Limonite
Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. Limonite is named for the Ancient Greek word for 'wet meadow' or 'marshy lake,' alluding to its occurrence as bog iron ore in meadows and marshes.
- field
- Mineralogy, Metallurgy
- known_for
- One of the three principal iron ores; source of nickel; pigment since Neolithic times
- composition
- Mixture of hydrated iron(III) oxide-hydroxides, including goethite, lepidocrocite, akaganeite, and jarosite
- specific_gravity
- 2.7 to 4.3
- hardness
- 1 to 5
- streak
- Yellowish brown on unglazed porcelain
Lore & Background
Limonite usually forms from the hydration of hematite and magnetite, from the oxidation and hydration of iron-rich sulfide minerals, and chemical weathering of other iron-rich minerals such as olivine, pyroxene, amphibole, and biotite. It is often the major iron component in lateritic soils, and limonite laterite ores are a source of nickel and potentially cobalt and other valuable metals. It is often deposited in run-off streams from mining operations. One of the first uses of limonite was as a pigment. The yellow form produced yellow ochre for which Cyprus was famous, while the darker forms produced more earthy tones. Roasting the limonite changed it partially to hematite, producing red ochres, burnt umbers and siennas. Bog iron ore and limonite mudstones are mined as a source of iron. Limonite was also mined for its ancillary gold content, as the oxidation of sulfide deposits containing gold often resulted in the concentration of gold in the iron oxide and quartz of the gossans. Limonite was one of the earliest materials used as a pigment by humans, and can be seen in Neolithic cave paintings and pictographs. In Africa, where the first evidence of iron metallurgy occurs, limonite is the most prevalent iron ore. Complex systems developed, notably in Tanzania, to process limonite.
Reader's Guide
Its importance extends beyond iron: nickel-rich limonite ores represent the largest reserves of nickel, and as saprolite deposits have been exhausted in many mining sites, limonite has become the most prominent source of nickel for use in energy-dense batteries. Limonite also served as a pigment from Neolithic times onward, producing yellow ochre and, after roasting, red ochres. Gold-bearing limonite gossans were productively mined in several regions, including Shasta County, California, and near Rio Tinto in Spain. The mineral's amorphous nature and variable composition—a mixture of goethite, lepidocrocite, akaganeite, and jarosite—make precise identification possible only with X-ray diffraction. Its pseudomorphs after pyrite and other minerals illustrate chemical weathering processes. Limonite's legacy is thus tied to both ancient ironworking and modern nickel production.
Did You Know?
- Limonite is named from the Ancient Greek word for 'wet meadow' or 'marshy lake,' due to its occurrence as bog iron ore.
- The streak of limonite on an unglazed porcelain plate is always yellowish brown, distinguishing it from hematite (red streak) and magnetite (black streak).
- Limonite pseudomorphs can form after pyrite, hematite, magnetite, siderite, and almandine garnets.
- Gold-bearing limonite gossans were productively mined in Shasta County, California, and near Rio Tinto in Spain.
Frequently Asked Questions
Who is Limonite?
Limonite is an iron ore composed of a blend of hydrated iron(III) oxide-hydroxides, including goethite, lepidocrocite, akaganeite, and jarosite. It holds the distinction of being one of the three principal iron ores in metallurgy.
What are Limonite's powers and role?
Beyond its role as a key iron ore, limonite serves as a nickel source and has been ground into pigment since Neolithic times. Physically, it spans a wide range, with specific gravity from 2.7 to 4.3 and hardness between 1 and 5.
Where does Limonite appear in the field?
Limonite is most often found as bog iron ore in meadows and marshy lake settings, which is precisely the environment that inspired its name. It straddles both mineralogy and metallurgy as a practical, earth-bound material.
Why is Limonite important to the overall story?
As one of the three main iron ores, limonite has underpinned human metallurgy for thousands of years. Its use as a pigment stretching back to the Neolithic period makes it among the oldest materials people have deliberately worked with.
What does Limonite's name mean and how do you identify it?
The name derives from an Ancient Greek word for 'wet meadow' or 'marshy lake,' pointing to the bog environments where it typically forms. A quick field test is its streak: it leaves a characteristic yellowish-brown mark on unglazed porcelain.
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