Editorial Feature

Limonite - Mining Fundamentals

Limonite is an iron ore, containing a hydrated iron (III) oxide-hydroxide mixture in different compositions. It is one of the two major iron ores, the other being hematite - used in the production of iron.

It was named by Johann Friedrich Ludwig Hausmann in 1813 after the Greek word for meadow, indicating its common occurrence in lowlands.

Limonite was initially considered to be an amorphous equivalent of lepidocrocite and goethite. However, X-ray studies have revealed that limonite is actually goethite. Limonite acts as an iron ore, as well as a pigment.

Formation

Limonite is a common iron ore, which forms the coloring matter in different types of soils. It is usually formed from the chemical weathering of iron rich minerals, such as biotite, amphibole, pyroxene, and olivine, the hydration/oxidation of iron rich sulfide minerals, or from the hydration of magnetite and hematite.

Limonite is commonly deposited in run-off streams resulting from mining operations. It also occurs in lateritic soils, and forms as pseudomorphs over minerals, such as siderite, marcasite, and pyrite. These pseudomorphs can be identified by their yellow staining and partially rusted appearance.

Properties

Limonite is a relatively dense amorphous ore with a specific gravity, ranging from 2.7 to 4.3. The color of limonite varies from a bright lemon-yellow to a dark grayish-brown. The limonite streak on an unglazed porcelain plate is usually brownish, which differentiates it from the mineral hematite, with a red streak, or magnetite, with a black streak.

Limonite attains magnetic properties when heated, and has an opaque appearance. The hardness of limonite varies from 4 to 5.5.

References

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