limonite - teaching student specimens of limonite - UNIT OF 10 smaller SPECIMENS
FeO(OH).nH20 - hydrous iron oxide
Limonite was mined by George Washington's soldiers as a source of iron for cannon balls. Bog iron was the primary source of Viking era iron. Limonite is concentrated in bogs by two processes. In the acidic environment of a bog, a chemical reaction forms insoluble iron compounds which precipitate out. A more important process involves the anaerobic bacteria Gallionella and Leptothrix. Growing under the surface of the bog, they concentrate iron as part of their life processes. These bacteria leave an iridescent oily film on the surface of the bog, called jambrák (iron slick) in Iceland.
Limonite also importantly forms at the surface above veins of metallic ores where pyrite is common. The pyrite oxidizes, changing from iron sulfate to iron oxide and takes in water to become limonite. This imparts a yellow brown "iron hat" to these veins, a clue to prospectors that valuable minerals lie below.
Limonite is a mixture of hydrated iron oxides. Though much of what is called limonite is actually goethite, the name is retained for hydrous iron oxides of uncertain identity.
As limonite gradually picks up water, it eventually converts to yellow ochre, a pigment favored by Native Americans. Initially brown and vitreous, limonite gradually takes on a mustard yellow tint and becomes earthy. These specimens were collected from the iron hat above a vein. Though limonite is brown, it gives a mustard yellow streak, an identifying characteristic. Its hardness ranges from 5 to 5 1/2 on the Mohs scale of mineral hardness, so it might or might not scratch glass.
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