petrographical and mineralogical characteristics of rocks may...

PETROGRAPHY AND MINERALOGY

38

Petrographical and mineralogical characteristics of rocks may help in

understanding the process of lateritisation/bauxitisation. Although genesis of bauxite

is subjected to the process of alteration and it mainly decipher by the composition of

preexisting rocks and also processes involved in lateritisation/bauxitisation. This

chapter deals the petrography and mineralogy of rocks of the study area along with

their texture, forms and mineral transformations.

Scope of the chapter comprises thin section studied under polarizing

microscope and polished section studies in reflected light microscope. Heavy minerals

separation and their mineralogical studies under stereo zoom microscope were also

performed. X-ray diffraction studies were taken up on a few samples of bauxite and

laterite collected from the study area for detailed mineralogy. Few samples are also

subjected to infra-red and thermo-gravimetric analytical studies to corroborate the

results of XRD analysis.

5.1 MINERALOGICAL CHARACTERS OF LATERITE/BAUXITE

Mainpat bauxite deposit is lateritic bauxite deposit in which gibbsite is the

predominant aluminous mineral followed by boehmite and diaspore, like other

bauxite deposits of Central India and West Coast. In laterite, goethite and hematite are

the chief minerals but their percentage varies with variety of laterite. Kaolinite is main

silicate mineral of laterite profile and it is predominant in saprolite/clay horizon.

Gibbsite is subordinate in this horizon and occasionally halloysite and

montmorillonite are also there. It is also quite significant that quartz is negligible in

these laterite and bauxite.

Laterites and bauxites of the study areas can be divided mineralogically into

three different varieties as follows-

Variety of ore Al mineral Fe mineral Silicate mineral

Bauxite 50-80% --together-------- upto ----------20%

Aluminous laterite 40-60-% 0-10% 20-40%

Ferruginous laterite 20-40% 40-80% 0-10%


39

The major constituent minerals were noticed in the thin sections of laterites and

bauxite of the study area is as follows:

5.1.1 ALUMINOUS MINERALS

Gibbsite Al (OH)3

It occurs in sample of bauxite at white patches or some time as very porous

white mineral. In nodules and concretions light coloured bands of gibbsitic

composition are seen. Gibbsite crystals have not been identified megascopically in

any samples. The size of the gibbsite grains ranges from 10 mμ to 100 mμ (milli

microns). The largest ones have been observed on the walls of small cavities, vugs

and fissures filling in the bauxite in massive as well as pisolitic bauxites crystals can

be seen. The crystals are commonly colourless to pale brown in thin sections and

exhibit moderate to weak birefringence upto low second order. Large tabular crystals

show polysynthetic twining with well defined thin lamellae.

Gibbsite present in other bauxite deposit of central India, is generally of two

generation viz. one formed directly from alteration of primary mineral of parent rock

and other from disilication of kaolinite (Roy Chowdhary et al, 1964, 1968). In

Mainpat bauxite, second generation gibbsite is clearly seen in thin section (Plate

No.1- a, c; 3- a, b, c, d; and 4- a, b, c), where as first generation gibbsite is difficult to

identify. It may be probably due to very fine-grained nature. Alteration of kaolinite

into gibbsite and its occurrence in the form of minute crystal can be seen.

Boehmite AlO(OH)

After gibbsite, boehmite is the most common mineral of Mainpat bauxite. The

size of boehmite crystals is less then 10 mμ. It is enriched in pisoidal as comparison to

gibbsite. Boehmite occurs in nucleus of pisoids as minute grain (Plate No.1- a, b, c, d;

Plate No. 2- c, d; Plate No. 3- a, b).

Diaspore AlO (OH)

Diaspore is not a rare alumina mineral for these bauxites, however they are

characterized by having upto 7% diaspore. The crystals of this mineral are 10-15 mμ


40

in size. The matrix between the pisoids contains less diaspore and more kaolinite

(Plate No. 2- a and 4- b).

5.1.2 IRON MINERALS

Goethite Fe2O3. H2O

This is the main iron mineral occurs in laterites of the study area. However, it

can not be identified megascopically. Its presence may be ascertained by its fine

dissemination in the bauxite giving aphanitic texture and characteristic yellowish

brown colour. This is the main constituent of ferruginous laterite occurring in pisoids,

nodules and concretions. It is also seen as encrustations and fracture filling between

grains of other minerals. In thin section of ferruginous laterite, goethite along with

hematite appears as dark red and brown spherical bodies with faint boundaries.

Solution movement can also be seen along it. In thin section of pisolitic

laterite/bauxite, it appears as concentric rings.

Hematite Fe2O3

Hematite is frequent iron mineral but occurs in comparatively minute

proportion with goethite. Its presence in bauxite is inferred from its typical red colour.

Hematite rich portions of laterite and bauxite appear as black or reddish brown

patches in thin sections. It is also seen as scattered grains (Plate No. 1 c, 2 a, 4 a, d).

5.1.3 CLAY MINERALS

Kaolinite Al4(OH)8(Si4O10)

Kaolinite is the most common clay mineral occurring with bauxite. High-

grade, low silica bauxites contain less than 10% kaolinite where as it is present in

trace amount in the most desilicated deposits. The kaolinite content is generally

lowest in the core part of the profile. It increases downward and becomes predominant

in the saprolite horizon.

Kaolinite is found evenly distributed with aphanitic textures. It is also

concentrated in the matrix between pisoids, nodules and concretions. Kaolinite


41

appears in thin sections as worm like structures and its transformation into gibbsite

can also be seen (Plate No. 7 a, b, c, 8 a, c, d and 9 a, b, c).

Apart from kaolinite, some samples of bauxite were also found to contain

montmorillonite and halloysite, but their presence was detected only from XRD

results.

5.1.4 TITANIUM MINERALS

Anatase (FeTiO2) and rutile are the chief titanium minerals but their presence

was not recorded during microscopic studies. They are opaque minerals and appear as

completely black in thin section. Generally 1-7% anatase was determined by X-ray

diffraction in bauxite of Mainpat.

Rutile (TiO2) is also widespread in the laterite profile, and its average quantity

is generally upto 1%.

5.1.5 OTHER MINERALS

Other minerals present in laterite/bauxite are quartz and heavy minerals.

Quartz is practically absent in bauxite of Mainpat, in some laterites a little quartz upto

5% is noticed. It appears in thin sections as minute irregular grains, between the

crystals of gibbsite, goethite and hematite.

Heavy minerals in bauxite include zircon, tourmaline and opaque minerals.

This assemblage is almost akin to heavy minerals of the bedrocks.

5.1.6 HEAVY MINERAL STUDIES

The process of heavy mineral separation in brief and than their description.

The petrological characters of these transparent heavies are as follows-

Zircon (ZrO2SiO2): These are mostly colourless and brown in ordinary light (In

stereo zoom carlziess: yellow, lemon yellow, orange yellow, colourless to bluish and

colourless) and generally exhibit euhedral crystal outline. It is common in all the

samples, the zircon grain are usually rounded to sub-rounded, however prismatic,

angular to sub angular shapes are also common. Few grains also show well developed

crystal faces. Zircon is identified by straight extinction and high order of polarisation


42

colour. The inclusion and concentric type of zoning are common features. The

inclusion is mainly opaque, etched and pitted marks on the surface of the grains (Plate

No 6 a, b).

Sphene (CaO.TiO2.SiO2): It is identified by their typical form lozenge and wedge

shaped and crystallised in monoclinic system. It is brown and greyish-black colour in

reflected light, pleochoism in general rather week and cleavage (110) perfect (Plate

No. 5 c and d).

Rutile (TiO2): It is identified by their deep red and orange colour, dark boundaries

and weak pleochroism. They are generally prismatic in shape, however, sub rounded

grains are also observed. They are mostly yellow orange in colour. The characteristic

features of the rutile grains as colour and dark boundaries.

Andalusite (Al2SiO5): The metamorphic mineral andalusite occurs as colourless toi

pinkish subhedral crystal. It can be identified by high relief, straight extinction, and

two sets of cleavage.

Anatase (FeTiO2): It is characterised by octahedral prismatic in shape, indigo blue

colour, high relief and straight extinction (Plate no 5 a and b).

5.1.7 OPAQUE HEAVY MINERALS

Magnetite (Fe3O4): It is bluish black in reflected light. Angular to well rounded

particles are abundant. They are distinguishing from ilmenite with difficulty. They

derived from mafic igneous and high rank metamorphic rocks (Plate No. 5 e and f).

Ilmenite (FeO.TiO2): It is brownish to purplish black in reflected light. Common in

sediments as irregular to well rounded grain. They derived from mafic igneous and

high rank metamorphic rocks.

Hematite (Fe2O3): The hematite colour Indian red colour black in reflected light.

They altered product. Hematite is found in minor amount (Plate No 6 c, d, e and f).

Leucoxene (TiO2): They are dull white in reflected light. Appears as rounded grains

sometimes with mat surface minutely pitted. Opaque minerals commonly from the

bulk of heavies and useful for provenance.


43

5.2 X-RAY DIFFERECTION STUDIES (XRD)

In many geologic investigations, XRD complements other mineralogical

methods, including optical light microscopy, Electron Probe Micro-Analysis (EPMA),

and Scanning Electron Microscopy (SEM). X-ray diffraction is a versatile, non-

destructive analytical technique for identification and semi-quantitative estimation of the

various crystalline compounds, known as 'phases', present in solid materials and

powders. It provides a fast and reliable tool for routine mineral identification. It is

particularly useful for clays, ultra fine-grained minerals and mixtures or intergrowths

of minerals, which may not lend themselves to analysis by other techniques.

Laterite/Bauxite, composed of minerals may be studied by various analytical

techniques. One such technique, X-ray powder diffraction (XRD), is an instrumental

technique that is used to identify minerals, as well as other crystalline materials. Some

mineralogical samples analyzed by XRD are too fine grained to be identified by

petrological microscope.

XRD analysis for mineral identification of bauxite/laterite samples was carried

out in Laboratory of IIT, Powai (Mumbai) and GSI, Jaipur (Western Region) with the

help of X-ray Defractometer (X’Pert Pro of Panalytical). Identification is achieved by

comparing the X-ray diffraction pattern - or 'diffractogram' - obtained from an

unknown sample with an internationally recognised database containing reference

patterns for more than 70,000 phases. However, due to unavailability of mineral

separation unit bulk XRD analysis was carried out.

XRD analyses for identification of different mineral phases taken up and

gibbsite, boehmite, anatase and hematite minerals have been identified in most of the

bauxitic/lateritic samples. Detailed peak data given in tables (Table No. 5.1 to 5.6).

Diffractrogram with assigned peak is also seen (Figure No. 5.1 to 5.6). Characteristics

the spacing of different mineral phases are as follows-

Gibbisite: According to Hanawalts Number (Bayliss et al. 1980) Basic d-spacing of

the synthetic gibbisite are 4.82X, 4.34 4, 4.30 2, 2.372, 2.442, 2.031, 3.351, 1.981.


44

However, little shift of the peak has been observed. The shift may be due to the

occurrence of water molecule.

Boehmite: The significant peaks of the Boehmite are 6.11X, 3.16 7, 2.356, 1.863, 1.853,

1.452, 1.312, 1.661. Although the intensity of the peaks was found weak in comparison

to gibbsite. In a few samples boehmite recorded with a minor shift of the peaks. This

is also because of interference of the water molecule. Quantitatively the boehmite

occurs in less proportion.

Anatase: The chemical composition also reflected the higher content of TiO2.

Anatase as titanium mineral is notice in all the samples during XRD studies of sample

from the study area. The characteristic peaks of the anatase are 3.52X, 1.894, 2.382,

1.702, 1.672, 1.481, 2.431, 2.331. Quantitatively, the anatase occurs in association with

gibbsite and boehmite with very low proportion. Anatase also characterized by some

shoulder peaks in diffractrogram.

Hematite: In the samples, hematite found as inclusion with gibbsite, boehmite and

anatase. The occurrence of ferrous minerals causes scattering in diffratrogram. The

characteristic peaks of the hematite are 2.69X, 1.696, 2.515, 1.844, 1.484, 1.454, 2.203,

3.663.

5.3 PETROGRAPHY OF LATERITE AND BAUXITE

Microscopic study of thin sections of laterites and bauxites of Mainpat plateau

reveal that Petrographical character of the area is not much varied. However different

petrographic characters were noted in aluminous laterite/bauxite, ferruginous laterite

and highly ferruginous pisolitic laterite.

5.3.1 BAUXITE AND ALUMINOUS LATERITE

Thin section studies of the massive and vermiform bauxite/aluminous-laterite

show the presence of gibbsitic material intimately associated with ferruginous

material. In transmitted light the gibbsitic material is brownish yellow to greenish

yellow in colour and looks like an amorphous mass. Where as ferruginous material

forms numerous dust like patches. These materials are also traversed by numerous


45

solution and pathways, along the center of which is noticed dark to almost blackish

ferruginous material. The gibbsite mass adjacent to these solution tracks shows lighter

shades of colour and looks like Fe poor material. In the gibbsitic material, crystalline

variety of gibbsite occurs as small grains throughout. Comparatively bigger and

euhedral grains of gibbsite are seen as filling in circular to oval shaped cavities.

Quartz, if present shows indication of chemical reaction and it is very fine grained

(Plate No. 1 a, b and c, 2 b, c and d).

Pisolitic variety of bauxite shows either a gibbsitic core or ferruginous core

with indication of solution movement. Alternate coloured rings seen in hand specimen

appear in thin sections as gibbsitic and goethite/hematitic layers. Movement and

deposition of aluminous solution within ferruginous core can be seen in the form of

very fine grained gibbsitic and sometimes boehmitic material (Plate No 1 a and b, 2 c,

d, 8 d and 9 b).

It appears from thin section study that the pisoids might have been developed

from rhythmic precipitation of iron-aluminium hydroxide-gel into spherical or oval

bodies.

These pisoids are often heavily cracked which are filled with amorphous clay.

Fine-grained gibbsite is found sporadically distributed within the pisoids.

Apart from typical massive and pisolitic texture, some thin section of bauxite

show segregation of gibbsite and goethite/hematite in the form of colloform bands

(Plate No 1 b and 2 d). Valeton (1972), reffered this type of texture as gel-like texture,

formed by absolute enrichment of Al or Fe resulting in destruction of relic textures,

followed by rearrangement in spherical fabrics. Rao and Murthy, (1982) noticed

similar type of texture in Amarkantak bauxite. They considered this texture to be

formed by colloidal deposition of aluminous and ferruginous material followed by

recrystallisation. The colloidal growth may be due to the variation of rhythmic

differentiation of Fe+3

and Al+3

of the hydroxyl-gel.


46

5.3.2 FERRUGINOUS LATERITE

Ferruginous laterite shows interrelationship between iron minerals and alumina

minerals. In thin sections, ferruginous laterites show solution, movement and

segregation of iron. Movement of iron solution along cracks within pisoids is visible.

Iron solution also takes the irregular shape occupying cavities and voids. Similarly

gibbsite can be seen occupying cavities within ferruginous material. Formation of

gibbsite can also be seen between ferruginous pisoids (Plate No 1 d and 2 b and d).

5.3.3 HIGHLY FERRUGINOUS LATERITE

In highly ferruginous pisolitic laterite, pisoids are of dominant iron bearing

minerals. Thin sections of this laterite appear completely reddish brown. However,

faint boundaries between pisoids can be seen. Numerous cracks and solution

movement can be seen within individual pisoids (Plate No 3 a and 4 a and d).

5.4 CONCLUSIONS

Following important conclusions were drawn from the observation and study

of rocks.-

i. Mainpat bauxite is gibbsitic bauxite having gibbsite 5% to 30%.

ii. Gibbsite has a gradual upward increase in the profile with a corresponding

decrease in kaolinite.

iii. Kaolinite content is generally lowest in the middle part of the profile

(aluminous laterite and bauxite). It increases slightly in laterites and becomes

predominant in this saprolite horizon.

iv. Gibbsitization of kaolinite is prominent in laterites and bauxite. However, first

generation gibbsite is not clearly seen. Gibbsite occurs throughout as small

crystals or grains. These two features indicated fine grained nature of parent

rock.

v. Quartz is practically absent in bauxites, however, quartz is abundant in the bed

rock.


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vi. Average goethite content is less than hematite in bauxites.

vii. Pisolitic bauxite of the study area is of higher grade (Al2O3 more than 55%).

viii. Mineral transformation- gibbsite and goethite together with clay are intimately

associated, the early stage of bauxitization. Cryptocrystalline-gibbsite

developed from kaolinite, during the next stage of bauxitization. The gibbsite

which was initially cryptocrystalline grew into good crystals on further

desilication of kaolinite. Well defined crystals were formed in the last stage of

maturity. Iron hydroxides viz. goethite developed during the process of

deferrification of the system. The mobilized iron is seen in the form of goethite

veins.


48

Table 5.1: List of Peaks Generated by XRD for Different Mineral Phases

(Sample no. 1)

2-θ d(A) Intensity % Area

Name of the

Mineral

Phase

27.799 3.207 5.8 139

Unassigned

Peak

30.021 2.974 36.5 1409 Grassularite

33.74 2.654 100 3008 ---do----

35.459 2.529 11.6 621 ---do----

37.021 2.426 28 773 ---do----

38.58 2.332 15.3 379 ---do----

41.601 2.169 18.5 574 ---do----

47.08 1.929 22.2 750 ---do----

53.317 1.717 10.1 310 ---do----

55.72 1.648 18.5 669 ---do----

57.96 1.59 33.9 1050 ---do----

62.539 1.484 6.9 342 ---do----

68.94 1.361 5.8 113 ---do----

70.76 1.33 17.5 472 ---do----

72.78 1.298 15.3 601 ---do----

74.798 1.268 8.5 252 ---do----


49

Fig

ure

5.1

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no.

1)


50

Table 5.2: List of Peaks Generated by XRD for Different Mineral Phases (Sample no. 2)

2-θ d(A) Intensity % Area Name of the Mineral

Phase

18.46 4.802 100 6958 Gibbisite

20.48 4.333 20.3 1842 ---do---

25.44 3.498 13.9 1032 Anatase

27.06 3.292 4.8 460 Gibbisite

28.179 3.164 4.1 253 ---do---

33.279 2.69 4.3 395 ---do---

36.72 2.445 5.7 647 ---do---

37.219 2.414 3.8 1340 ---do---

37.84 2.376 10 537 ---do---

40.258 2.238 3.1 244 ---do---

41.899 2.154 3.8 342 ---do---

44.3 2.043 7.2 485 ---do---

45.6 1.988 4.8 310 ---do---

47.56 1.91 3.3 522 ---do---

48.101 1.89 3.8 664 Anatase

50.759 1.797 5.3 498 Gibbisite

52.34 1.747 6.5 539 ---do---

54.081 1.694 5.7 836 ---do---

54.58 1.68 5.5 1000 Anatase

57.96 1.59 3.1 303 Gibbisite

62.8 1.478 4.3 357 ---do---

63.998 1.454 6 515 ---do---

64.78 1.438 3.3 166 ---do---

66.299 1.409 4.5 187 ---do---

66.797 1.399 3.6 230 ---do---

68.958 1.361 2.4 224 ---do---

71.659 1.316 2.6 336 ---do---

79.26 1.208 2.2 288 ---do---


51

Fig

ure

5.2

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no. 2)


52

Table 5.3: List of Peaks Generated by XRD for Different Mineral Phases (Sample no.12)


Phase

14.44 6.129 4.9 1533 Boehmite

18.32 4.839 100 23329 Gibbisite

20.32 4.367 5.7 1893 Gibbisite

25.26 3.523 4 1130 Anatase

28.08 3.175 3.7 1033 Boehmite

36.54 2.457 2.7 1572 Gibbisite

37.1 2.421 2 887 Gibbisite

37.68 2.385 2.9 1070 Anatase

38.26 2.350 1.8 719 Gibbisite

44.16 2.049 2.1 601 Gibbisite

45.401 1.996 1.7 500 Gibbisite

50.639 1.801 2.1 567 Gibbisite

52.259 1.749 2.1 589 Gibbisite


53

Fig

ure

5.3

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no.1

2)

:


54



Phase

14.579 6.071 1.4 328 Boehmite

16.46 5.381 1.3 227 unassigned Peak

18.28 4.849 100 14093 Gibbisite

20.241 4.384 3.7 672 Gibbisite

21.321 4.164 3.2 485 Gibbisite

24.002 3.705 1.3 331

25.299 3.518 3 407 Anatase

26.919 3.309 1.6 356 Boehmite

28.001 3.184 1.4 228

33.12 2.703 4.1 719 Hematite

35.661 2.516 2.8 324 Hematite

37.04 2.425 3.6 1139

44.179 2.048 1.9 333 Gibbisite

45.642 1.986 1.6 306

47.42 1.916 1.4 219 Gibbisite

49.481 1.841 1.6 253

50.478 1.806 2 294 Gibbisite

52.099 1.754 1.6 308

53.56 1.71 2.2 582

54.039 1.696 2.4 839 Hematite

62.54 1.484 1.9 431

64.04 1.453 2.3 553

66.602 1.403 1.8 456

78.88 1.213 1.7 461


55

Fig

ure

5.4

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no.1

5)

:


56



Phase

14.461 6.12 0.9 258 Boehmite

18.32 4.839 100 25907 Gibbisite

20.3 4.371 3.4 1150 Gibbisite

25.3 3.517 3.7 1169 Anatase

28.101 3.173 1.2 313 Boehmite

33.14 2.701 1.4 500

37.12 2.42 3.5 1487 Gibbisite

44.218 2.047 1.6 574

50.599 1.802 1.5 467


57

Fig

ure

5.5

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no.1

8)

:


58



Phase

14.36 6.163 46 2366 Boehmite

18.2 4.87 100 4165 Gibbisite

20.22 4.388 31.5 1677 Gibbisite

25.2 3.531 29.4 1313 Anatase

26.401 3.373 6.4 420 Gibbisite

26.821 3.321 6.4 507 Gibbisite

28.04 3.18 27.7 1199 Boehmite

36.557 2.456 8.9 493 Gibbisite

37.64 2.388 17 765 Gibbisite

38.22 2.353 20.4 935 Boehmite

39.98 2.253 5.5 216

41.679 2.165 6.8 173

44.139 2.05 10.2 446 Gibbisite

45.42 1.995 7.2 485 Gibbisite

47.298 1.92 6 536

47.959 1.895 7.7 571 Anatase

48.839 1.863 12.8 934 Boehmite

49.099 1.854 11.5 1025 Boehmite

50.499 1.806 7.2 303

52.118 1.753 7.2 352 Anatase

54.4 1.685 8.1 1058 Anatase

57.86 1.592 5.1 287

62.541 1.484 6 332 Anatase

63.819 1.457 8.9 455 Boehmite

64.677 1.44 5.1 256

66.06 1.413 5.1 338

67.619 1.384 6 116

68.761 1.364 4.7 150

71.819 1.313 8.9 621 Boehmite


59

Fig

ure

5.6

: X

-Ray

Dif

frac

tion C

har

t (S

ample

no.2

2)

:


60

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