limonite - hand/display specimen of limonite - hydrated iron oxides - from the cap of a hydrothermal vein
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 Keeler Gold Mine in the Inyo Range in Inyo County, California. The mine produced from the early 1930s until gold production was stopped during WW II. The mill, in the planning stages in 1934, was converted during the war to process tungsten ores mined near Darwin, and was permanently shut down in 1944.
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. These examples are well on their way to becoming yellow ochre.
The field photos show a limonite cap on a mineralized vein at the Keeler Mine that turned out to not be productive. The miners dug out the limonite and went down ten feet, found nothing, and moved to a larger vein on the other side of the hill. That one had a huge limonite cap and was productive.
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