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Slagy brown iron stone with original lettering from the Brauner Johannes mine in Suhl (collection of the Natural History Museum Schleusingen)

Limonite , also known as brown iron ore or brown iron stone , is a very common iron and water-containing mineral mixture of various iron oxides such as goethite , lepidocrocite and various other hydrated iron oxides . In addition, additions of hematite were found at some sites . Therefore, limonite is no longer viewed as a separate mineral today.

Limonite releases water when it burns, which means it is dehydrated . To determine whether an ocher-colored rock or a potential colored earth contains limonite, a sample is held over a gas flame until it begins to glow red. If it turned red or purple after cooling , it was limonite. This also explains why many yellow clays produce reddish pottery or brickwork (e.g. terracotta ) after firing . The pigments of burnt ocher , burnt sienna or burnt umber are based on the same effect .

Education and Locations

Limonite precipitation at the bottom of a stream in the Caldera de Taburiente

Limonite is a component of sedimentary iron-bearing rocks . These are called iron stones or outdated as iron limestone called. Examples of such rocks are Raseneisenerz , oolitic iron ores, Minette , rubble ore and Bohnerz .

Limonite also occurs in secondary deposits (deposits that were formed by weathering of primary minerals). It is often found as a weathering product of iron-containing minerals ( olivine , pyroxenes , amphiboles , biotite , but also of magnetite ) in mafic volcanics such as melaphyre and basalt or in felsic igneous rocks such as diorite and granite . In everyday life, we encounter limonite mainly as rust on iron objects.

Limonite occurs in different forms: Brown glass head (as a drip, kidney or slag-like variety - yellow-brown to almost black), coarse brown iron stone (yellow-brown to black, not to be confused with brown stone , a manganese oxide). Sedimentary deposits are often made of Eisenkalk (light-dark rusty brown rehbraun to a limonite impregnated limestone) and limonite highly enriched Letts layers , either as a firm, solid limonite or are formed as Letts. The latter are also obtained as colored earths ( pigments such as ocher and sienna ). Ocher is a powdery weathering form of crystalline Limonits, also known as color earth as Umbra traded and mining also Brauneisenmulm is called. It can also be found in the weathering zone of ore veins accompanied by a base (e.g. in the "iron hat" of the copper / zinc veins of the Ore Mountains ).

In addition to magnetite and hematite , limonite is the most important iron ore that is still extracted today in large open-cast mines in many countries. FeO (OH) is also the end product that is created when iron rusts.

Well-known sites in Germany are mainly Auerbach in the Upper Palatinate and Sulzbach-Rosenberg in Bavaria, Eschweiler (in the Korkus and Propsteier Forest ) in North Rhine-Westphalia and Kamsdorf (near Saalfeld ), Brotterode-Trusetal ( Laudenbach ), Atterode (Stahlberg, Hohe Klinge ) and Albrechts (the "Altenfeld") in Thuringia.


In ancient times , brown iron ore played a lesser role in iron production than hematite , which was easier to smelt in the old racing fires . It was only with the introduction of the piece, blue and high ovens in the 16th century that brown iron ore mining began to flourish.

In its capacity as a proportion of earth pigments, however, it is one of the earliest materials used by humans and can be proven in Stone Age cave paintings and rock art.

Limonite deposits in rocks

The minerals of the limonite complex are a natural coloring component in many rocks and are often embedded in a very finely divided form. In many sandstones, they cause the yellowish to rust-brown color. In carbonate rocks, yellow tones in all shades are also often caused by this. Limonite and related minerals are an important, but not the only yellow-tinged rock component.

Of particular importance is the fact where limonite and other iron minerals are present in building and decorative stone in such a finely divided form that they cannot be perceived by the eye, but later produce a yellow coloration through subsequent environmental effects or further processing. For example, this phenomenon occurs with some granites and certain types of Carrara marble , which show significant discolouration in surfaces of different sizes after laying in strongly alkaline adhesives or when exposed to atmospheric influences. As is common with all other materials, in connection with such phenomena, the suitability of the respective rock for the specific application must be clarified by the experts.

See also


  • Martin Okrusch, Siegfried Matthes: Mineralogy. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 22, 36, 39, 282 .
  • Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .

Web links

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