Alkaline earth metal
Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it 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.
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 beets (see strontian process). At the peak of production of television cathode-ray tubes, as much as 75% of strontium consumption in the United States was used for the faceplate glass. With the replacement of cathode-ray tubes with other display methods, consumption of strontium has dramatically declined. …
Strontium is named after the Scottish village of Strontian (Scottish Gaelic: Sròn an t-Sìthein), where it was discovered in the ores of the lead mines.
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. This allowed Crawford 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). In 1793 Thomas Charles Hope, a professor of chemistry at the University of Glasgow studied the mineral and proposed the name strontites. He confirmed the earlier work of Crawford and recounted: "... Considering it a peculiar earth I thought it necessary to give it an name. …
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. It is softer than calcium and harder than barium. Its melting (777 °C) and boiling (1377 °C) points are lower than those of calcium (842 °C and 1484 °C respectively); barium continues this downward trend in the melting point (727 °C), but not in the boiling point (1900 °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). Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C.
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. 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. 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. …
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 and is found chiefly in two mineral forms: the sulfate (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. Because of the way it reacts with air and water, strontium only exists in nature when combined to form minerals. Naturally occurring strontium is stable, but its synthetic isotope Sr-90 is only produced by nuclear fallout.
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.
The mean strontium content of ocean water is 8 mg/L. 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. …
Consuming 75% of production, the primary use for strontium was in glass for colour television cathode-ray tubes, where it prevented X-ray emission. This application for strontium has been declining because CRTs are being replaced by other display methods. This decline has a significant influence on the mining and refining of strontium. 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.
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. This approach helps to identify the ancient migration patterns and the origin of commingled human remains in battlefield burial sites.
87Sr/86Sr ratios are commonly used to determine the likely provenance areas of sediment in natural systems, especially in marine and fluvial environments. …
Natural strontium is a mixture of four stable isotopes: 84Sr, 86Sr, 87Sr, and 88Sr. Of these isotopes, 88Sr is the most abundant, making up about 82.6% of all natural strontium, though the abundance varies somewhat due to the production of radiogenic 87Sr as the daughter of long-lived beta-decaying 87Rb. This is the basis of rubidium–strontium dating.
Of the unstable isotopes, the primary decay mode of the isotopes lighter than 86Sr 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 50.56 days and is used to treat bone cancer due to strontium's chemical similarity and hence ability to replace calcium. While 90Sr (half-life 28.91 years) has been used similarly, it is more an isotope of concern in fallout from nuclear weapons and nuclear accidents due to its longer life (both are produced as fission products). Its presence in bones, where all strontium accumulates, can cause bone cancer, cancer of nearby tissues, and leukemia. The 1986 Chernobyl nuclear accident contaminated about 30,000 km2 with greater than 10 kBq/m2 of 90Sr, or an estimated 5% of the reactor's total content of 90Sr.
Acantharea, a relatively large group of marine radiolarian protozoa, produce intricate mineral skeletons composed of strontium sulfate. In biological systems, calcium is substituted to a small extent by strontium. 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.
The human body absorbs strontium as if it were its lighter congener calcium. Stable strontium isotopes pose a significant health threat because the average human has an intake of about two milligrams of strontium a day. According to the CDC, strontium behaves very much like calcium. A large portion of the strontium will accumulate in bone. In adults, strontium mostly attaches to the surfaces of bones. In children, whose bones are still growing, strontium may be used by the body to create the hard bone mineral itself. As a result the strontium will be stored in the bone for years. Because of the way bone grows, strontium will be locally dissolved from bone and recirculate through the bloodstream, where it may be reused by growing bone, or be eliminated. This process accounts for the slow removal of strontium from the body.
The biological half-life of strontium in humans has variously been reported as from 14 to 600 days, 1,000 days, 18 years, 30 years and, at an upper limit, 49 years. …