Limonite And Goethite In The Laterite Profile

Limonite

Limonite is a non-crystalline mineral colloid. It can be found as stalactitic botryoidal or mammillary forms, but most commonly in laterite areas simply as an earthy mass. The colour in its earthy form is brownish yellow to ochre brown.

The term “Limonite” for a specific mineral species is no longer acceptable. At present the term “Limonite” designates various iron hydroxides that show a range of values for the water of crystallisation and may also carry some adsorbed water. Traditional composition of limonite is approximately 2Fe2O3.3H2O with the water of crystallisation at about 14.5%. However, the amount of water of crystallisation varies considerably in limonite. For this reason, some authorities consider limonite as a colloidal and amorphous form of goethite with adsorbed and capillary water.

Limonite is always of secondary origin, derived from the breakdown of ironbearing minerals (iron-bearing olivines and pyroxenes in the case of Sorowako area ultramafics). It can be formed at low temperatures and pressures.

Goethite

Goethite is an orthorhombic mineral of relatively high specific gravity (4.28) and medium hardness (5.0 – 5.5). It occurs in botryoidal forms and as earthy masses. The colour is yellowish, reddish and blackish brown. Goethite is most commonly found associated with limonite, less commonly with hematite.

Ferrous iron released from the weathering of the primary mafic minerals oxidised to ferric iron and is precipitated as a hydroxide. The hydroxide has a poor degree of crystallinity and appears amorphous. It occurs as concretions as well as claylike earthy mass. The concretions have concentric layers indicating rhythmic precipitation, often around a nucleus or core. Goethite concentrates as a residual mineral due to its insolubility under prevailing pH-Eh conditions in the laterite environment.

See Also : Silicification in the Laterite Profile

Although the standard formula of goethite is Fe2O3.H2O, significant quantities of  Al2O3, Cr2O3 and SiO2 may be present. Thus, a plot of Al2O3+Cr2O3 against the Fe2O3 shows an inverse relationship since both alumina and chrome replace the ferric iron in the goethite structure. Otherimpurities that may also be present include Mg, Ni, Co, Mn, Ti, Cu, Zn, Cd and Pb. While the incorporation of Al in goethite may reach as high as 33 mol%, otherelements are incorporated in considerably less quantities. Experiments by Gerth (1990) indicate a maximum incorporation of Ni in synthetic goethite of 6-7 mol%.

Compositions of some Goethite/limonite.

Goethite is the most common iron sesquioxide in soils that are well drained and where oxygen is abundantly available. In soils where oxygen supply is limited, its polymorph, lepidocrocite is more likely to form.

Schwertmann et al (1989) assert that goethite and hematite cannot be interconverted through a simple solid-state dehydration-hydration mechanism at ordinary temperatures. They maintain that their conversion takes place only through the processes of dissolution and reprecipitation (aided by appropriate reducing and
oxidising conditions).

Goethite in soils may also be accompaniedby its polymorph, lepidocrocite. However, its occurrence is less common and generally limited to soils that are generally deficient in oxygen due to say water saturation. In hand specimens lepidocrocite occurs as bright orange mottles or bands. (Reference: Waheed Ahmad, 2008, Fundamentals of chemistry, mineralogy, weathering processes, formation, and exploration).

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