limonite - teaching student specimens of limonite partly converted to yellow ochre - UNIT OF 5 SPECIMENS

Limonite is a mixture of hydrated iron oxides, varying in composition. It is an important ore of iron. Though much of what is called limonite is actually goethite, the name is retained for natural hydrous iron oxides of uncertain identity. Limonite typically forms in the upper part of a mineralized vein.

Pyrite is common in metallic deposits. As it oxidizes, it changes from iron sulfate to iron oxide and takes up water to become limonite. The limonite "iron hat" or gossan that caps these veins often forms a yellow brown streak across a mountainside, obvious in the Southwest where vegetation is often sparse. Miners were attracted by the iron hat, in hopes of finding an enriched mineral zone below. 

Limonite is dark brown and vitreous until it takes on water. Its color changes to mustard yellow and the mineral finally becomes earthy yellow ochre, a pigment valued by Native Americans and by artists today.

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.

These specimens are partly converted to yellow ochre, changed from dark brown to yellow brown as they picked up water. They give the typical yellow brown streak.

Collected at Keeler Gold Mine, Inyo county, California. The field photos show limonite at the top of a mineralized vein and a prospect shaft with discarded limonite at this mine.

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