Limonite

Limonite is an iron ore made up of a mix of hydrated iron(III) oxide-hydroxides, with a composition that can vary widely. Its name comes from the Ancient Greek words for "wet meadow" or "marshy lake," referencing how it often forms as bog iron ore in such environments.

This ore is relatively dense, with a specific gravity between 2.7 and 4.3, and is usually medium to dark yellowish brown. Its streak on unglazed porcelain is always yellowish brown, which sets it apart from hematite (red streak) and magnetite (black streak). Hardness ranges from 1 to 5. Under a microscope, limonite appears red, yellow, or brown, with a high refractive index of 2.0–2.4, and while its minerals are strongly birefringent, the grains are typically too small to detect this.

Originally considered a single mineral, limonite is now a field term for a mixture of hydrated iron oxide minerals like goethite, lepidocrocite, akaganeite, and jarosite. Determining the exact makeup requires X-ray diffraction. Individual minerals within limonite can form crystals, but limonite itself does not, though it may appear fibrous or microcrystalline. It often occurs as concretions, compact or earthy masses, and sometimes in mammillary, botryoidal, reniform, or stalactitic shapes. Because it is amorphous and forms in hydrated areas, it often looks like clay or mudstone. Limonite also forms pseudomorphs after other minerals, such as pyrite, where chemical weathering hydrates the pyrite crystals but preserves their external shape. Pseudomorphs have also formed from hematite, magnetite, siderite, and iron-rich silicates like almandine garnets.

Limonite usually forms when hematite and magnetite become hydrated, when iron-rich sulfide minerals oxidize and hydrate, or through chemical weathering of minerals like olivine, pyroxene, amphibole, and biotite. It is a major iron component in lateritic soils, and limonite laterite ores are a source of nickel and sometimes cobalt and other metals as trace elements. It often deposits in run-off streams from mining operations.

Nickel-rich limonite ores are the largest reserves of nickel, classified as lateritic nickel ore deposits. One of its earliest uses was as a pigment: the yellow form produced yellow ochre (for which Cyprus was famous), while darker forms gave earthy tones. Roasting limonite partially converts it to hematite, producing red ochres, burnt umbers, and siennas. Bog iron ore and limonite mudstones are mined for iron. Iron caps or gossans of siliceous iron oxide form from intense oxidation of sulfide ore deposits, and prospectors used these gossans as guides to buried ore. Limonite was also mined for its gold content: oxidation of gold-bearing sulfide deposits concentrated gold in the iron oxide and quartz of gossans, and gold from primary veins concentrated into limonites in deeply weathered rocks. In Brazil, deeply weathered iron formations concentrated gold with limonite in the resulting soils.

Limonite was one of the earliest pigments used by humans, appearing in Neolithic cave paintings and pictographs. While the first iron ore was likely meteoric iron, and hematite was easier to smelt, limonite is the most prevalent iron ore in Africa, where the earliest evidence of iron metallurgy is found. Before smelting, heating the ore drove off water, converting more limonite to hematite. The ore was then pounded while heated above 1250 °C, causing metallic iron to stick together and non-metallic impurities to fly off as sparks. Complex systems, notably in Tanzania, were developed to process limonite. However, hematite and magnetite remained preferred for bloomery smelting; it was only with the development of blast furnaces (in China by the 1st century BCE and in Europe around 1150 CE) that limonite could be used to best advantage. Bog iron ore and limonite were mined in the US, but this ended with advanced mining techniques. Gold-bearing limonite gossans were productively mined in Shasta County, California, near Rio Tinto in Spain, and at Mount Morgan in Australia. In the Dahlonega gold belt in Lumpkin County, Georgia, gold was mined from limonite-rich lateritic or saprolite soil. As saprolite deposits have been exhausted at many sites, limonite has become the most prominent source of nickel for energy-dense batteries.