form the oxide spontaneously at room temperature. It should be kept under kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal will ignite spontaneously in air at room temperature. Volatile strontium salts impart a crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four radiostable isotopes.

Applications

As a pure metal strontium is being used in strontium 90%-aluminium 10% alloys of an eutectic composition for the modification of aluminium-silicon casting alloys. The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission.

Other uses:

• ⁸⁹Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic prostate cancer. The strontium acts like calcium and is preferentially incorporated into bone at sites of increased osteogenesis. This localization focuses the radiation exposure on the cancerous lesion.
• ⁹⁰Sr has been used as a power source for radioisotope thermoelectric generators (RTGs). ⁹⁰Sr produces about 0.93 watts of heat per gram (it is lower for the grade of ⁹⁰Sr used in RTGs, which is strontium fluoride). However, ⁹⁰Sr has a lifetime approximately 3 times shorter and has 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.
• ⁹⁰Sr is also used in cancer therapy. Its beta emission and long half-life is ideal for superficial radiotherapy.
• Strontium is one of the constituents of AJ62 alloy, a durable magnesium alloy used in car and motorcycle engines by BMW.
• Since Strontium is so similar to calcium, it is incorporated in the bone. All four isotopes are incorporated, in roughly similar proportions as they are found in nature (please see below). 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. This approach helps to identify the ancient migration patterns as well as the origin of commingled human remains in battlefield burial sites. Strontium thus helps forensic scientists too.
• Strontium is used in studies of neurotransmitter release in neurons. Like calcium, strontium facilitates synaptic vesicle fusion with the synaptic membrane. But unlike calcium, strontium causes asynchronous vesicle fusion. Therefore, replacing calcium in the culture medium with strontium allows scientists to measure the effects of a single vesicle fusion event, e.g., the size of the postsynaptic response elicited by the neurotransmitter content of a single vesicle.

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. A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system (Krom et al, 1999; Krom et al, 2002; Talbot et al. 2000). Due to the differing ages of the rocks that constitute the majority of the Blue and White Nile catchment areas the changing provenance of sediment reaching the River Nile delta and East Mediterranean Sea can be discerned through Sr isotopic studies. Such changes are climatically controlled in the Late Quaternary.

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 (English et al, 2001; Benson et al, 2003). ⁸⁷Sr/⁸⁶Sr ratios in teeth may also be used to track animal migrations (Barnett-Johnson, 2007; Porder et al., 2003) or in criminal forensics.

Strontium atoms are used in an experimental atomic clock with record-setting accuracy.

Compounds

• Ferrite magnets and refining zinc.
• Strontium titanate has an extremely high refractive index and an optical dispersion greater than that of diamond, making it useful in a variety of optics applications. This quality has also led to it being cut into gemstones, in particular as a diamond simulant. However, it is very soft and easily scratches so it is rarely used.
• Strontium carbonate, Strontium nitrate, and Strontium sulfate are commonly used in fireworks for red color.
• Strontium aluminate is used as a bright phosphor with long persistence of phosphorescence.
• Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight.
• Strontium oxide is sometimes used to improve the quality of some pottery glazes.
• Strontium is also commonly used in aerosol paint, such as the Spanish Montana (Montana Hardcore). This is one of the most likely sources of exposure to the public.
• Strontium ranelate is used in the treatment of osteoporosis. It is a prescription drug in the EU and UK, but not in the USA, and is marketed by Servier, a French drug house, as Protelos.

History

The mineral strontianite is named after the Scottish village of Strontian, having been discovered in the lead mines there in 1787. Adair Crawford recognized it as differing from other barium minerals in 1790. Strontium itself was discovered in 1798 by Thomas Charles Hope, and metallic strontium was first isolated by Sir Humphry Davy in 1808 using electrolysis and announced by him in a lecture to the Royal Society on 30th June 1808.

Strontium was among the radioactive materials released by the 1957 Windscale fire.

Occurrence

In 2005, China was the top producer of strontium with almost two-thirds world share followed by Spain and Mexico, reports the British Geological Survey.

Strontium commonly occurs in nature, the 15th most abundant element on earth, averaging 0.034% of all igneous rock and is found chiefly as the form of the sulfate mineral celestite (SrSO₄) and the carbonate strontianite (SrCO₃). Of the two, celestite occurs much more frequently in sedimentary deposits of sufficient size to make development of mining facilities attractive. Strontianite would be the more useful of the two common minerals because strontium is used most often in the carbonate form, but few deposits have been discovered that are suitable for development. The metal can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride:

Sr²⁺ + 2 e^- → Sr

2 Cl^- → Cl_{2 (g)} + 2 e^-

Alternatively it is made by reducing strontium oxide with aluminium in a vacuum at a temperature at which strontium distills off. Three allotropes of the metal exist, with transition points at 235 and 540 °C. The largest commercially exploited deposits are found in England.

See also Strontium minerals.

Isotopes

The alkali earth metal strontium has four stable, naturally occurring isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸⁷Sr (7.0%) and ⁸⁸Sr (82.58%). 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. Thus, there are two sources of ⁸⁷Sr in any material: that formed during primordial nucleo-synthesis along with ⁸⁴Sr, ⁸⁶Sr and ⁸⁸Sr, as well as 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 atomic radius similar to that of calcium, it readily substitutes for Ca in minerals.

Sixteen unstable isotopes are known to exist. Of greatest importance are ⁹⁰Sr with a half-life of 28.78 years and ⁸⁹Sr with a half-life of 50.5 days.

• ⁹⁰Sr is a by-product of nuclear fission which is found in nuclear fallout and presents a health problem since it substitutes for calcium in bone, preventing expulsion from the body. This isotope is one of the best long-lived high-energy beta emitters known, and 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 1986 Chernobyl nuclear accident contaminated a vast area with ⁹⁰Sr. ⁹⁰Sr confined inside a concave silver plaque is also used for the medical treatment of a resected pterygium.
• ⁸⁹Sr is a short-lived artificial radioisotope which provides a health benefit since it substitutes for calcium in bone. In circumstances where cancer patients have widespread and painful bony metastases (secondaries), the administration of ⁸⁹Sr results in the delivery of radioactive emissions (beta particles in this case) directly to the area of bony problem (where calcium turnover is greatest). The ⁸⁹Sr is manufactured as the chloride salt (which is soluble), and when dissolved in normal saline can be injected intravenously. Typically, cancer patients will be treated with a dose of 150 MBq. The patient needs to take precautions following this because their urine becomes contaminated with radioactivity, so they need to sit to urinate and double flush the toilet. The beta particles travel about 3.5mm in bone (energy 0.583 MeV) and 6.5mm in tissue, so there is no requirement to isolate patients who have been treated except to say they should not have any one (especially young children) sitting in their laps for 10-40 days. The variation in time results from the variable clearing time for ⁸⁹Sr which depends on renal function and the number of bony metastases. With a lot of bony metastases, the entire ⁸⁹Sr dose can be taken up into bone and so the entire radioactivity is retained to decay over a 50.5 day half-life. However, where there are few bony metastases, the large proportion of ⁸⁹Sr not taken up by the bone will be filtered by the kidney, so that the effective half-life (a combination of the physical and biological half-life) will be much shorter.

Precautions

In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard.

Effect on the human body

The human body absorbs strontium as if it were calcium. Due to the elements being sufficiently similar chemically, the stable forms of strontium might not pose a significant health threat, but the radioactive ⁹⁰Sr can lead to various bone disorders and diseases, including bone cancer. The strontium unit is used in measuring radioactivity from absorbed ⁹⁰Sr.

A recent in-vitro study conducted the NY College of Dental Sciences using strontium on osteoblasts showed marked improvement on bone-building osteoblasts (http://iadr.confex.com/iadr/2007orleans/techprogram/abstract_89231.htm.)

An innovative drug made by combining strontium with ranelic acid has aided in bone growth, boosted bone density and lessened vertebral, peripheral and hip fractures. Women receiving the drug showed a 12.7% increase in bone density. Women receiving a placebo had a 1.6% decrease. Half the increase in bone density (measured by x-ray densitometry) is attributed to the higher atomic weight of Sr compared with calcium, whereas the other half a true increase in bone mass. It means that strontium ranelate creates new, stronger bone. Strontium ranelate (marketed under the trade names Protelos, Osseor, Protos, Bivalos, Protaxos, Ossum) is registered for treatment of osteoporosis in many countries all over the world. Strontium ranelate has been shown to strengthen bones, according to presentations given at the IOF World Congress on Osteoporosis, in June 2006. It also reduced bone resorption.

Strontium ranelate is registered as a prescription drug in Europe and many countries worldwide. It needs to be prescribed by a doctor, delivered by a pharmacist, and requires strict medical supervision. Currently, (early 2007) it is not available in Canada or the United States.

Several other salts of strontium such as strontium citrate or strontium carbonate are often presented as natural therapies and sold at a dose that is several hundred times higher than the usual strontium intake. Despite the lack of strontium deficit referenced in the medical literature and the lack of information about possible toxicity of strontium supplementation, such compounds can still be sold in the United States under the Dietary Supplements Health and Education Act of 1994. However, their long-term safety and efficacy have never been evaluated on humans using large-scale medical trials. However other long term mammalian studies have shown promising results (e.g. Delannoy et al. Metabolism, 2002 and Farlay et al. J Bone Miner Res, 2005).

Allegedly, an attempt was made in 1968 to poison Alexander Dubček with Sr-90, but it failed.

It is thought that, contrary to popular belief, Gladiators were mostly vegetarian which ensured a greater intake of strontium leading to stronger bones and therefore more resistance to the otherwise bone-breaking and crushing attacks of other gladiators.

See also

• Strontium compounds''

References

• Los Alamos National Laboratory – Strontium. .
• Dasch, J. (1969). Strontium isotopes in weathering profiles, deep-sea sediments, and sedimentary rocks. Geochimica et Cosmochimica Acta, Vol. 33, pp. 1521-1552.
• Krom et al. (1999). The characterisation of Saharan Dusts and Nile particulate matter in surface sediments from the Levantine basin using Sr isotopes. Marine Geology, Vol. 155, pp. 319-330.
• Krom et al. (2002). Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development. Geology, Vol. 30, pp. 71-74.
• Talbot et al., (2000). Strontium isotope evidence for late Pleistocene reestablishment of an integrated Nile drainage network. Geology, Vol. 28, pp. 343-346.
• Barnett-Johnson, R., Grimes, C.B., Royer C.F., Donohoe, C.J. (2007) Identifying the contribution of wild and hatchery Chinook salmon (Oncorhynchus tshawytscha) to the ocean fishery using otolith microstructure as natural tags. Canadian Journal of Fisheries and Aquatic Sciences, Vol.64, pp. 1683-1692.
• Benson, L., Cordell, L., Vincent, K., Taylor, H., Stein, J., Farmer, G., and Kiyoto, F. (2003) Ancient maize from Chacoan great houses: where was it grown?: Proceedings of the National Academy of Sciences, Vol. 22, pp. 13111-13115.
• English, N.B., Betancourt, J.L., Dean, J.S. and J. Quade (2001) Strontium Isotopes Reveal Distant Sources of Architectural Timber in Chaco Canyon, New Mexico. Proceedings of the National Academy of Sciences of the United States of America, Vol. 98, pp. 11891-11896
• Porder, S., Paytan, A., and E.A. Hadly (2003) Mapping the origin of faunal assemblages using strontium isotopes. Paleobiology, 29: 197 - 204.