STRONTIUM ORE

Strontium ( /ˈstrɒnʃiəm/ STRON-shee-əm, /ˈstrɒntiəm/ STRON-tee-əm, or /ˈstrɒnʃəm/ STRON-shəm) 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 28.90 years. Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was first discovered.

Detailed descrption

Strontium is a grey, silvery metal that is softer than calcium and even more reactive in water, with which it 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, at room temperature it will only form the oxide spontaneously.^[2]

Because of its extreme reactivity with oxygen and water, this element occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestite. 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 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 stable isotopes.^[2]

History

Strontium is named after the Scottish village of Strontian, having been discovered in the ores taken from the lead mines.^[3] In 1790, Adair Crawford, a physician engaged in the preparation of barium, recognised that the Strontian ores exhibited different properties to those normally seen with other "heavy spars" sources. This allowed him to conclude "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined". The new mineral was named strontites in 1793 by Thomas Charles Hope, a professor of chemistry at the University of Glasgow.^[4] 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.^[5] In keeping with the naming of the other alkaline earths, he changed the name to strontium.^[6][7][8]

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 Dubrunfaut in 1849^[9] 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 19th century. Prior to the World War I the beet sugar industry used 100000 to 150000 tons of strontium hydroxide for this process per year. ^[10] 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 celestite deposits in Gloucestershire started.^[11] These mines supplied most of the world strontium supply from 1884 to 1941 ^[12]

Occurrence

According to the British Geological Survey, China was the top producer of strontium in 2007, with over two-thirds world share, followed by Spain, Mexico, Turkey, Argentina and Iran.^[13]

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.^[14] The metal can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride:

Sr²⁺ + 2 e⁻ → Sr

2 Cl⁻ → Cl₂ (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.

Isotopes

Main article: Isotopes of strontium

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 in stars 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.

Applications

As a pure metal strontium is used in strontium 90%-aluminium 10% alloys of an eutectic composition for the modification of aluminium-silicon casting alloys.^[15] Strontium is 2% by weight of AJ62 alloy, a durable, creep-resistant magnesium alloy used in car and motorcycle engines by BMW.

Strontium is used in scientific 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.^[16][17]

Compounds

The primary use for strontium compounds is in glass for colour television cathode ray tubes to prevent X-ray emission.^[18][19] All parts of the CRT tube have to 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 has to use a different glass mixture, in which strontium and barium are the X-ray absorbing materials. The average values for the glass mixture determined for a recycling study in 2005 is 8.5% strontium oxide and 10% barium oxide.^[20]

Other applications are as follows:

• Ferrite magnets and refining zinc.^[2]
• 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.^[2]
• Strontium carbonate, strontium nitrate, and strontium sulfate are commonly used in fireworks for red color sometimes for other colors too.
• 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 ranelate is used in the treatment of osteoporosis. It is a prescription drug in the EU, but not in the USA.
• Strontium barium niobate can be used in outdoors holographic 3D displays as a "screen".^[21]
• Strontium phosphide is an inorganic compound with the formula Sr₃P₂ and is used as a laboratory reagent and in the manufacture of chemically reactive devices.

Radioactive strontium isotopes

⁸⁹Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic bone 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 form of ⁹⁰Sr used in RTGs, which is strontium fluoride).^[22] 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.

Because strontium is so similar to calcium, it is incorporated in the bone. All four stable 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.

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.^[23] A good example of a fluvial-marine system to which Sr isotope provenance studies have been successfully employed is the River Nile-Mediterranean system,^[24] 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 Sr isotopic studies. Such changes are climatically controlled in the Late Quaternary.

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