limonite - teaching student specimens of limonite, with part converted to yellow ochre - UNIT OF 5 SPECIMENS
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Limonite was mined by George Washington's soldiers as a source of iron for cannon balls and bog iron was the primary source of Viking era iron. In bogs, limonite is concentrated 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 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.
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 illustrate the process and 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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