Minerals associated with laterites Primary Mafic Minerals Primary mafic minerals in the ultramafic rocks of the Sorowako area consist essentially of olivine and pyroxenes, as follows: Chrysolite olivine: (Mg,Fe)2SiO4 Orthopyroxene, Enstatite: MgSiO3 Orthopyroxene, Hypersthene: (Mg,Fe)SiO3 Clinopyroxene, Diopside: CaMg(SiO3)2 Clinopyroxene, Hedenbergite: CaFe(SiO3)2 Clinopyroxene, Augite: CaFe(SiO3)2 with (Mg,Fe)(Al,Fe)2SiO6 Primary Spinel Minerals Primary spinels in the ultramafic rocks consists essentially of magnetite and chromite: Magnetite: Fe3O4 or FeO.Fe2O3 Chromite FeCr2O4 or FeO.Cr2O3 Hydrothermal Mafic Minerals
Transcript
Minerals associated with laterites
Primary Mafic MineralsPrimary mafic minerals in the ultramafic rocks of the Sorowako area consist essentially of olivine and pyroxenes, as follows:
Primary Spinel MineralsPrimary spinels in the ultramafic rocks consists essentially of magnetite and chromite:
Magnetite: Fe3O4 or FeO.Fe2O3Chromite FeCr2O4 or FeO.Cr2O3
Hydrothermal Mafic MineralsMafic minerals, formed during the process of serpentinisation and hydrothermal alteration, include serpentine, talc and chlorite:
Serpentine: H4Mg3Si2O9 or 3MgO.2SiO2.2H2OTalc: H2Mg3Si4O12 or 3MgO.4SiO2.H2OChlorite: H8Mg5Al2Si3O18 or 5MgO.Al2O3.3SiO2.4H2O
Secondary Spinel
During the formation of hydrothermal serpentine, unwanted iron from olivine and pyroxene structures is thrown out as magnetite (Fe3O4). Thus, highly serpentinised rocks are generally more magnetic than unserpentinised rocks.
Secondary OxidesSecondary oxides includes essentially Hematite and its magnetic variety, Maghemite:
Hematite: Fe2O3Maghemite: Fe2.66O4
Secondary HydroxidesEssentially of iron, aluminium and manganese:
Serpentinisation of olivines requires: Addition of water Leaching of magnesia (or addition of silica) Release of iron in the (Mg,Fe) olivine Conversion of released iron from the ferrous to ferric state to form fine-grained magnetite. Thus,
serpentinised rocks are generally more magnetic.
3Mg2SiO4 + 4H2O + SiO2 = 2H4Mg3Si2O9forsterite water silica serpentine
2Mg2SiO4 + 3H2O = H4Mg3Si2O9 + Mg(OH)2forsterite water serpentine
SERPENTINE GROUP MINERALS
Serpentine is formed by hydrothermal alteration of ferromagnesian minerals such as olivines, pyroxenes and amphiboles. Pure magnesian serpentine contains about 13% water of crystallisation that is expelled at very high temperatures of +800ºC.
Lizardite: This is the most common form of serpentine and is massive. However, microscopically, it may be finely fibrous and felted. Lattice structure consists of planar layer arrangement. Colour is usually light to medium green but quite variable due to the presence of other minerals. [Lizardite should not be confused with serpentine rock, which is also massive but forms large rock masses].
Antigorite: This is the micaceous, scaly, lamellar or foliated form of serpentine. Lattice structure consists of undulating layer arrangement. The laminae are generally inseparable but may be separable in some thinly foliated varieties. Colour is light to medium green but quite variable (presence of magnetite may impart a grey, brown or black colour while the presence of hematite may give it shades of brown and red).
Chrysotile: Delicately fibrous with the fibres usually flexible and easily separable. Lattice structure consists of rolled layer arrangement. Chrysotile commonly occurs in veins or matted masses. Colour is usually yellowish green, white or grey. Much of what is popularly called “asbestos” is actually this serpentine variety. However, some amphiboles, as described below, are also included in the commercial “asbestos”.
MAGNETITE
Magnetite occurs in ultramafic rocks as an accessory mineral, generally amounting to less than 1%. Its mineral structure allows for easy substitution by Mg2+ for Fe2+ as part of a continuous solid solution between pure magnetite (FeO.Fe2O3) and magnesioferrite (MgO.Fe2O3). Likewise, some Al3+ may substitute for Fe3+. Some manganese and chrome are also generally present in the magnetite structure. Magnetite is also a common alteration product during serpentinisation of ferromagnesian olivines and pyroxenes. Most serpentines are iron-free and the ferrous iron contained in the olivines and pyroxenes is usually oxidised to magnetite. For this reason serpentinised rocks are generally more magnetic compared to their unserpentinised equivalents.During lateritic weathering, magnetite readily alters to hematite, goethite, limonite,and other iron hydroxides.
CHROMITE
Chromite is a very common mineral associated with peridotite rocks and the serpentines and serpentinites derived from them. It is a high-temperature mineral and forms very early during magmatic differentiation. It forms lenses, layers, pods, and occasionally large masses within the peridotite and is frequently exploited as an ore of chromium. However, most frequently it occurs as disseminated grains throughout the ultramafic body.
OXIDES
The only oxides that are commonly associated with ultramafic rocks and laterites are the oxides of iron, manganese and silicon.
Hematite can be of primary origin and can also form during the process of chemical weathering of the ultramafic rocks. It is the mineral responsible for the red colour of the iron-bearing soils. The strong red colour of hematite may mask the presence of any goethite. The development of secondary hematite versus goethite depends on soil temperature and the presence of moisture. Hematite to goethite ratio in the soil is higher for warmer latitudes and arid areas. Locally, the hematite to goethite ratio is higher at the top of ridges compared to the bottom of valleys.
Maghemite is a magnetic variety of hematite and is believed to have formed as a weathering product by the oxidation of magnetite [2Fe3O4 + O = 3Fe2O3]. The crystal structure of maghemite is closer to that of spinels such as magnetite but with a deficiency of iron (Fe2.66O4 compared with magnetite’s Fe3O4). Iron deficiency amounts to 11.33%. The spinel structure of maghemite inverts to the hematite structure (Fe2O3) on heating. Birkeland (1999), however, states that maghemite in soils comes from the conversion of Fe-oxides during forest fires. Maghemite is most abundant near the soil surface and is capable of giving a strong magnetic signal during geophysical surveys.
Silica is never present in the ultramafic rocks in a free form but exists as silicates. The leaching of ultramafics under tropical conditions releases large quantities of both silica and magnesia. In the early stages of leaching, magnesia is more soluble leaving behind encrustations and deposition of silica in the laterite profile. Occasionally, such silica deposition can form highly siliceous zones ranging in competency from friable silica to very hard and compact masses that may require blasting. More commonly, though, silica is deposited along fractures and openings in the peridotite and will ultimately result in the formation of silica boxwork as the peridotite converts to
limonite. Practically all of silica formed in the laterite environment is of low temperature amorphous variety
HYDROXIDES
GOETHITEGoethite 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.Goethite in soils may also be accompanied by 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.
LIMONITELimonite 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.
MANGANESE WAD
Manganese wad is amorphous-looking material commonly found as thin coatings on joints and fractures, as spots, and as reniform masses. The colour is dull black, bluish or brownish black. It is very soft and easily soils the hand when touched. The material is generally loosely aggregate and feels light. The wad is rich in hydroxides of manganese (MnO2 and MnO) and can contain appreciable amounts of other metals such as Fe, Al, Co and Ni. Significant amounts of the water of hydration of 10-20% may be present.
LITHIOPHORITE
Lithiophorite is a hydrous manganese-oxide with some lithium in it. Frequently, quantities of lithium can be very low. Various formulae have been advanced for this mineral, all containing the main components of Mn, O, and OH and with minor Al and Li as additional cations: (Al,Li)MnO2(OH)2.
ASBOLAN / ASBOLITE
The term asbolan or asbolite is used for the “Earthy Cobalt’ which is an amorphous substance and contains appreciable amounts of cobalt up to 32%.
HETEROGENITE
It is an amorphous substance that occurs in globular or reniform (kidney-shaped) masses and has the approximate composition CoO.2Co2O3.6H2O. The compositionis not definite and the material is regarded a colloid. It has been reported in Bulong area ores in Western Australia.
CLAY GROUP
The word clay is used in two senses: Fine-grained particles that are less than 0.002mm. These particles could be made up of any
composition. A group of sheet silicates with fairly well defined composition whose mineral structure could be
explored through X-ray diffraction methods.The clay minerals described here fall in the second category. Common clay minerals are hydrated silicates of aluminium, iron and magnesium.
1. Kaolinite GroupThis group includes the minerals kaolinite, dickite and nacrite with the general formula: 4Al2Si2O9. Kaolinite group minerals are commonly formed by the weathering of alkali feldspars under acidic conditions.
2. Smectite Group (Montmorillonite Group)This group includes montmorillonite (Mg-smectite), nontronite (Fe-smectite) and beidellite (Al-smectite). The three sets of cations, other than main structural Al and Si, can include: Na or ½ Ca; Mg or Fe++; Al or Fe+++. The smectites are made up of repeating TOT layers with 10ºA basal spacing. The layers have a net negative charge that is balanced by the cations. The amount of water residing between the layers can vary considerably and can change the size of the unit layer to as high as 15.2ºA. This water can be introduced and removed at room temperatures.Smectites are formed by the alteration and weathering of basic rocks that are low in K but contain Ca and Mg. The conditions should be alkaline.
3. Illite GroupThe Illite group includes illite and hydromicas (and perhaps glauconite). The basal spacing is 10ºA, or similar to that of the smectites. The inter-layer cation is predominantly K. The layers are well bonded and it is not easy for the water to get in. Thus, Illite group clays do not swell up when moistened.
4. Chlorite GroupIn the Chlorite group, a single sheet of positively charged octahedral layer is bonded to the negatively charged TOT structure with electrostatic bond. The basal spacing is 14ºA. Chlorites are
5. Mixed-Layer Clay MineralsFrequently, clays of one group are intimately mixed with that of another. In this situation, the group is referred to as “Mixed-Layer” Clay minerals. Thus, layers of illite can alternate with those of smectite to yield what is referred to as illite/smectite clay. This mixing is not physical but occurs at unit cell level. Other mixed layer clays include: chlorite/smectite, chlorite/vermiculite, and mica/vermiculite. Most clay-rich sediments when deposited in sedimentary environment are rich in smectite and have little illite. After the burial of the sediments, the quantity of illite layers increases due to the recrystallisation of smectite.
GARNIERITES
Jules Garnier (1839–1904) discovered a nickel bearing silicate in New Caledonia in 1864. This was named as garnierite (1867). By 1875, mining of this mineral as an ore of nickel had already started. The term garnierite has been used as a field term to include all hydrous nickel magnesian silicates. The first member of the garnierite group (chrysoprase, green silica) was defined in the 18th century. Faust (1966) showed that most garnierites are structurally related to talcs and serpentines. Kato (1961) found New Caledonian garnierites to be similar in structure to serpentine, talc and chlorites.
Figure 1. Garnierites of Serpentine-Talc structure
ASBESTOS
The name is used for fibrous varieties of several mineral species. All are silicates and common varieties include tremolite, actinolite, crocidolite and chrysotile. The first three belong to the amphibole group while the last one belongs to the serpentine group of minerals.Due to the nature of their soft, highly flexible fibres, asbestos minerals have been felted and woven like a fabric. Their primary use has been in the area of fireproofing and heat insulation.
Asbestiform Minerals
The term “asbestiform” is used generally for minerals that appear fibrous in nature and resemble asbestos in appearance. More commonly, the following minerals of the serpentine and amphibole groups are included in the category of asbestiform minerals:• Serpentine minerals: Chrysotile• Amphibole minerals: Tremolite, Actinolite, Crocidolite (Riebeckite)
Figure 2. Chrysotile (serpentine group)
CHEMICAL MOBILITY OF ELEMENTS IN GROUND WATER
Many metallic elements are soluble in ground water although the solubilities are extremely low compared to common salts.Generally speaking, solubilities are a function of the temperature of water and pH and Eh conditions. The common acid in ground water is Humic Acid that is derived from the decaying of ground vegetation and assists considerably in dissolving certain elements in ground water. Several researchers have produced estimates of mobilities of various elements associated with lateritic environments; these are briefly discussed below:
Soluble, Supergene and Residual elementsMobilities of elements commonly found in ultramafic/laterite association could be classified as follows:• Highly soluble and highly mobile• Non-soluble and non-mobile• Limited solubility and limited mobility
Highly soluble and highly mobile elements: Ca, Na, Mg, K, Si Easily leached out of the weathering profile Highly soluble in tropical ground waters that are slightly acidic Removed from laterite environment and taken to lakes and the seaNon-soluble (residual) elements: Al+++, Fe+++, Cr+++, Ti, Mn+++
Insoluble in ground water at ordinary pH/Eh conditions These elements make up the bulk of the residual soil Elements with limited solubility and mobility: Ni++, Co++, Mn++
Partly soluble in acidic groundwater Insoluble in the presence of more soluble elements (Si, Mg) Partial solubilities lead to supergene (secondary) enrichment
Residual concentration of non-mobile elements
As mobile elements leave the saprolite/soil through chemical leaching, nonmobile elements begin to increase in relative proportion. The following data is provided from Petea, a serpentinised peridotite area located northeast from Soroako:
Table 5.1: Concentration Factors for various elements
Supergene enrichment
Certain elements such as Ni, Co and Mn are somewhat soluble in the acidic waters percolating down the laterite profile but become insoluble as the waters reach below and are neutralised when highly soluble magnesia goes into solution.
CLASSIFICATION OF SOILS
SOIL HORIZONSMajority of the soils are stratified and are divided into soil horizons. These horizons differ from each other on the basis of physical or chemical characteristics.These distinct horizons result due to leaching, residual concentration, and downwardmigration of certain elements. All the processes of chemical weathering ⎯ and to some extent even of physical weathering ⎯ play an important role in developing distinctive soil profiles. Lateral continuity of these horizons is subject to continuity of underlying parent rock, topographic morphology, and prevailing climatic and environmental conditions. Commonly, the following classification is used for naming soil horizons with respect to laterites:O horizon:Surface accumulation of organic material. The horizon is further subdivided on the basis of degree of decomposition of the organic material.A horizon:Zone of mixed organic material and mineral fraction, the latter being dominant. Located at the surface or below the O-horizon.B horizon:Lying below the O or A horizon, B horizon is commonly marked by the residual concentration of sesquioxides of Fe, Al and Mn. More soluble
components have been leached away from this zone. No evidence remains of the original rock structure or mineralogy.C horizon:A subsurface horizon (excluding R horizon), which is the source of the soil and is in various stages of weathering. C horizon must be in situ. Designation Cr indicates the “saprolite” zone.R horizon:Fresh, consolidated bedrock below the soil profile.
A simplified equivalency of the above mentioned soil horizons with conventional laterite profile terminology is shown below:
Figure 5.12: Comparison of conventional laterite classification and equivalent soil horizons Conventional laterite terms Soil horizon terms
RELATIVE CHANGES IN SOIL PROFILEFor soils that are essentially residual in nature ⎯ including most of the laterites ⎯ changes in the soil profile can be ascertained after comparison with the underlying unweathered bedrock. Various experts in the field of soil development have used various oxides such as Al2O3, Fe2O3, and other non-mobile oxides as index markers. Relative concentrations of these index markers can reveal the relative gains and losses of various components in the original bedrock and the soil in the various horizons.Under conditions of chemical weathering that are typical of tropical areas with very humid conditions:
Mobile elements are leached out of the weathered bedrock (these include Ca, Na, K, Mg and Si)
Non-mobile elements undergo residual concentration (these include Al, Fe, Cr, Ti, Mn and Co)
Semi-mobile elements are leached out of the upper part of the laterite profile and concentrated in the lower part through supergene enrichment (essentially Ni but, to a lesser degree, also Co and Mn)
Depth Profiles of Major ElementsFigure-13 shows depth profiles for Fe, Al2O3, SiO2 and MgO, the four major elements in the laterite profile with concentrations greater than 5%. The profiles represent averages for several holes drilled in the Petea area.Note that above the Transition zone, iron and alumina are residually concentrated while silica and magnesia are chemically leached out. The Transition zone marks a sudden change in the relative proportions of all four major elements.
Depth Profiles of Minor ElementsFigure-14 shows the depth profiles for Cr2O3, MnO, Ni and Co, the four minor elements in the laterite profile with concentrations of generally less than 3%. According to the plots, chrome and manganese show residual concentrations above the Transition zone; nickel shows supergene enrichment in the Saprolite zone; while cobalt shows residual concentration towards the lower part of the Limonite zone.Note that profiles for the remaining elements such as CaO, K2O, Na2O, and TiO2 are not shown due to extremely low concentrations encountered in the laterite.
Relative Concentrations of ElementsFigure-15 displays the relative concentrations of the six elements that are normally enriched in the laterite profile through residual or supergene enrichment. “Relative concentration” of an element is defined as its present concentration in the profile against its background value in the bedrock. Since goethite/limonite represents the ultimate product of lateritic weathering of ultramafic rocks, the relative concentrations of various elements in the limonite zone can be computed based on their bedrock levels, as shown in Table-6.
