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Latin selene meaning "Moon"
Minerals; Inorganic; Column Six
History and authority: Selenium was discovered by Berzelius in 1818 and was introduced to Homoeopathy by Hering. Allen's Encyclop. Mat. Med. Vol. VIII, 576.
Description of the substance
Selenium occurs as 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.
Selenium has 28 isotopes, of which 5 are stable.
While free selenium is nontoxic, many of its compounds are extremely toxic, and have modes of action similar to that of arsenic. Hydrogen selenide and other compounds are very toxic. Plants grown in selenium-rich soils, such as locoweed, can cause serious effects on animals feeding on the plants.
Selenium is found in a few rare minerals such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode metal from electrolytic copper refineries now provide the source of most of the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds.
It also exhibits a photovoltaic effect, converting light to electricity, and a photoconductive effect, electrical conductance increasing as selenium is exposed to light. Below its melting point, selenium is a p type semiconductor.
Selenium is a very rare mineral. It is scarce wherever it is found and it is not found too often. The color is a distinctive red-gray with a metallic luster. This is quite different from its closest related element, sulfur, which is vitreous and yellow. It is more similar in color to native tellurium which follows selenium in the same Periodic Table of Elements column. Elements found in the same column of the Periodic Table of Elements tend to have similar properties although those properties also tend to strengthen or diminish either up or down the column.
On the left is the column from the Periodic Table of Elements in which selenium appears. The farther down the column, the more metallic in character the element becomes. So that oxygen is the least metallic and polonium is the most metallic of the elements in this column. The dividing line between metals and non-metals is between selenium and tellurium; making these elements semi-metals, an element that has significant characteristics of both metals and non-metals. Tellurium is more metallic in nature than selenium but is still a semi-metal.
Although selenium has a metallic luster, it is not metallic in crystal structure or bonding characteristics. Its bonds are more covalent in nature and its structure is more spread out than in metallic minerals such as copper and iron. Both of these elements have lighter atoms than selenium and yet in crystalline form each is nearly twice the density of crystalline selenium. With metallic bonded crystals, atoms are comparatively much closer and are nearly in contact with each other with respect to covalently bonded crystals.
The element selenium has many industrial uses. Most notable is the use of selenium for photvoltaic and photoconductive purposes. This makes it valuable for use in photoelectric cells and exposure meters for photographic purposes. Selenium is also a good tracer element for medical purposes and as a isotope tracer in ground water for hydrogeologic purposes since there are as many as six stable natural isotopes of selenium. The more isotopes, the more control a scientist has over aberrant abundances.
Selenium the mineral, or native selenium, does not usually form good crystals but when it does they are steep rhombohedrons or tiny acicular (hair-like) crystals. Massive specimens are also known. Selenium is an interesting element that rarely forms good specimens. A specimen of good quality is therefore a real treat for a collector of native elements.
Color is reddish-gray to red.
Luster is metallic.
Transparency is opaque.
Crystal System is trigonal; bar 3 2/m
Crystal Habits include steep rhombohedrons or tiny acicular (hair-like) crystals. Massive specimens are also known..
Streak is gray.
Hardness is 2
Specific Gravity is 4.8 (average for metallic minerals).
Clausthalite (PbSe) is a member of the Galena Group of minerals. Its properties are very similar to galena as it shares basically the same structure. However it can be distinguished from the far more common galena by its greater density and lack of good crystals. The two minerals are in a series in which the sulfur and selenium ions substitute for each other.
Clausthalite and the mineral crookesite, a copper thallium selenide are the two most common selenium minerals. However they are not ores of selenium due to their rarity and the fact that selenium is mostly acquired through the processing of copper sulfide ores. The selenium is found as a trace in many copper sulfide minerals especially pyrite and in coal