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II. The Nature of Laterite

II. The Nature of Laterite

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6



S. SIVARAJASINGHAM ET AL.



and tortuous tubular cavities either empty, filled, or partially filled with

a greyish-white clay passing into an ochreous, reddish and yellowish

brown dust; or with a lilac-tinted litheomargic earth. The sides of the

cavities are usually ferruginous and often of a deep brown or chocolate

color; though generally not more than a line or two in thickness, their

laminar structure may frequently be distinguished by the naked eye. . . .

The interior of the cavities has usually a smooth polished superficie, but

sometimes mammillary, and stalactiform on a minute scale . . . . The

surface masses of the softer kinds present a variegated appearance. The

clay and lithomarge exhibit lively colored patches of yellow, lilac, and

white, intersected by a network of red, purple, or brown. The softness

of this rock is such that it may be cut with a spade; hardening by

exposure to the sun and air, like the laterite of Malabar.” (Omissions are

by the present authors.) Vesicular laterite may be soft or of varying

hardness and commonly has earthy material in the cavities. It usually

occurs near the surface.

Cellular slag-like laterite is a scoriaceous mass. The many empty

cavities are separated by ferruginow material similar in appearance to

that which separates the earthy substance in vesicular laterite. Cellular

laterite is usually dark colored and may have a dull or lustrous surface.

It is of varying hardness and is brittle, being usually easily shattered

when struck a sharp blow with a hammer. According to Fox (1936),

cellular laterite is formed by removal of kaolin and other earthy material

from the cavities in vesicular laterite when the latter is exposed to

erosion and leaching at the surface. Falconer (1911), from his observations in nothern Nigeria gave a similar explanation, though he avoided

using the term laterite for “surface ironstone.”

Nodular laterite consists of individual concretions, pisolites or other

crudely round masses, usually the size of a pea but commonly larger or

smaller; it is generally ferruginous. The nodules may occur as a superficial covering or as a component in one or more horizons in the soil,

varying in concentration from low or insignificant amounts to very high

amounts. The nodules vary in hardness; some can be readily cut by a

knife but most are hard and brittle.

When the nodules of a layer are cemented together, hard “pisolitic”

or “concrete-like” laterite is formed. It occurs mainly at or near the

surface. The individual nodules may either be joined directly to one

another or be discrete entities in a cementing matrix of similar, but

usually less ferruginous, material.

Recent studies by Alexander and Cady ( 1962) present enlightening

detail on the physical arrangement of discrete components. Though

various specimens exhibit a great variety of micromorphological features,



7



LATERITE



certain structures are common to many, but not necessarily all, varieties.

Commonly under magnification in thin sections, tiny bodies ranging from

perfect spheres to oblong rounded forms may be seen embedded in a

matrix of fine particles; the matrix may be either very dense or spongelike. The rounded bodies may be individual units or, commonly, may be

aggregates of smaller spherical units closely packed. Such rounded bodies

may be widely spaced or closely packed in the matrix. Their boundaries

may be smooth and definite or irregular and indefinite in various specimens. The matrix may be unorganized, may have a gridlike rectangular

or reticulate network of oriented material, or may be largely oriented. Oriented material commonly lines pores and may appear as skins on the nodules. Crystalline oriented material is common as pseudomorphs after primary minerals, as porefillings, and as discrete bodies ranging from barely

visible units to relatively large homogeneous masses. Rock structure may

be preserved or may be entirely absent. Quartz particles may be

included, and in some specimens weatherable minerals encased in a

protective covering of weathered material have been observed.



B. CHENICAL

CHARACTERISTICS

Materials identified in the field as laterite have a wide range of

chemical characteristics. A prominent feature common to all laterites,

nevertheless, is a high content of either iron or aluminum or both

relative to other constituents (Alexander et al., 1956). This is clearly

illustrated by the following analyses, which are thought to be typical

examples (Table I ) .

Bases are almost completely absent. Combined silica is generally

TABLE I

Chemical Composition ( % ) of Selected Laterites

Constituent

Quartz

Feldspar

SiO,

A1203

Fe7.03

TiO,

CaO

H,O (loss on ignition)



Site:a



1



2



3



4



5



0.76

NDb

1.77

4.32

80.02

6.06



ND

ND

1.93

62.32

1.88

11.87



4.32

2.35

17.08

20.83

40.18

1.72



7.06



ND

ND

0.37

43.83

26.61

4.45

0.86

23.88



21.54



11.05



ND

ND

31.37

19.22

38.51

1.12

0.10

9.10



-



99.99



100.00



97.53



99.42



-



-



99.54



-



-



Site: 1, Coolgardie, Australia (Simpson, 1912). 2, Satara, Bombay, India

(Warth and Warth, 1903). 3, Bagru Hill, Bihar, India (Fox, 1936). 4, Cheruvannur,

India; Buchanan’s original site (Fox, 1936). 5, Djougou, Dahomey; laterite on

granite (Alexander and Cady, 1962).

0 ND, not determined.



8



S. SIVAFUJASINGHAM ET AL.



low (sites 1, 2, and 3, Table I ) , but some varieties, such as the original

laterite of Buchanan (site 4, Table I ) , may have significant amounts.

This is probably largely in the form of kaolin, which has been found in

recent work by Alexander and Cady (1!362) to be the principal or only

identifiable silicate clay mineral in samples from Africa. Alumina may

be the principal sesquioxide (site 3, Table I ) , but more commonly iron

oxide (site 1, Table I ) or iron oxide and alumina together (sites 2, 4,

and 5, Table I ) are the major constituents. Combined water, determined

by loss on ignition, is appreciable but is generally higher in aluminous

than in ferruginous varieties, as is shown in Table I. Titanium is also

common in significant amounts in most varieties and may be a major

constituent (site 3, Table I ) . Vanadium and chromium are found, but

rarely in appreciable quantities.

Quartz may be absent or present in only limited amounts, but on

rocks high in quartz it is commonly a significant or major component, as

on the granite of site 5 of Table I, for which petrographic studies showed

that much of the total silica was contributed by quartz. Quartz is also

common in laterite over nonquartzose rock, where it appears to be

derived mainly from wind-blown or detrital material from outside sources.

Ten samples of detrital laterite from various parts of India had an average

of ,2074 quartz (Warth and Warth, 1903). Pendleton and Sharasuvana

(1912, p. 10) have emphasized that differences in amount of quartz

commonly contribute to major variation in SiO, among samples, even

in the same profile.

The impoverishment in combined silica and bases and concentration

of sesquioxides during weathering and laterite formation on a dolerite

is illustrated in Table 11. The “primary laterite” of Harrison is not to be

confused with ‘laterite” as used in this review. It was a weathered earthy

product that lay between the surficial hard laterite crust and the unweathered dolerite rock. Major differences in proportions of iron and

aluminum and in amount of combined water between weathered material

and laterite presumed to have formed in similar material are common, as

in Table 11, but it is rarely possible to be certain that the laterite crust has

indeed formed in material like that of the underlying weathered product.

No consistent relationship seems to exist between the relative amounts

of silica, iron, and alumina and the degree to which the physical properties of laterite are developed. The shortcoming of any chemical classification was shown by Fox (1936) from the analyses of laterite samples

from Buchanan’s original sites (site 4, Table I ) . These would have been

called ‘lateritic lithomarge” in Fermor’s ( 1911) classifkation because of

the high content of combined silica, though the material was vermicular

and was being quarried for building purposes.



9



LATERITE



The analyses considered so far refer to bulk samples of massive

laterite without distinction between segregated nodular material and the

matrix. The nodular material is, however, found either to be similar in

composition to the matrix or to contain less combined silica and more

TABLE I1

Chemical Composition ( ”/o ) of Dolerite, “Primary Laterite,” and Associated Laterite

Ironstone at Eagle Mountain, British Guianaa

Constituent



Dolerite rock



“Primary

laterite”



Laterite

ironstone



2.40

49.60

17.29

2.90

8.26

0.35

0.53

0.05

6.95

8.80

0.18

2.81



2.86

0.50

46.80

23.64

2.50

22.96

0.69

Nil

Nil

Nil

Nil

Nil



0.14

0.62

10.54

74.43

0.65

9.60

3.91

Nil

Trace

0.02

Nil

Nil



99.95



99.91



H,O (loss on ignition)

TiO,

MnO

MgO

CaO

K,O

Na,O

a



-



-



100.12

From Harrison (1933).



TABLE I11

Selected Chemical Constituents (%) of Nodules of Laterite at 5 Sites and of the

Matrix in Which the Nodules Were Embedded at One of Them

5

Constituent Site:a

SiO,

A1203

Fe,%

TiO,

MnO,



1



2



3



4



Soft

nodules



Matrix



8.0

4.7

67.9



29.7

29.1

21.7

2.0

2.3



26.1

14.2

20.7

6.3

13.1



49.8

2.7

28.9

9.8

1.1



39.3

19.3

30.1

0.9

0.1



54.8

20.6

13.5

1.0

0.1



0.5

Nil



a Site: 1, Natal; ferruginous nodules (Beater, 1940). 2, Welimada, Ceylon;

aluminous and silicious nodules (Joachim and Kandiah, 1941). 3, Peradeniya,

Ceylon; manganiferous nodules (Joachim and Kandiah, 1941). 4, Hambantota,

Ceylon; titaniferous nodules (Joachim and Kandiah, 1941), 5, Congo; soft nodules

and matrix (Alexander and Cady, 1962).



ferric oxide. Site 5 of Table 111 illustrates the latter in soft nodules of a

ground-water laterite of the Congo. Prescott and Pendleton (1952, p. 21)

believed that nodules usually contain less free alumina than the more

massive forms and that they are low in manganese.



10



S. SIVARAJASINGHAM ET



AL.



Nodules studied by Alexander et al. (1956) were high in sesquioxides

and low in silica. Other workers have, however, reported appreciable

contents of both quartz and combined silica (Joachim and Kandiah, 1941;

Waegemans, 1954). The data in Table 111 illustrate the wide range of

silica, alumina, iron, titanium, and manganese in nodules from different

places. The work of Bennett and Allison (1928) also reveals variations in

the composition of nodules in different soils.



L

C. M W ~ L O G I C ACHARAC~ERISTLCS

Chemical analysis alone is not sufficient to reveal the nature and

origin of laterite ( Harrison, 1910; Campbell, 1917). Laterites having

similar physical properties, such as hardness or morphology, may differ

greatly in chemical composition, and, conversely, laterites having similar

chemical compositions may have greatly different physical properties.

Petrographic studies of thin sections ( Harrison, 1910, 1933), adsorption

of dyes (Hardy and Rodrigues, 1939), differential thermal analysis

(Humbert, 1948; Bonifas, 1959), and X-ray analysis (Alexander et al.,

1956; Bonifas, 1959) have been used to supplement chemical determinations.

Free alumina is mostly in the form of gibbsite (A1203.3H20),as

boehmite ( A1203.H20), or as an amorphous hydrated form which has

been called cliachite and a variety of other names (Hanlon, 1944;

Palache d aE., 1944). Iron is found in the form of goethite (FeO-OH),

hematite ( FeeOa),and as amorphous oxides or unidentifiable coatings on

other minerals (Alexander et al., 1956). Free silica is mostly inherited

quartz (Alexander et al., 1956),though Harrison ( 1933, p. 40) reported

PLATE 1

Photomicrographs illustrating features of weathering and laterite formation

A. Weathering diorite, North Carolina. Crossed Nicols. The lath-like forms are

gibbsite pseudomorphs after feldspar. Some dark areas are allophane and some are

iron oxide.

€3. Soft laterite from granite, Nigeria. Crossed Nicols. The light areas of the

crystal aggregate (upper left) and of the filled channel (lower right) are gibbsite

formed upon weathering of kaolinite. Dark areas are iron-impregnated clays and

iron oxides, which are isotropic or have a very low birefringence.

C. Hard laterite from granite, Nigeria. Plain light. The dark areas are impregnated with iron by local redistribution from the light areas. The higher population of quartz grains (white areas) in the part that has lost iron indicates that these

parts have collapsed.

D. Hard laterite from granite, Kigeria. Crossed Nicols. The yellow parts are

crystalline goethite which forms a continuous network, especially on the walls of

the small channel at the left. The dark streak through the center is a former channel

filled with fine-grained hematite. White spots are quartz grains.



A



C



B



D



PLATEI



This Page Intentionally Left Blank



LATERITE



13



secondary quartz in laterites presumed to have been derived from basic

igneous rocks containing only small amounts of quartz. Chalcedony and

opaline forms have been observed also. Presence of colloidal silica has

been suspected. Besides quartz, other resistant minerals like magnetite,

zircon, sphene, anatase, and ilmenite have been found.

Combined silica is known to occur in kaolin and comparable silicate

clay minerals that can be identified but is present in amorphous or

subcrystalline forms as well. Montmorillonite and illite types of clays

have not been identified in significant amounts.

The distribution and form of constituents within the laterite has

special significance. Recent studies by the authors have shown that the

finely divided matrix is commonly largely unoriented and unidentifiable

by petrographic techniques, though identsable portions range from none

to most of the material. Grains and large patches of oriented aggregates

ranging from kaolin slightly stained with iron oxide through kaolin

highly impregnated with iron oxide to almost pure goethite or hematite

are among the more prominent identifiable constituents. Variable optical

density within the matrix is related to the degree of iron impregnation

(Plate 1, C and D). Hardness of the mass appears to be related to the

degree of crystalinity and continuity of the crystalline phase of the

impregnating iron, which is largeIy in the form of goethite. Tiny spherical

bodies comparable to incipient nodules of centripetal enrichment described by Bryan (1952) are largely unoriented earthy material having

a higher degree of iron impregnation than the surrounding matrix. Some

have films of crystalline goethite on the surface or as concentric shells

within the body. Other spherical bodies embedded in the matrix may

be concretionary or pisolitic forms of gibbsite, boehmite, goethite, or

hematite. Gridlike networks of oriented materials are found in the matrix

of many specimens and are composed principally of goethite (Alexander

et al., 1956) or hematite ( Sivarajasingham, 1961).

Gibbsite pseudomorphs after feldspar (Plate 1A) and goethite pseudomorphs after ferromagnesian minerals are common constituents. These

appear to be most abundant in young laterites whereas concretionary or

pisolitic nodular forms of the same minerals are more conspicuous in

older varieties (Alexander et al., 1956). Gibbstite and boehmite are

found commonly as fillings in cracks and pores, both in the matrix and

in nodules. Pores and cracks may also be lined with oriented kaolin

having varying degrees of iron impregnation, or even with oriented

goethite (Plate 1D). In hard laterite crusts, hematite may be found as

pore linings, as bands, or as discrete masses (Plate 1D). In varieties

derived from quartzose material, quartz grains are generally distributed

randomly through the matrix (Plate 1C) and earthy nodular bodies.



14



S. SIVARAJASINGHAM ET AL.



Quartz, apparently derived from outside sources, may also be present in

laterite over nonquartzose rock (Alexander et aZ., 1956).

111. The Environment of Laterite



Laterite is widely distributed in Africa, Australia, Southeast Asia, and

South America; Prescott and Pendleton (1952) have given a comprehensive review of its geographical distribution. The observed distribution

of laterite relative to present environment, however, is not necessarily a

criterion of conditions under which laterite forms (Hallsworth and

Costin, 1953, p. 25; Pendleton, 1936, p. 107).

A. CLIMATE

Pendleton (1941) considered that an effective rainfall capable of

supporting a forest is necessary and that laterite probably never develops

in a climate that would support a savannah-type climax vegetation.

Maignien (1958, p. 15) concluded that the Sudano-Guinea climate of

80 or more inches of rain per year with 2 to 4 months relatively dry is

optimum for mobilization, accumulation, and induration of iron in

laterite. Humbert ( 1948, pp. 281-282) concluded that continuously wet

conditions do not favor laterite formation. The occurrence of a period of

drought, which is observed in many areas where laterite occurs, was

though to be a requisite by Maclaren (1906). This was disputed by

Scrivenor ( 1909), however, who declared that laterite occurs in Malacca,

where there is no such alternation of seasons. Campbell (1917) and

Humhert (1948, p. 282) considered that a regular alternation of wet

and dry seasons is not necessary if there are periods of wetness and

dryness, even though irregularly distributed. Mohr and van Baren (1954,

pp. 69-71) have suggested, on the basis of effective rainfall and evapotranspiration, that a monthly rainfall not exceeding 60 mm. would constitute an adequately “dry” season to provide no excess of rain over

evapotranspiration in tropical regions. Simpson ( 1912) thought that

laterite was forming under the semiarid conditions of western Australia.

It appears clear that some minimum amount of water is necessary for

weathering, the removal of bases and combined silica, and segregation

of iron so evident in the resultant material. It also appears reasonable

that periods of drying should favor the crystallization of goethite or

similar minerals, which appear to be associated with hardening. Observations by the authors suggest that approximately equal wet and dry

seasons favor crust formation and that some degree of alternating wet

and dry season is probably essential for this process. It is not clear whether

the material may or may not be conditioned for this final segregation

and crystallization in permanently wet conditions. To some degree,



LATERITE



15



conflicting views may be the result of two confounding factors: (1) the

occurrence of laterite under climates that are unlike those under which

the laterite formed, and ( 2 ) differential effectiveness of climatic wetness

or dryness in combination with differences of topography, parent material, and time.

Laterite is found where temperatures are warm or are believed to

have been warm at the time of formation. This does not preclude the

possibility, however, that time, not temperature, is the controlling

factor. If the principal effect of high temperature is to accelerate rates of

reactions, the time interval necessary for laterite formation in temperate

regions may exceed the period of time that appropriate landscapes have

been stable. Similarly, observations of Oldham ( 1893), Joachim and Kandiah (1935), and Prescott and Pendleton (1952) that laterite has not

formed under the cool temperatures of highlands in the tropics may be

due less to cool temperature than to landscape instability. Nevertheless,

field observation generally agree with the hypothesis that warm temperatures favor the formation of laterite.



B . VEGETATION

A forest vegetation was considered necessary for laterite formation by

Glinka (1927, pp. 33-34). More detailed observations have shown that,

while laterite occurs in regions with a rain-forest vegetation, welldeveloped laterite is most commonly found under a low forest and that

hard surficial laterite is a very common feature of the open savannah

adjacent to the forest. Maignien (1958, p. 16) concluded that laterite is

most extensive and strongly expressed at the boundary of forest and

savannah. DHoore (1954,1957, p. 55) found iron mobilized to a greater

degree and to greater depth under tropical grasses than under forest.

Buchanan’s type of soft laterite is found in Ceylon in the forested areas

of the southwestern lowland.

Humbert (1948) suggested that laterite forms in a climate that has a

wet and a dry season, and his descriptions and illustrations indicate that

the laterite he observed was in an open savannah that was gradually

replacing forest, a common condition where a dry season is prominent.

The change from soft laterite to the hard form within a few years after

man has cleared the forests has been reported by Alexander and Cady

( 1962) in French Guinea and other parts of West Africa. Blackie’s (1949)

description of soils of Fiji and Aubert’s (1950) description of Dahomey

also confirm the hardening of laterite following a change from forest to

savannah caused by man’s activities. It would appear from the literature

that laterite is most extensive in areas of savannah but that laterite forms

under forest though its hardening is favored by lack of forest cover.



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