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Goethite is an iron oxides mineral. Iron oxides are composed of Fe, O and OH. They are compounds that can be found in all different compartments of the global system such as the atmosphere, pedosphere, biosphere, hydrosphere and lithosphere. The weathering process of magmatic rocks gives rise to the formation of iron oxides. These iron oxides are redistributed through various redistribution processes including wind, water and human activities as iron metals and oxides are becoming more and more valuable to industrial development. Goethite is considered one of the most thermodynamically stable iron oxides. For that reason, it is thought that Goethite might have been the first oxide to form. Goethite’s characteristic is responsible for the colors in rocks and soils

Chemical Composition Goethite has a chemical composition of α-Fe3+OOH. Like most of other iron oxides, goethite’s Fe is in the trivalent state. According to Mohapatra (2008), impurities in Goethite such as SiO¬¬2, ¬CaO, MnO and Al2O3 might be as high as 5%. Base on the study of samples collected from Orissa, India, goethite is categorized into several microstructural and morphological types. Each type of goethite has a unique chemical composition. Goethite has a high tendency for substitution and a large absorbency character. This is the main factor that contributes to the impurities in the mineral. Like other mineral, goethite’s chemical composition is strongly dictated by environmental conditions where the mineral is formed, such as temperature and pressure. Variation in goethite’s composition is strongly dependent on the mineral’s occurrence. Goethite’s occurrences are put into two groups: open space filling textures and replacement textures.

Atomic Structure Goethite belongs to the orthorhombic crystallographic system with the space group Pnma. Goethite also has the hexagonal close packing (ABAB [001]) configuration (Fig.1). Its unit cell dimensions are a: 0.9956nm, b: 30215nm, c: 0.4608nm, Z =4. Goethite structure consists of mirror planes and glide planes: a with glide component a/2 and n with glide component (b+c)/2. The structure of goethite is mainly constructed by the hexagonal close packing of anions stacking with different ordered arrangements of the cations. Because of its anion framework, goethite’s topology can be altered easily. For example, within its process of dehydroxylation to hematite, goethite’s pair of water molecules is removed from the a-axis although the anion array remains the same. The array of O2- and OH- is stacked along the [010] direction while Fe3+ ions occupying half the octahedral interstices within a layer. With the Fe ions arranged in the double rows, the empty sites at the crystal surfaces serve as double rows of separation (Fig.2 a,c). Surrounding a Fe ion are three O2- and three OH- to give FeO3(OH)3 octahedral. These octahedral are the terms that are often used to describe goethite’s structure. The edge sharing running parallel to the direction [100] forms the double chains of octahedral. Then another corner-sharing will link these chains to their adjacent double chains with one chain being displaced by b/2 with respect to its neighbors. Ultimately, the orthorhombic symmetry is formed by this arrangement. The double chains of octahedral are twisted slightly with respective to each other due to the different distance between the OI-OI bond (oxygens shared by octahedral of two different double chains) and the OII-OII bond (oxygens shared by octahedral of the same chain Physical Properties Goethite has the color of dark brown and black in its massive crystal state. However, it could be found yellow in powder . As mentioned, goethite is a FeIII oxide; this gives the mineral a high stability and thus a low solubility. Naturally, goethite is formed in small sized crystals due to its high energy of crystallization. This property of goethite can also be related to its high specific surface area . Table 3 lists basics properties of goethite.

Table 3 Goethite Density (g/cm3)	4.26 Cleavage {010}	Perfect Cleavage {100}	Good Streak	Brownish yellow Fracture	Uneven Optical Class	Biaxial(-) Uniaxial (-) Octahedral Occupancy	½ Color	Brown, yellow, red Hardness	5-5.5 Type of magnetism	Antiferromag Standard free energy of formation(kj/mol) -488.6 Curie Temperature (K)	400 Source:

Geologic Occurrence Goethite is a natural occurring iron oxy-hydroxide mineral. Typically, goethite forms under the oxidizing conditions as a weathering product of iron-bearing minerals (Morris, 1985). It is usually found along with other types of mineral deposits such as iron, manganese and bauxite ores. Goethite is also one of the most common iron oxides in soils due to its high stability. Very often, goethite appears in soils of cool to temperate, humid climates. Goethite is usually associated with hematite and ferrihydrite in warmer and cooler regions respectively. Owing to its large surface area and typical structural arrangement, goethite has a large absorbency. It might be found with impurities including Al¬2O3, CaO, MnO. The impurity level might reach 5% (Klein & Hurlbut, 1985). Large quantities of goethite have been found as residual lateritic mantles resulting from the weathering of serpentine. Globally, goethite can be found mostly in France, Germany, Bohemia, Czechoslovia and England. Goethite, along with other iron oxides, is utilized in water treatment system as well as detoxification of air pollution.

Biographic Sketch Goethite was named after the German poet, novelist, playwright, philosopher and geoscientist Johann Wolfgang von Goethe (1749-1832). Besides being one of the most influential German poets, he also held the position of minister for mines for the Duke of Weimar. For minerals was one of his passions, his collection of minerals can still be visited at his house in Weimar, Germany.

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