Limonite is an iron ore consisting of a mixture of hydrated iron(III)
oxide-hydroxides in varying composition. The generic formula is frequently
written as FeO(OH)·nH2O, although this is not entirely accurate as the ratio of
oxide to hydroxide can vary quite widely. Limonite is one of the two
principal iron ores, the other being hematite, and has been mined for the
production of iron since at least 2500 BCE.
Names Limonite is named from the Greek word
for meadow, in allusion to its occurrence as bog iron ore in meadows
and marshes. In its brown form it is sometimes called brown hematite or brown
iron ore. In its bright yellow form it is sometimes called lemon rock or yellow
iron ore. Characteristics
Limonite is relatively dense with a specific gravity varying from 2.7 to
4.3. It varies in colour from a bright lemony yellow to a drab greyish brown.
The streak of limonite on an unglazed porcelain plate is always brownish, a
character which distinguishes it from hematite with a red streak, or from
magnetite with a black streak. The hardness is variable, but generally in
the 4 - 5.5 range. Although originally defined as a single
mineral, limonite is now recognized as a mixture of related hydrated iron oxide
minerals, among them goethite, akaganeite, lepidocrocite, and jarosite.
Individual minerals in limonite may form crystals, but limonite does not,
although specimens may show a fibrous or microcrystalline structure, and limonite
often occurs in concretionary forms or in compact and earthy masses; sometimes
mammillary, botryoidal, reniform or stalactitic. Because of its amorphous
nature, and occurrence in hydrated areas limonite often presents as a clay or
mudstone. However, there are limonite pseudomorphs after other minerals such
as pyrite. This means that chemical weathering transforms the crystals of
pyrite into limonite by hydrating the molecules, but the external shape of the
pyrite crystal remains. Limonite pseudomorphs have also been formed from
other iron oxides, hematite and magnetite; from the carbonate siderite
and from iron rich silicates such as almandine garnets.
Formation 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. It is often deposited in run-off streams from
mining operations. Uses of limonite
One of the first uses 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, although commercial mining of them has ceased in
the United States. Iron caps or gossans of siliceous iron
oxide typically form as the result of intensive oxidation of sulfide ore
deposits. These gossans were used by prospectors as guides to buried ore. In
addition the oxidation of those sulfide deposits which contained gold, often
resulted in the concentration of gold in the iron oxide and quartz of the
gossans. Goldbearing limonite gossans were productively mined in the Shasta
County, California mining district. Similar deposits were mined near Rio
Tinto in Spain and Mount Morgan in Australia. In the Dahlonega gold belt in
Lumpkin County, Georgia gold was mined from limonite-rich lateritic or
saprolite soil. The gold of the primary veins was concentrated into the
limonites of the deeply weathered rocks. In another example the deeply weathered
iron formations of Brazil served to concentrate gold with the limonite of
the resulting soils. History
While the first iron ore was likely Meteoric iron, and hematite was far
easier to smelt, in Africa, where the first evidence of iron metallurgy
occurs, limonite is the most prevalent iron ore. Before smelting, as the ore
was heated and the water driven off, more and more of the limonite was
converted to hematite. The ore was then pounded as it was heated above 1250 °C,
at which temperature the metallic iron begins sticking together and
non-metallic impurities are thrown off as sparks. Complex systems developed,
notably in Tanzania, to process limonite. Nonetheless, hematite and
magnetite remained the ores of choice when smelting was by bloomeries, and it
was only with the development of blast furnaces in 1st century BCE in China and
about 1150 CE in Europe, that the brown iron ore of limonite could be used to
best advantage. As regards to the use of limonite for
pigments, it was one of the earliest man-used materials and can be seen in
Neolithic cave paintings and pictographs.
Notes See also
Bog iron Iron ore
Ore genesis External links
Mineral galleries Mindat
Gold and limonite