limonite - display specimen of limonite from the top of a mineralized vein, showing conversion to yellow ochre
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.
This limonite was collected from the mill site of the Copper World mine in San Bernardino County, California. The ore body was discovered in 1868 and in 1869 a few tons of very high grade ore was shipped to San Francisco. By late 1899, the Copper World was one of the four largest copper mines in the United States. The mine operated continuously during the periods of 1906 to 1908, 1916 to 1918 and in 1944 and produced copper, lead, silver and some gold. The irregular ore body was formed where dolomite was in contact with sill-like bodies of quartz monzonite, a common association for copper mineralization.
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. Limonite's hardness ranges from 5 to 5 1/2 on the Mohs scale of mineral hardness, so it might or might not scratch glass.
Excellent example of the transition from limonite to ochre. This is an almost cubic example and typical of what is found at the top of a vein.
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