Gossan (eiserner hut or eisenhut) is intensely

oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

weathered ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...

or decomposed rock, usually the upper and exposed part of an

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

deposit or mineral

vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...

. In the ''classic'' gossan or iron cap all that remains is

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

s and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, often in the form of

boxwork In geology, boxwork is defined as a honeycomb-like structure that can form in some fractured or jointed sedimentary rocks. If the fractures in the host rock are mineralized, they can become more resistant to weathering than the surrounding rock, ...

s (which are quartz-lined cavities retaining the shape of the dissolved ore minerals). In other cases, quartz and iron oxides,

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

, and

jarosite Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct dur ...

, exist as pseudomorphs, replacing the

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

and primary ore minerals. Frequently, gossan appears as a red "stain" against the background rock and soil, due to the abundance of oxidized iron; the gossan may be a

topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary scienc ...

positive area due to the abundance of

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

-resistant quartz and iron oxides. Although most gossans are red, orange, or yellow, black gossans from

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

oxides such as

pyrolusite Pyrolusite is a mineral consisting essentially of manganese dioxide ( Mn O2) and is important as an ore of manganese.. It is a black, amorphous appearing mineral, often with a granular, fibrous, or columnar structure, sometimes forming reniform ...

manganite Manganite is a mineral composed of manganese oxide-hydroxide, MnO(OH), crystallizing in the monoclinic system (pseudo-orthorhombic). Crystals of manganite are prismatic and deeply striated parallel to their length; they are often grouped togethe ...

, and especially

psilomelane Psilomelane is a group name for hard black manganese oxides including hollandite and romanechite. Psilomelane consists of hydrous manganese oxide with variable amounts of barium and potassium. Psilomelane is erroneously, and uncommonly, known as ...

form at the oxidized portion of manganese-rich mineral deposits. In the 19th and 20th centuries, gossans were important guides to buried ore deposits used by

prospectors Prospecting is the first stage of the geological analysis (followed by exploration) of a territory. It is the search for minerals, fossils, precious metals, or mineral specimens. It is also known as fossicking. Traditionally prospecting rel ...

in their quest for metal ores.Johnson, Joseph Colin Francis (1897) ''Getting Gold: A Practical Treatise for Prospectors, Miners, and Students'' Lippincott, Philadelphia, Pennsylvania
page 15
An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below the ''iron cap''.

Name

The name ''gossan'' appears to come from Cornish miner's slang."gossan" ''

Century Dictionary and Cyclopedia ''The Century Dictionary and Cyclopedia'' is one of the largest encyclopedic dictionaries of the English language. It was compared favorably with the ''Oxford English Dictionary,'' and frequently consulted for more factual information than woul ...

'' Vol. III, page 2581 The terms "iron cap" and "iron hat", common in America, are direct translations from the German "''Eiserner Hut''" or "''Eisenhut''", which term was also used in America.

Notes

References

*Guilbert, John M. and Charles F. Park Jr (1986) ''The Geology of Ore Deposits,'' W. H. Freeman, pp 799–830, Economic geology Mineralogy Cornish words and phrases {{rock-stub