Elemental potassium does not occur in nature because it reacts violently with water. As various compounds, potassium makes up about 1.5% of the weight of the Earth's crust and is the seventh most abundant element. As it is very electropositive and highly reactive potassium metal is difficult to obtain from its minerals.
Selenium occurs naturally in a number of inorganic forms, including selenide, selenate, and selenite. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.
Selenium has a biological role, and it is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine. In these compounds selenium plays a role analogous to that of sulfur.
Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain plants. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.
See also Selenide minerals.
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− → 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. The largest commercially exploited deposits are found in England.
- ↑ Mark Winter. "Potassium: Key Information". Webelements.
- ↑ "Public Health Statement: Selenium" (PDF). Agency for Toxic Substances and Disease Registry. Retrieved on 2009-01-05.
- ↑ British Geological Survey (2009). World mineral production 2003–07. Keyworth, Nottingham: British Geological Survey. ISBN 978-0-85272-639-6, http://www.bgs.ac.uk/mineralsuk/downloads/wmp_2003_2007.pdf. Retrieved on 6 April 2009.
- ↑ Ober, Joyce A.. "Mineral Comodity Summaries 2008: Strontium" (PDF). United States Geological Survey. Retrieved on 2008-10-14.