Tellurium

Tellurium has two allotropes, crystalline and amorphous. When crystalline, tellurium is silvery-white with a metallic luster. It is a brittle and easily pulverized metalloid. Amorphous tellurium is a black-brown powder prepared by precipitating it from a solution of tellurous acid or telluric acid (Te(OH)6). Tellurium is a semiconductor that shows a greater electrical conductivity in certain directions depending on atomic alignment; the conductivity increases slightly when exposed to light (photoconductivity). When molten, tellurium is corrosive to copper, iron, and stainless steel. Of the chalcogens (oxygen-family elements), tellurium has the highest melting and boiling points, at 722.66 K (841.12 °F) and 1,261 K (1,810 °F), respectively.

With an abundance in the Earth's crust comparable to that of platinum (about 1 µg/kg), tellurium is one of the rarest stable solid elements. In comparison, even the rarest of the stable lanthanides have crustal abundances of 500 µg/kg (see Abundance of the chemical elements). This rarity of tellurium in the Earth's crust is not a reflection of its cosmic abundance. Tellurium is more abundant than rubidium in the cosmos, though rubidium is 10,000 times more abundant in the Earth's crust. The rarity of tellurium on Earth is thought to be caused by conditions during preaccretional sorting in the solar nebula, when the stable form of certain elements, in the absence of oxygen and water, was controlled by the reductive power of free hydrogen. Under this scenario, certain elements that form volatile hydrides, such as tellurium, were severely depleted through evaporation of these hydrides. Tellurium and selenium are the heavy elements most depleted by this process. Tellurium is sometimes found in its native (i.e., elemental) form, but is more often found as the tellurides of gold such as calaverite and krennerite (two different polymorphs of AuTe2), petzite, Ag3AuTe2, and sylvanite, AgAuTe4. The city of Telluride, Colorado, was named in hope of a strike of gold telluride (which never materialized, though gold metal ore was found). Gold itself is usually found uncombined, but when found as a chemical compound, it is most often combined with tellurium. Although tellurium is found with gold more often than in uncombined form, it is found even more often combined as tellurides of more common metals (e.g. melonite, NiTe2). Natural tellurite and tellurate minerals also occur, formed by oxidation of tellurides near the Earth's surface. In contrast to selenium, tellurium does not usually replace sulfur in minerals because of the great difference in ion radii. Thus, many common sulfide minerals contain substantial quantities of selenium and only traces of tellurium. In the gold rush of 1893, miners in Kalgoorlie discarded a pyritic material as they searched for pure gold, and it was used to fill in potholes and build sidewalks. In 1896, that tailing was discovered to be calaverite, a telluride of gold, and it sparked a second gold rush that included mining the streets.

The largest consumer of tellurium is metallurgy in iron, stainless steel, copper, and lead alloys. The addition to steel and copper produces an alloy more machinable than otherwise. It is alloyed into cast iron for promoting chill for spectroscopy, where the presence of electrically conductive free graphite tends to interfere with spark emission testing results. In lead, tellurium improves strength and durability, and decreases the corrosive action of sulfuric acid.

Tellurium (Latin tellus meaning "earth") was discovered in the 18th century in a gold ore from the mines in Kleinschlatten (today Zlatna), near today's city of Alba Iulia, Romania. This ore was known as "Faczebajer weißes blättriges Golderz" (white leafy gold ore from Faczebaja, German name of Facebánya, now Fața Băii in Alba County) or antimonalischer Goldkies (antimonic gold pyrite), and according to Anton von Rupprecht, was Spießglaskönig (argent molybdique), containing native antimony. In 1782 Franz-Joseph Müller von Reichenstein, who was then serving as the Austrian chief inspector of mines in Transylvania, concluded that the ore did not contain antimony but was bismuth sulfide. The following year, he reported that this was erroneous and that the ore contained mostly gold and an unknown metal very similar to antimony. After a thorough investigation that lasted three years and included more than fifty tests, Müller determined the specific gravity of the mineral and noted that when heated, the new metal gives off a white smoke with a radish-like odor; that it imparts a red color to sulfuric acid; and that when this solution is diluted with water, it has a black precipitate. Nevertheless, he was not able to identify this metal and gave it the names aurum paradoxum (paradoxical gold) and metallum problematicum (problem metal), because it did not exhibit the properties predicted for antimony. In 1789, a Hungarian scientist, Pál Kitaibel, discovered the element independently in an ore from Deutsch-Pilsen that had been regarded as argentiferous molybdenite, but later he gave the credit to Müller. In 1798, it was named by Martin Heinrich Klaproth, who had earlier isolated it from the mineral calaverite. The 1960s brought an increase in thermoelectric applications for tellurium (as bismuth telluride), and in free-machining steel alloys, which became the dominant use.