Description
The mineral goethite is an iron- and oxygen-containing hydroxide with the chemical formula Fe3+O.OH. This mineral should not be confused with limonite, the imperfectly crystallized form of goethite. It has a hardness of 5 to 5.5 on the Mohs scale of mineral hardness. Its density is 3.3 to 4.3, and the mineral is not radioactive. The mineral was identified in 1806 and is named after the German writer Johann Wolfgang von Goethe, who was interested in mineralogy. Goethite is a common iron ore, occurring worldwide through the weathering of other iron-bearing minerals such as magnetite and pyrite. In Europe, it occurs in regions including Alsace-Lorraine (France), Westphalia (Germany), and Bohemia (Czech Republic). It is also used as a clay pigment; goethite has been found as a pigment in the cave paintings at Lascaux in France. Limonite, the imperfectly crystallized form of goethite, is a constituent of the rock umber. In 2004, the mineral was also identified on Mars by NASA's "Spirit" robot. This could indicate the presence of water on Mars in the past, as goethite only forms in the presence of water. The teeth of the common limpet, a sea snail that lives along the European Atlantic coast, contain goethite fibers. This makes the tooth material perhaps the strongest found on Earth, and certainly stronger than spider silk, which was previously the strongest known material. The large teeth are just as strong as the small ones, which is remarkable because large structures usually contain many faults and therefore break more easily. The limpet uses its hard teeth to scrape its food loose from the hard substrate on which it lives.
