NICKEL LATERITES ⎯
A TRAINING MANUAL
CHEMISTRY, MINERALOGY &
FORMATION OF Ni LATERITES
Waheed Ahmad
July 30, 2001
LIST OF CONTENTS
CHEMISTRY
Valencies; Metals & oxides; Important oxides; Mineralogical formulas in terms of
oxides; Atomic weights; Ionic radii; Ionic replacements; Chemical mobility of
elements; Chemical composition in terms of elements; Chemical composition on
terms of oxides; Conversion of elemental percentages to oxide percentages.
ROCK-FORMING MINERALS
Silicon tetrahedron; Primary mafic minerals; Primary feldspars; Primary spinels;
Secondary minerals; Nickel hydrosilicates; Garnierites; Pyroxene composition
fields; Iron oxides; Chromite; Asbolite
OLIVINE GROUP
Structure; Chemical composition; Formation of olivines; Nickel in the olivines;
Alteration and weathering;
SERPENTINISATION OF OLIVINES
Conditions for serpentinisation; Serpentinisation by volume change;
Serpentinisation at constant volume; Dissociation of serpentine
ULTRAMAFIC ROCKS
Terminology; Classification; Field occurrence; Nickel in ultramafic rocks
MAGMATIC DIFFERENTIATION
Continuous-reaction series; Discontinuous-reaction series
PHASE DIAGRAMS
Ice-water-vapour system; 2-component system with solid solution;
2-component system without solid solution; SiO2-MgO system; Phase chemistry
in PT Inco furnaces
WEATHERING
Processes of change; Chemical weathering; Hydrolysis; Oxidation; Hydration;
Solution; Factors that influence chemical weathering; Stability of minerals; pH
conditions; Eh conditions; Rate of removal; Role of climate; Role of topography
WEATHERING OF ULTRAMAFICS
CLASSIFICATION OF SOILS
LATERITIC SOILS
Definition; Lateritic profile; Limonite zone; Intermediate zone;
Saprolite zone; Rate of laterisation
2
CHEMISTRY
Chemical valencies of elements
Valence (or valency) is the capability of an atom to combine in particular
proportion with another atom.
Cations: Generally metallic elements (Na, K, Cu, Mg, Ca, Fe, Mn, Ni, Co, Pb)
Anions: Generally non-metallic elements (O-2, S-2, F-, Cl-, Br-, I-)
Valency
Other atoms
Monovalent
1
Divalent
2
Trivalent
3
Tetravalent
4
Pentavalent
5
Hexavalent
6
Examples
H, Li, Na, K, Cu1
Mg, Ca, Co, Cu2, Fe2, Mg, Mn2, Ni, O, Pb2, Zn).
Bi, Cr3, Al, Fe3, Sb3
C, Si, Ti, Zr, Mn4, Pb4
P, As, Sb5
Cr6
Many atoms such as Fe, Mn, Cu, Cr, Pb, and Sb have multiple valencies: they
are capable of combining with other atoms in different proportions.
Metals and oxides
Metals
Al
(Aluminium)
C
(Carbon)
Ca
Co
Cr
Fe
(Calcium)
(Cobalt)
(Chromium)
(Iron)
H
K
Mg
Mn
(Hydrogen)
(Potassium)
(Magnesium)
(Manganese)
Na
Ni
P
S
Si
(Sodium)
(Nickel)
(Phosphorous)
(Sulphur)
(Silicon)
Oxides and names
Al2O3 (Alumina)
CO
(Carbon monoxide)
CO2 (Carbon dioxide)
CaO (Calcium oxide)
CoO (Cobalt oxide)
Cr2O3 (Chrome)
FeO (Ferrous oxide) ⎯ Iron protoxide
Fe2O3 (Ferric oxide) ⎯ Iron sesquioxide
H2O (Water)
K2O (Potassium oxide)
MgO (Magnesia)
MnO (Mangannous oxide)
MnO2 (Mangannic oxide) ⎯ Pyrolusite
Na2O (Sodium oxide)
NiO (Nickel oxide)
P2O5 (Phosphorous pentaoxide)
SO2 (Sulphur dioxide)
SiO2 (Silica)
3
Important oxides in mineralogical studies
Univalent metal:
H2O
K2O
Na2O
Cu2O
= Water
= Potassium oxide
= Sodium oxide
= Cuprous oxide
Divalent metal:
MgO
MnO
NiO
CoO
CaO
CuO
FeO
ZnO
= Magnesia
= Manganous oxide
= Nickel oxide
= Cobalt oxide
= Calcium oxide
= Cupric oxide
= Iron protoxide (ferrous iron)
= Zinc oxide
Trivalent metal:
Cr2O3
Al2O3
Fe2O3
= Chrome
= Alumina
= Iron sesquioxide (ferric iron)
Tetravalent metal: SiO2
CO2
MnO2
SO2
TiO2
= Silica
= Carbon dioxide
= Mangannic oxide (Pyrolusite)
= Sulphur dioxide
= Titanium oxide
Pentavalent metal: P2O5
= Phosphorous oxide
Representation of mineralogical formulae in terms of oxides
Most rock-forming minerals can be represented in terms of oxides:
Mineral name
Formula
Forsterite olivine
Fayalite olivine
Enstatite/Bronzite
Serpentine
Talc
Magnetite
Hematite
Goethite
Limonite
Chromite
Mg2SiO4
Fe2SiO4
MgSiO3
H4Mg3Si2O9
H2Mg3(SiO3)4
Fe3O4
Fe2O3
H2Fe2O4
H6Fe4O9
FeCr2O4
Representation by oxides
=
=
=
=
=
=
=
=
=
=
4
2MgO.SiO2
2FeO.SiO2
MgO.SiO2
3MgO.2SiO2.2H2O
3MgO.4SiO2.H2O
FeO.Fe2O3
Fe2O3
Fe2O3.H2O
2Fe2O3.3H2O
FeO.Cr2O3
Atomic weights
Atomic weights of elements are relative weights in comparison to Hydrogen as
1.0 or Oxygen as 16.0.
H
1.0
O
16.0
Mg
24.3
Al
26.9
Si
28.0
Ca
40.0
Cr
52.0
Mn
54.9
Fe
55.8
Ni
58.6
Co
58.9
Ionic radii of Common rock-forming elements
Ions are measured in terms of their radii. The unit of radius is an angstrom.
One angstrom is one ten millionth of a millimetre (1 ºA = 10-7 mm or 10-10 m):
Si
0.42
Al
0.51
Cr6
0.52
Mn4
0.60
Co3
0.63
Cr3
0.63
Fe3
0.64
Mg
0.66
Fe2
0.74
Zr
0.79
Mn2
0.80
Na
0.97
Ca
0.99
K
1.33
O
1.40
S-2
1.84
Ti
0.68
Ni
0.69
Co
0.72
Ionic replacements
Certain cations can replace each other to form isomorphous series. The
replacement requires the following conditions to be met:
•
•
•
•
They have about the same ionic radii
The minerals they form have the same crystal structure
The difference in charge is no more than one valency
[if an element of a smaller positive charge comes in, then an
additional cations are needed to balance the charge in the lattice]
The conditions are suitable (the crystals and the remaining liquid
are in contact with each other and the composition of the liquid has
an opportunity to alter the composition of the crystals)
Fe replaces Mg in olivines
Magnesian and ferrous olivines form a continuous series of solid solutions. Both
have orthorhombic crystal structure, both are divalent, and the ionic radii of
magnesium and ferrous iron are fairly close (0.66°A and 0.74°A respectively).
Mg2SiO4
Fe2SiO4
(Mg,Fe)2SiO4
Magnesian Olivine, Forsterite
Iron Olivine, Fayalite
Solid solution of Mg and Fe olivines
5
A typical olivine in ultramafic rocks is Fo90Fa10. Such an olivine is a solid solution
of 90% Forsterite and 10% Fayalite. The proportion of oxides and elements in
such an olivine can be written out as follows:
Fe = 5.48%
FeO = 7.05%
Fe2SiO4 = 10% (Fayalite)
Atoms of Fe = 0.09821
Mg = 31.10%
MgO = 51.57%
Mg2SiO4 = 90% (Forsterite)
Atoms of Mg = 1.2798
Fe:Mg = 1:5.7
FeO:MgO = 1:7.3
Fa:Fo = 1:9
Fe:Mg = 1:13
Fe replaces Mg in pyroxenes
Magnesian and ferrous pyroxenes form a continuous series of solid solutions.
Both end members have similar crystal structure, are divalent, and the ionic radii
of magnesium and ferrous iron are fairly close (0.66°A and 0.74°A respectively).
MgSiO3
FeSiO3
(Mg,Fe)SiO3
Magnesian pyroxene, Enstatite
Iron pyroxene, Ferrosilite
Solid solution of Mg and Fe pyroxenes, Hypersthene
A pyroxene from Poro, New Caledonia, has the composition: (Mg 0.9 Fe 0.1) SiO3.
The above composition can be expressed in different ways, as follows:
Fe = 5.39%
Mg = 21.14%
FeO = 6.94%
MgO = 35.05%
FeSiO3 = 10% (Ferrosilite) MgSiO3 = 90% (Enstatite)
Atoms of Fe = 0.09659
Atoms of Mg = 0.86996
Fe:Mg = 1:3.92
FeO:MgO = 1:5.05
Fa:Fo = 1:9
Fe:Mg = 1:9
Al replaces Si in plagioclases
Aluminium can replace silicon atoms in plagioclase feldspars since both have
nearly the same ionic radii (Si=0.42°A; Al=0.51°A). However, aluminium is
trivalent (Al+++) while silicon is tetravalent (Si++++). Thus, if a cation of lower
positive charge replaces a cation of higher positive charge, another cation must
be introduced into the crystal lattice to balance the deficit.
In the case of albite (NaAlSi3O8), every fourth silicon ion (++++) is replaced by
aluminium (+++) and sodium ions (+).
In the case of anorthite (CaAl2Si2O8), every second silicon ion is replaced by
aluminium and a divalent calcium ion.
Ni replaces Mg in olivines
During the crystallisation of olivines, small quantities of nickel present in the
magma can replace Mg in the lattice structure. Such replacement is possible for
the following reasons:
6
•
•
•
Ionic radii of Mg and Ni are fairly close (Mg=0.66°A; Ni=0.69°A)
Their valencies are the same (Mg++ ; Ni++)
Their crystals belong to the same orthorhombic system
The solid solution of Ni and Mg in olivines can be perceived as the solid solution
of nickel-olivine in magnesian-olivine. If the proportion of nickel in forsterite is
0.3%Ni, then the relative quantities of nickel and magnesium in the rock can be
worked out in terms of elements, oxides or the type of olivine, as follows:
Ni = 0.3%
NiO = 0.382%
Ni2SiO4 = 0.554% (Ni-olivine)
Mg = 34.368%
MgO = 56.983%
Mg2SiO4 = 99.446% (Fo)
In terms of numbers of atoms:
Ni:Mg = 1:115
NiO:MgO = 1:150
Ni-ol:Mg-ol = 1:180
Ni:Mg = 1:275
Nickel and iron replace Mg in olivines
An olivine from Poro, New Caledonia, is: (Mg1.813 Fe0.18 Ni0.007) SiO4.
The olivine can be assumed to be a solid solution of three different olivines. The
proportion of various elements, oxides, and olivines is given below:
Ni-olivine
Fe-olivine
Mg-olivine
0.007/2 = 0.35%
Ni = 0.28%
NiO = 0.356%
0.18/2 = 9.0%
Fe = 6.855%
FeO = 8.822%
1.813/2 = 90.65%
Mg = 30.07%
MgO = 49.856%
Nickel and iron replace Mg in pyroxene
An pyroxene from Poro, New Caledonia, is: (Mg0.908 Fe0.09 Ni0.002) SiO3.
The pyroxene can be assumed to be a solid solution of three different pyroxenes.
The proportion of various elements, oxides, and pyroxenes is given below:
Ni-pyroxene
Fe-pyroxene
Mg-pyroxene
0.002 / 1 = 0.20%
Ni = 0.1136%
NiO = 0.1446%
0.09 / 1 = 9.0%
Fe = 4.866%
FeO = 6.263%
0.908 / 1 = 90.8%
Mg = 21.3795%
MgO = 35.447%
7
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 increase at higher temperatures and in the
presence of low-pH (acidic) waters. The common acid in ground water is Humic
Acid that is derived from the decaying of ground vegetation.
For components of ultramafic rocks, reasonable solubilities exist at temperatures
and conditions commonly found in the tropical climate.
Mobilities of elements commonly found in the mafic and ultramafic rocks are
classified as follows:
•
•
•
Highly soluble and highly mobile
Non-soluble and non-mobile
Limited solubility and limited mobility
Highly soluble and mobile elements: Ca, Na, Mg, K, Si
Easily leached out of the weathering profile.
Highly soluble in tropical ground waters (slightly acidic)
Non-soluble (residual) elements: Al, Fe+++, Cr, Al, Ti, Mn, Co
Insoluble in ground water.
These elements make up the bulk of the residual soil.
Elements with limited solubility and mobility: Ni, Fe++
Partly soluble in acidic groundwater.
Insoluble in the presence of more soluble elements (Si, Mg).
Partial solubility of Ni leads to supergene (secondary) enrichment.
Relative Mobilities of elements (given below in the order of decreasing values)
Ca++ >
3.0
•
•
•
•
•
•
•
Na+ >
2.4
Mg++ >
1.3
K+ >
1.25
SiO2 >
0.20
Fe2O3 >
0.04
Ca++, Mg++, Na+ are highly soluble and readily lost during leaching
K+ is readily leached but fixed again as K-bearing clays
Fe++ (ferrous iron) is readily leached and can be mobile
Si++++ is slowly lost under leaching conditions
(crystalline quartz has one-tenth the solubility of amorphous silica).
Ti++++ is generally immobile, except if released as Ti(OH)4
Fe+++ (ferric iron) is immobile under oxidising conditions
Al+++ is immobile in the normal pH range of 4.5 – 9.5
8
Al2O3
0.02
Chemical composition in terms of elements
Relative proportions of elements in compounds are expressed in percentages:
H2O (water)
Total weight = 1+1+16 = 18
H = 2 / 18
= 11.1%
O = 16 / 18 = 88.9%
SiO2 (silica)
Total weight = 28 + 16 + 16 = 60
Si = 28 / 60 = 46.7%
O = 32 / 60 = 53.3%
FeO (iron protoxide)
Total weight = 55.8 + 16 = 71.8
Fe = 55.8 / 71.8
= 77.7%
O = 16 / 71.8
= 22.3%
Fe2O3 (iron sesquioxide)
Total weight = 55.8 + 55.8 + (3 x 16) = 159.6
Fe = 111.6 / 159.6 = 69.9%
O = 48 / 159.6
= 30.1%
Hematite
Fe3O4 (FeO.Fe2O3)
Magnetite
2Fe2O3.3H2O
Limonite
Total weight = 55.8 + 55.8 + 55.8 + (4 x 16) = 231.4
Fe = 167.4 / 159.6 = 72.3%
O = 64 / 231.4
= 27.7%
Total weight = (4 x 55.8) + (9 x 16) + (6 x 1) = 373.2
Fe = 223.2 / 373.2 = 59.8%
O = 144 / 373.2
= 38.6%
H = 6 / 373.2
= 1.6%
Mg2SiO4 (olivine)
Total weight = 24.3 + 24.3 + 28.0 + (4 x 16) = 140.6
Mg = 48.6 / 140.6 = 34.6%
Si = 28 / 140.6
= 19.9%
O = 64 / 140.6
= 45.5%
MgSiO3 (pyroxene)
Total weight = 24.3 + 28 + (3 x 16) = 100.3
Mg = 24.3 / 100.3 = 24.2%
Si = 28 / 100.3
= 27.9%
O = 48 / 100.3
= 47.9%
H4Mg3Si2O9
Total weight = 4 + (3 x 24.3) + (2 x 28) + 144 = 276.9
H = 4 / 276.9
= 1.4%
Mg = 72.9 / 276.9 = 26.3%
Si = 56 / 276.9
= 20.2%
O = 144 / 276.9
= 52.0%
Serpentine
9
Chemical composition in terms of oxides
Atomic weights of important oxides
SiO2 = 28 + 32
MgO = 24.3 + 16
FeO = 55.8 + 16
Fe2O3 = (2 x 55.8) + (3 x 16)
NiO = 58.6 + 16
H2O = 1 + 1 + 16
= 60
= 40.3
= 71.8
= 159.6
= 74.6
= 18
Forsterite olivine
Mg2SiO4
=
2MgO.SiO2
= 140.6
MgO = 80.6 / 140.6 = 57.3
SiO2 = 60 / 140.6 = 42.7%
Fayalite olivine
Fe2SiO4
=
2FeO.SiO2
= 203.6
FeO = 143.6 / 203.6 = 70.5%
SiO2 = 60 / 203.6 = 29.5%
Enstatite/Bronzite
MgSiO3
=
MgO.SiO2
= 100.3
MgO = 40.3 / 100.3 = 40.2%
SiO2 = 60 / 100.3 = 59.8%
Serpentine
H4Mg3Si2O9 =
3MgO.2SiO2.2H2O
MgO
SiO2
H2O
= 276.9
= 43.7%
= 43.3%
= 13.0%
Talc
H2Mg3(SiO3)4 =
3MgO.4SiO2.H2O
MgO
SiO2
H2O
= 378.9
= 31.9%
= 63.3%
= 4.8%
Hematite
Fe2O3
=
Fe2O3
= 100%
Magnetite
Fe3O4
=
FeO.Fe2O3
FeO
Fe2O3
= 231.4
= 31.0%
= 69.0%
Goethite
FeO(OH)
=
Fe2O3.H2O
Fe2O3
H2O
= 177.6
= 89.9%
= 10.1%
Limonite
2Fe2O3.3H2O =
2Fe2O3.3H2O
Fe2O3
H2O
= 373.2
= 85.5%
= 14.5%
10
Conversion of elemental percentages to oxide percentages
Ni to NiO:
Atomic weight of Ni = 58.6
Atomic weight of NiO = 74.6
Conversion factor for Ni to NiO = 74.6 / 58.6 = 1.273
Co to CoO:
Atomic weight of Co = 58.9
Atomic weight of CoO = 74.9
Conversion factor for Co to CoO = 74.9 / 58.9 = 1.272
Fe to FeO:
Atomic weight of Fe = 55.8
Atomic weight of FeO = 71.8
Conversion factor for Fe to FeO = 71.8 / 55.8 = 1.287
Fe to Fe2O3:
Atomic weight of Fe = 55.8
Atomic weight of Fe2O3 = 159.6 / 2 = 79.8
Conversion factor of Fe to Fe2O3 = 79.8 / 55.8 = 1.430
Mg to MgO:
Atomic weight of Mg = 24.3
Atomic weight of MgO = 40.3
Conversion factor for Ni to NiO = 40.3 / 24.3 = 1.658
Si to SiO2:
Atomic weight of Si = 28.0
Atomic weight of SiO2 = 60
Conversion factor for Si to SiO2 = 60 / 28 = 2.143
Al to Al2O3:
Atomic weight of Al = 26.9
Atomic weight of Al2O3 = 101.8 / 2 = 50.9
Conversion factor for Al to Al2O3 = 50.9 / 26.9 = 1.892
Cr to Cr2O3:
Atomic weight of Cr = 52.0
Atomic weight of Cr2O3 = 152 / 2 = 76
Conversion factor for Cr to Cr2O3 = 76 / 52 = 1.462
Mn to MnO:
Atomic weight of Mn = 54.9
Atomic weight of MnO = 70.9
Conversion factor for Mn to MnO = 70.9 / 54.9 = 1.291
Ca to CaO:
Atomic weight of Ca = 40.0
Atomic weight of CaO = 56
Conversion factor for Ca to CaO = 56 / 40 = 1.400
Oxide is always heavier than the metallic element since oxide contains oxygen in
addition to the metal. Thus:
•
To convert into oxide, multiply by the factor
•
T convert into metal, divide by the factor
11
ROCK-FORMING MINERALS
SiO4
Silicon tetrahedron
The basic structure of all silicate minerals is the silicon tetrahedron where on
silicon atom is linked to four oxygen atoms.
In the olivines, the silicon tetrahedra exist independently.
In the pyroxenes, the silicon tetrahedra are linked together to make a chain.
In the amphiboles, two adjacent chains are so linked as to make a ring structure.
In the micas, the silicon tetrahedra are arranged in sheets.
Fields of
Mafic minerals
SiO2
PYROXENES
En
Kayolinite-Smectite
Fo
CLAYS
Serp.
OLIVINES
MgO
Al2O3
Ferrosilite
Nontronite
Fa
FeO
Fe2O3
12
Primary Mafic minerals
Olivines
Forsterite (Mg)
Chrysolite (Mg,Fe)
Fayalite (Fe)
Mg2SiO4
(Mg,Fe)2SiO4
Fe2SiO4
Pyroxenes
Orthorhombic
Enstatite (Mg)
Bronzite (Mg,Fe)
Hypersthene (Fe,Mg)
MgSiO3
(Mg,Fe)SiO3
(Fe, Mg)SiO3
Monoclinic
Clinoenstatite (Mg)
Pigionite (Mg,Ca)
Diopside (Ca,Mg)
Hedenbergite (Ca,Fe)
Augite (Ca,Mg/Mg,Fe)
Acmite (Na,Fe)
Jadeite (Na,Al)
Spodumene (Li,Al)
MgSiO3
intermediate between clinoenstatite & diopside
CaMg(SiO3)2
CaFe(SiO3)2
CaMg(SiO3)2 with (Mg,Fe)(Al,Fe)2SiO6
NaFe(SiO3)2
NaAl(SiO3)2
LiAl(SiO3)2
Triclinic
Rhodonite (Mn)
Babingtonite (Ca,Fe,Mn)
MnSiO3
(Ca,Fe,Mn)SiO3.Fe2(SiO3)3
Amphiboles
Orthorhombic
Anthophyllite (Mg,Fe)
(Mg,Fe)SiO3
[Si4O11]
(Mg,Fe)SiO3
(Fe,Mg)SiO3
CaMg3(SiO3)4
Ca(Mg,Fe)3(SiO3)4
(K2,Na2,Mg,Ca,Mn)SiO3
[Si4O11]
[Si4O11]
[Si4O11]
[Si4O11]
[Si4O11]
(Fe,Na2)(Si,Ti)O3.Na(Al,Fe)(SiO3)2
[Si4O11]
Monoclinic
Cummingtonite (Mg,Fe)
Grunerite (Fe,Mg)
Tremolite (Ca,Mg)
Actinolite (Ca,Mg,Fe)
Hornblende
Triclinic
Aenigmatite
Micas
Biotite, muscovite, phlogopite
13
Primary Feldspars
Plagioclase feldspars
Anorthite (Ca)
Bytownite
Labrodarite
Andesine
Oligoclase
Albite (Na)
Potash feldspars
Orthoclase
Microcline
CaAl2Si2O8
Solid solutions of calcic
to sodic plagioclases
NaAlSi3O8
KAlSi3O8
KAlSi3O8
An = 100 – 90%
An = 90 – 70%
An = 70 – 50%
An = 50 – 30%
An = 30 – 10%
An = 10 – 0%
(monoclinic)
(triclinic)
Primary spinels (R++O. R+++2O3)
Common spinel (Al)
Iron spinel (magnetite)
Chrome spinel (chromite)
MgAl2O4
Fe3O4
Fe.Cr2O4
14
[MgO.Al2O3]
[FeO.Fe2O3]
[FeO.Cr2O3]
Secondary minerals
Hydrous mafic minerals
Serpentine (Mg) Chrysotile
Serpentine (Fe) Greenalite
Tremolite (Mg/Ca)
Talc (Steatite or soapstone) (Mg)
Kerolite (more hydrous form of talc)
Chlorite (Penninite / Clinochlore / Prochlorite)
Sepiolite
Clay minerals (hydrous silicates)
Kayolinite / Nacrite / Dickite (+Al)
Hallyosite (endellite) (Al)
Illite (K-Al)
Smectite (-Al)
Montmorillonite / Saponite / Pyrophyllite (Al,Ca,Mg)
Nontronite (Fe)
Saponite (Mg)
Mg3Si2O5(OH)4
Fe3Si2O5(OH)4
Mg5Ca2Si8O22(OH)2
Mg3Si4O10(OH)2
Mg3Si4O10(OH)2.nH2O
Mg5Al2Si3O10(OH)8
Mg4Si6O15(OH)2.4H2O
Al2Si2O5(OH)4
Al2Si2O5(OH)4.2H2O
KAl3Si3O10(OH)2
Al2Si4O10(OH)2
Fe2Si4O10(OH)2
Mg3Si4O10(OH)2
Non-silicate oxides and hydroxides
(also a magnetic polymorph, “Maghemite”)
Hematite
Fe2O3
Goethite
Fe2O3.H2O
Limonite
2Fe2O3.3H2O
Brucite
Mg(OH)2
Boehmite
AlO (OH)
[Al2O3.H2O]
Gibbsite
Al(OH)3
[Al2O3.3H2O]
15
Nickel hydrosilicates
In nickel hydrosilicates, nickel replaces the Mg atoms in such minerals as
serpentine, talc, and chlorite. Pure nickel end members do not exist in nature and
most nickel hydrosilicates contain (Ni,Mg) in the place of Mg.
The term garnierite has been used as a field term to include all hydrous nickelmagnesian 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.
Brindley and Pham Thi Hang (1973) determined that garnierites fall into two
groups: they are either serpentine-like with 7°A basal spacing, or talc-like with
10°A basal spacing.
Springer (1974) proposed the following definition for garnierites: “nickelmagnesian hydrosilicates, with or without alumina contents, having X-ray
diffraction patterns typical for serpentine, talc, Sepiolite, chlorite, vermicolite, or
mixtures of these”.
The field of garnierite minerals has as its end members: kerolite (talc),
serpentine, pimelite and nepouite. Compositions of secondary nickel silicates are
shown below along with their magnesian end-members:
Mg-end member
Chrysotile
Mg3Si2O5(OH)4
Lizardite
Mg3Si2O5(OH)4
Talc
Mg3Si4O10(OH)2
Kerolite
Mg3Si4O10(OH)2.nH2O
Clinochlore
Sepiolite
Mg5Al2Si3O10(OH)8
Mg4Si6O15(OH)2.4H2O
Ni-end member (actually Ni,Mg)
Pecroaite
Ni3Si2O5(OH)4
Nepouite
Ni3Si2O5(OH)4
Willemsite Ni3Si4O10(OH)2
Pimellite
Ni3Si4O10(OH)2.nH2O
Garnierite
Ni3Si4O10(OH)2.H2O
Nimite
Ni5Al2Si3O10(OH)8
Falcondite Mg4Si6O15(OH)2.4H2O
Garnierites are largely of supergene origin being precipitated in the lower parts of
the weathered ultramafic profiles, from downward or laterally-moving solutions.
Garnierites occur as fillings in open spaces (fractures, joints), or as coatings in
joint and fracture surfaces. They range widely in colour from green (light and
dark), to yellowing green, to light blue and turquoise blue. The rich green
varieties contain more nickel.
16
Garnierites
SiO2
10°A basal
spacing
Kerolite - Talc
10°A
GARNIERITES
Serpentine
Pimelite
7°A
Nepouite
7°A basal
spacing
MgO
NiO
Garnierite compositions
Sample-1, New Caledonia1
Sample-2, New Caledonia
Sample-3, New Caledonia
Sample-4, New Caledonia
Sample-5, New Caledonia
SiO2
53.0
49.0
53.2
49.8
37.4
MgO
18.1
18.9
15.0
13.5
2.7
FeO
0.08
0.18
0.03
0.21
0.31
NiO
20.9
21.7
24.5
29.2
49.6
Konde, pit-717
Konde, pit-717
Konde, pit-717
Konde, pit-717
Konde, pit-717
Konde, pit-717
Konde, pit-717
43.8
43.7
52.5
48.7
51.1
50.6
37.6
36.8
30.3
25.3
25.2
22.6
16.1
0.7
4.15
5.95
0.20
0.00
0.05
0.04
0.31
1.31
4.30
9.43
12.75
14.67
22.20
47.14
43.7
52.9
47.8
52.3
30.4
18.3
18.6
16.3
5.49
0.23
0.14
n.a.
5.50
16.80
19.6
20.80
(wall)
(centre)
Morro do Cerisco, Brazil
Morro do Niquel, Brazil
Riddle, Oregon, USA
Riddle, Oregon, USA
1
New Caledonian garnierite assays for samples 1-5 are from G. Troly et al., 1979.
17
Pyroxene
Composition fields
CaSiO3
Diopside
(CaMg)SiO3
Hedenbergite
(CaFe)SiO3
Augite
Ferro-augite
Pigionite
I
Hypersthene
(Mg,Fe)SiO3
Enstatite
MgSiO3
Ferrosilite
FeSiO3
Pyroxene compositions
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
NiO
CoO
MgO
CaO
Na2O
K2O
H2O
LOI
Totals
Soroako
Unserp.
ENSTATITE
55.1
0.05
3.23
0.58
Soroako
Unserp.
CLINOPYROX.
53.2
0.09
3.47
0.86
Poro
Harzburgite
Orthopyrox.
60.1
Tiebaghi
Harzburgite
Orthopyrox.
61.8
5.79
0.13
0.076
0.006
33.5
1.86
2.52
0.08
0.05
< 0.006
18.5
21.7
5.8
5.4
0.1
0.06
34.7
32.7
0.40
0.64
18
Iron oxides
Fe
Iron protoxide FeO
Magnetite Fe3O4
Iron sesquioxide Fe2O3
( Hematite)
O
Goethite Fe2O3.H2O
Limonite 2F2O3.3H2O
H2O
H
Magnetite
Magnetite (Fe3O4) occurs in the laterite profile, derived from two sources:
•
•
From the primary magnetite present in the ultramafics
Magnetite derived from the serpentinisation of the olivines
Magnetite eventually breaks down under the process of chemical weathering.
The initial product of weathering is maghemite (a magnetic polymorph of
hematite with the Fe2O3 formula). Maghemite eventually gets hydrated to
goethite in the upper part of the laterite profile.
19
Goethite
Ferrous iron released from the weathering of the primary mafic minerals oxidises
to ferric iron and is precipitated as an hydroxide. The hydroxide has a poor
degree of crystallinity and appears amorphous. It occurs as concretions as well
as clay-like 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.
Although the standard formula of goethite is Fe2O3.H2O, significant quantities of
Al2O3 and Cr2O3 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.
Compositions of goethites
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO2
NiO
CoO
MgO
CaO
Na2O
K2O
H2O
LOI
Totals
Soroako
Bonsora
3–6m
1.61
0.08
10.24
3.35
71.96
Soroako
Bonsora
6–9m
1.33
0.18
11.13
3.37
70.23
Soroako
Bonsora
9 – 12 m
2.71
0.09
11.95
3.15
68.79
0.082
0.41
0.04
0.36
0.08
0.13
0.48
0.02
0.00
0.03
0.46
0.01
0.00
0.02
0.47
0.01
0.00
0.03
2
Manganese in the original assays of all Bonsora samples is reported as total manganese or
Mn2O3 (MnO + Mn2O3).
20
COMPOSITION OF CHROMITE
Soroako
Podiform
Soroako
Disseminated
FeO
Cr2O3
14.93
49.4
19.56
42.6
Fe
Cr
Cr/Fe ratio
11.6
33.79
2.91
15.2
29.14
1.92
21
COMPOSITION OF ASBOLITES
Asbolane or manganese wad is a black, amorphous-looking material commonly
found as thin coatings on joints and fractures, and occasionally as nodules and
beads. The material is rich in manganese and contains appreciable amounts of
Fe2O3, Al2O3, CoO and NiO. Other elements are present only in very small
quantities. Significant amount of the water of hydration is also present.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO2
NiO
CoO
MgO
CaO
Na2O
K2O
H2O
LOI
Totals
Soroako
Limonite
3–6m
1.3
0.1
9.0
0.02
18.4
Soroako
Limonite
6–9m
0.8
0.1
15.0
0.2
14.3
Soroako
Limonite
9 – 12 m
1.2
n.d.
7.0
0.5
36.0
Soroako
Saprolite
31.0
1.65
7.12
0.2
0.03
0.1
0.2
33.6
3.44
7.38
0.5
0.01
0.1
0.1
33.0
2.29
4.96
0.5
0.1
n.a.
0.2
32.0
16.17
3.18
2.8
0.1
n.a.
0.01
1.8
n.a.
3.5
n.d.
14.2
New Cal.
(Wadsley,
1950)
1.63
19.22
15.95
39.29
6.99
4.723
11.974
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.
3
H2O -120°C
4
H2O +120°C
22
OLIVINE GROUP
Forsterite
Chrysolite
Fayalite
Olivine type
Formula
In terms of oxides
Magnesia
Mg2SiO4
2MgO.SiO2
Ferro-magnesian
(Mg,Fe)2SiO4
2(Mg,Fe)O.SiO2
Ferro
Fe2SiO4
2FeO.SiO2
Density
Hardness
3.22
7
4.39
6.5
Melting point
1890°C
1205°C
Structure
•
•
Olivines belong to the orthorhombic group (three unequal axes, all at
right angles)
Olivine structure consists of individual silicon-oxygen tetrahedral linked by
magnesium atoms, each of which has six nearest oxygen neighbours.
The oxygens lie in sheets parallel to the (100) plane and are arranged in
approximate hexagonal close packing (see Figure of olivine structure).
The silicon-oxygen tetrahedra point alternately either way along both the x
and y directions.
The magnesian atoms do not occupy one set of equivalent lattice
positions: half are located at centres of symmetry and half on reflection
planes, the former having as nearest neighbours two oxygens from two
adjacent tetrahedra, the latter, two oxygens from one adjacent
tetrahedron.
•
In the olivine series, the unit cell dimensions increase with the increasing
replacement of the smaller magnesium atom by the larger iron atom. Unit
cell size of Forsterite is 4.756 °A while that of Fayalite is 4.817 °A.
23
Chemical composition of olivines
•
Complete solid solution of the Mg and Fe olivines to give a complete
range of Mg-Fe olivines
•
Chemical composition of olivines is generally indicated by the percentage
of Forsterite (Fo) and Fayalite (Fa) present (example: Fo90Fa10)
•
Pure end members of the Mg-Fe olivines are rare in nature. Thus pure
forsterite (Fo100) or pure Fayalite (Fa100) is almost never seen in nature.
•
The composition of olivine that forms dunite at the type locality in New
Zealand (Dun Mountain) is Fo92.
•
Generally, the first olivines to crystallise from many basic magmas is in the
range of Fo88 to Fo82.
•
The average composition of peridotite olivine is Fo88.
•
Olivines with compositions in the range of Fo80 to Fo50 are very common
constituents of basic igneous rocks (gabbros, dolerites/diabases, basalts).
•
Olivines in the ferro-gabbros of the strongly differentiated Skaergaard
intrusion show a continuous variation from Fo39 to Fo2.
Olivine Compositions
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
NiO
CoO
MgO
CaO
Na2O
K2O
H2O
LOI
Totals
Soroako
Unserp.
40.3
0.02
0.41
0.02
Poro
Harzburgite
40.8
Tiebaghi
Harzburgite
39.2
8.92
0.13
0.37
0.013
50.8
0.07
7.8
9.0
0.5
0.3
49.2
51.4
0.23
24
Formation of olivines
•
Members of the olivine group are important rock forming minerals in the
basic and ultrabasic igneous rocks.
•
Basic and ultrabasic rocks generally contain magnesian-rich olivines. Ironrich olivines are found only in some alkaline rocks and iron-rich sediments
that have been metamorphosed.
•
Olivines are the first mafic mineral to crystallise from a basic magma.
•
Forsterite has a melting temperature of 1890 °C.
Fayalite has a melting temperature of 1205 °C.
•
The cation-oxygen bonds are weaker for the larger ferrous cation (ionic
radii of 0.74 °A for Fe++ compared to 0.66 °A for Mg++). As more of the
larger ferrous cations enter the olivine structure, there is a progressive
reduction in the melting temperature of intermediate compositions.
•
Formation of olivines from a pure olivine-composition liquid is shown in the
Figure. As temperature drops and the liquidus curve is reached, olivine of
composition intersecting the solidus curve is precipitated. The liquid
composition then moves along the liquidus curve as the composition of
new olivines moves along the solidus curve. This continues until all
olivines with the average composition of the starting liquid are crystallised.
•
If the original magma has more silica than can be used by the olivines
(>40%)5, then the more siliceous mafic minerals such as pyroxenes will be
formed after all olivines have been precipitated (in actual reality some
simultaneous or co-precipitation of olivines and pyroxenes may occur).
5
Pure magnesian olivine (forsterite) has maximum silica content of 42.7%. Iron-olivines,
however, require less silica to form. Most olivines have some iron in them. Thus, the silica
threshold above which pyroxenes begin to form is in fact lower than 42.7% and depends upon the
availability of iron in the magma.
25
Nickel in the olivines
•
Olivines can take up to 0.41% of NiO (0.322% Ni). Most of the nickel is
contained as a replacement of magnesium atoms by nickel atoms that are
of the same size. Generally, the Ni:Mg ratio in the olivines is the same as
in the basic magmas.
•
Entry of nickel into the olivine structure in excess of the magmatic Ni:Mg
ratio is due to nickel replacing some of the iron in the olivines (iron in
olivines lowers the thermal stability).
•
Nickel in the nickeliferous magnetite is also originally derived from the
nickeliferous olivines.
Alteration and weathering of olivines
•
Minerals of the olivine group are highly susceptible to alteration by
hydrothermal fluids and weathering processes.
•
Alteration reactions involve: hydration, silicification, oxidation and
carbonation.
•
Common alteration products are serpentine, chlorite, amphibole,
carbonates, iron oxides, and talc.
Alteration of
Forsterite olivine
H2O
+800°C:
Fo to En
625° - 800°C: Fo to En to Talc
500° - 625°C: Fo to Talc
200° - 500°C: Fo to Serpentine
Serpentine
Talc
MgO
Fo
26
En
SiO2
SERPENTINISATION OF OLIVINES
•
One of the most common products of hydrothermal alteration of olivines is
serpentine. The serpentine takes on two forms:
o Under static conditions, fibrous chrysotile is formed
o Under conditions of stress, flaky antigorite is formed
o Under certain conditions, structureless serpophite is formed
•
Alteration of olivines generally begins along randomly placed fractures in
the crystal. Eventually, the entire crystal may be altered and replaced ⎯
as a pseudomorph ⎯ by the alteration product.
•
Serpentinisation of olivines results in the:
o
o
o
o
Addition of water
Leaching of magnesia (or addition of silica)
Release of iron in the (Mg,Fe) olivine (serpentine has no iron in it)
Conversion of released iron from the ferrous to ferric state to form
magnetite. Thus, serpentinised rocks are generally more magnetic.
Conditions for serpentinisation
In the presence of water and free silica, olivine would alter to serpentine at
temperatures ranging from 200 to 500ºC. Above 500ºC, olivine cannot be
converted to serpentine and would change to other minerals as follows:
@ 200° to 500°C, olivine changes to serpentine
@ 500º to 625ºC, olivine changes to talc
@ 625º to 800ºC, olivine changes to enstatite and then to talc
@ 800º and above, olivine changes to enstatite (pyroxene)
Below is a comparison of olivine and serpentine compositions and densities:
Composition:
Composition as oxides:
MgO
SiO2
H2O
Density
Olivine
Mg2SiO4
2MgO.SiO2
Serpentine
H4Mg3Si2O9
3MgO.2SiO2.2H2O
57.3%
42.7%
43.0%
44.1%
12.9%
3.2
2.2 (relative change of 31%)
27
Serpentinisation accompanied by volume and weight increase
(No removal of MgO)
Since serpentine carries water and more silica than olivine, these two
components have to be added to serpentinise the olivines. As the entire water
and silica are used up and as serpentine has a lower density, considerable
volume increase takes place in such a process of serpentinisation.
3Mg2SiO4 + 4H2O + SiO2 = 2H4Mg3Si2O9
forsterite
water silica serpentine
420 gm
131 cc
552 gm
220 cc
(+31% total weight increase)
(+68% total volume increase)
1.26 gm Ni
0.3% Ni
1.26 gm Ni
0.228% Ni (relative decrease of 24%)
The problem with the above model is that most serpentinites do not show any
signs of significant volume increases. Similarly, pseudomorphs of serpentine
after olivine cannot be explained by the above mechanism.
Serpentinisation at constant volume
(Removal of MgO and SiO2)
To maintain a constant volume during the process of serpentinisation,
considerable quantities of magnesia and some silica need to be removed from
the system. The total removal amounts to nearly 31% of the weight of the original
olivines. This results in practically no volume change from olivine to serpentine.
5Mg2SiO4 + 4H2O = 2H4Mg3Si2O9 + 4MgO + SiO2
forsterite
water serpentine
700 gm
72 gm 552 gm
160 gm + 60 gm
Removed in solution
220 cc
220 cc
2.1 gm Ni
0.3% Ni
2.1 gm Ni (assuming all nickel stays in the serpentine)
0.38% Ni (relative increase of 27%)
If it is assumed that all original nickel stays in the newly formed serpentine, then
the relative proportion of nickel in the ultramafic is increased by about 27%.
28
However, it is very unlikely that all the nickel will stay behind since considerable
quantities of magnesia are being removed hydrothermally.
The model of serpentinisation involving equal volumes of original and altered
rock faces a different problem. To remove 31% of the original material requires
extremely large quantities of water since the solubilities of both magnesia and
silica are still limited in hydrothermal solution. In nature, no evidence exists of
either the availability of such large quantities of water or the nearby deposition of
such large quantities of magnesia and silica that are removed from the system.
Dissociation of Serpentine at high temperatures
At 500º to 625ºC, serpentine changes to talc.
Water of hydration and magnesia are released:
3MgO.2SiO2.2H2O
[serpentine]
3MgO.4SiO2.H2O + 3H2O + 3MgO
[talc]
At 625º to 800ºC, serpentine changes first to enstatite and then to talc.
In both reactions, water of hydration is given out and magnesia is formed.
2(3MgO.2SiO2.2H2O)
4(MgO.SiO2) + 4H2O + 2MgO
3MgO.4SiO2.H2O + 3H2O + 3MgO
At over 800ºC, serpentine changes to enstatite.
Water of hydration and magnesia are released:
3MgO.2SiO2.2H2O
[serpentine]
2(MgO.SiO2) + 2H2O + MgO
[enstatite]
29
[enstatite]
[talc]
ULTRAMAFIC ROCKS
Terminology
•
Ultramafic rocks are rich in mafic (ferro-magnesian) minerals such as
olivines, pyroxenes and amphiboles.
•
Almost all ultramafic rocks contain less than 45% silica (however, pure
orthopyroxenite is classified as an ultramafic rock but contains nearly 60%
silica).
•
All ultramafic rocks have colour indices of more than 70.
•
Ultramafic rocks generally lack any feldspars.
•
Ultramafic rocks have no exact counterparts among lavas6.
•
The density of ultramafic magma would be too high for it to rise through
the sialic portion of the earth.
Classification of ultramafic rocks
Dunite
Monomineralic ultramafic rock composed entirely of olivine (generally
magnesian). Accessory minerals may include: chromite, magnetite, ilmenite and
spinel.
Pyroxenite
Monomineralic ultramafic rock composed entirely of pyroxenes. The pyroxenites
are further classified into whether the pyroxene is orthorhombic or monoclinic:
•
•
Orthopyroxenites: Bronzitites
Clinopyroxenites: Diopsidites; diallagites
Hornblendite
Monomineralic ultramafic rock composed entirely of hornblende.
6
Such as rhyolites are volcanic counterparts of granites; dacites are volcanic counterparts of
granodiorites; andesites are volcanic counterparts of diorites; trachytes are volcanic counterparts
of syenites and basalts are volcanic counterparts of gabbros.
30
Serpentinite
Monomineralic rock composed entirely of serpentine. However, the rock may
have formed by the serpentinisation of dunite, pyroxenite, hornblendite or
peridotite.
Peridotite
Ultramafic rock that contains largely olivine but also other mafic minerals in
significant amounts. Depending upon the other mafic minerals, peridotite may be
classified as:
Pyroxene peridotite
Hornblende peridotite
Mica peridotite (such as kimberlite)
Pyroxene peridotites are one of the most common ultramafic rocks. Depending
upon the type of pyroxene, pyroxene peridotites are further classified into:
•
•
•
Harzburgite: olivine + orthopyroxene (enstatite or Bronzite)
Wehrlite: olivine + clinopyroxene (diopside or diallage)
Lherzolite: olivine + orthopyroxene + clinopyroxene
Field occurrence of ultramafic bodies
The field occurrences of the ultramafic rocks can be simplified to essentially three
types:
1. Ultramafic rocks associated with layered intrusions. There is clear
evidence in these locations for ultramafics to have derived through
gravity settling of heavy mafic minerals during crystallisation of a
basic magma (Skaergaard intrusion, the Great Dike in Africa)
2. Small bodies composed entirely of ultramafic rocks (lenses, sheets,
dikes, stocks, etc. Occasionally, a feeder to the magmatic chamber
is clearly present indicating that the ultramafics may have been
intruded as solid crystalline masses.
3. Very large ultramafic occurrences that are clearly associated with
ophiolites, subduction melange, outer island arcs and orogenic
belts (Ural area, Himalayas, New Zealand, New Caledonia,
Sulawesi, etc.).
Refer to Figures that show the distribution of major ultramafic belts of the world.
31
OLIVINE
ULTRAMAFIC
ROCKS
Dunite
90% OL.
Peridotites
Harzburgite
Lherzolite
Wehrlite
40% OL.
Pyroxenites
Orthopyroxenite
Websterite
Clinopyroxenite
Hornblendites
PYROXENE
HORNBLENDE
OLIVINE
ULTRAMAFICS
Dunite
IN TERMS OF
OL, OPX, CPX
90% OL
Wehrlite
Harzburgite
Lherzolite
OL+OPX+CP
40% Olivine
Olivine
Orthopyroxenite
Ortho10%
Pyroxenite
Olivine
Clinopyroxenite
Olivine Websterites
OL+OPX+CP
10%
ClinoPyroxenite
Websterites
ORTHOPYROXENE
CLINOPYROXENE
32
Nickel in the ultramafic rocks
•
Nickel in the ultramafic rocks is held primarily in the mafic minerals. The
proportion of nickel generally decreases as follows: olivine > opx > cpx.
•
Primary chromite and magnetite may also contain minor amounts of
nickel.
•
Within mafic minerals, nickel is held essentially by olivines that crystallise
first. Pyroxenes, that crystallise later, contain far less quantities of nickel.
•
Olivines can take up to 0.4%Ni (0.5% NiO). Much of the nickel is held as a
replacement of Mg atoms in the olivine structure.
•
Some nickel, however, may also be held as a replacement of the larger
Fe++ atoms in the olivines, particularly when the Ni:Mg ratio in the olivine is
higher than that in the original magma (iron in the olivines lowers the
thermal stability and allows nickel to come in with greater ease).
•
Nickel contents of olivines:
%Ni
Olivine in harzburgite, Poro, New Caledonia
0.39
Olivine in unserp. peridotite, Bonsora-W, Soroako
0.358
Olivine in unserp. peridotite, Konde pit 717, Soroako 0.313
Olivine in unserpentinised peridotite, Soroako
0.29
Olivine in harzburgite, Tiebaghi, New Caledonia
0.24
Olivine in “early rocks” of the Skaergaard Intrusion
0.20
•
Nickel contents of pyroxenes:
%Ni
0.10
0.067
0.0635
0.047
0.039
%NiO
0.127
0.085
0.081
0.06
0.05
%Ni
Chromite, unserp. peridotite, Konde pit 717, Soroako 0.0657
Chromite, unserp. peridotite, Bonsora-W, Soroako
0.0620
%NiO
0.084
0.079
Opx in harzburgite, Poro, New Caledonia
Opx in unserp. peridotite, Konde pit 717,Soroako
Opx in unserp. peridotite, Bonsora-W, Soroako
Opx in harzburgite, Tiebaghi, New Cal.
Cpx in unserpentinised peridotite, Soroako
•
%NiO
0.50
0.456
0.398
0.37
0.30
0.25
Nickel contents of chromites:
33
•
Nickel contents of ultramafics:
%Ni
Average dunite compositions (Edel’shtein, 1960)
0.26
Unserp. peridotite, Bonsora-W, Soroako
0.22
Average peridotite compositions (Edel’shtein, 1960) 0.16
Average pyroxenite compositions (Edel’shtein, 1960) 0.08
•
%NiO
0.33
0.28
0.20
0.102
Santos-Ynigo and Esguerra (1961) found highest nickel grades in laterite
associated with dunite, peridotite and serpentinite. The poorest grades
were associated with pyroxenite and conglomerates.
34
MAGMATIC DIFFERENTIATION
Magmatic differentiation is the process by which a homogeneous magma
crystallizes into unlike fractions and yields rocks of different compositions.
Magmatic differentiation is achieved through fractional crystallisation ⎯ a
process in which different crystals are formed at different temperatures during the
cooling of the magma. There is a tendency for the newly formed crystals to
remain in equilibrium with the liquid. Different minerals achieve this equilibrium in
two different ways:
1. Continuous-reaction series:
The first plagioclases to crystallise from a basic magma are rich in lime
(anorthitic composition). As the temperature drops, and as the
remaining liquid gets enriched in sodic composition, first-formed
crystals become progressively more sodic. In this way, a continuous
series of homogeneous solid solutions is produced. Crystallisation of
this type is referred to as a continuous-reaction series.
2. Discontinuous-reaction series
Certain ferro-magnesian minerals are incapable of solid solutions. As
the composition of the liquid changes significantly, the first-formed
crystals change into entirely new crystals with a completely different
crystal structure. During the cooling of a basic magma, the first crystals
to form belong to the olivine group. If the magma has more than 40%
silica, the olivines will react with the remaining liquid to form pyroxenes
that have a completely different crystal structure. In time, the
pyroxenes may react with the liquid to form amphiboles and them
biotite. Such crystallisation process that yields crystals of completely
different structure is called a discontinuous-reaction series.
Bowen was the first petrologist to propose the order of minerals in the
Continuous and Discontinuous reactions series that are shown below. During the
cooling of a basic magma, both series begin to crystallise at about the same
time. Thus gabbros contain olivine, magnesian pyroxene and calcic plagioclase.
Similarly, low-temperature minerals go together such as mica, alkalic feldspars
and quartz (granites and granodiorites).
Compatible minerals: (olivine + anorthite); (olivine + pyroxene); (quartz +
orthoclase)
Incompatible minerals: (forsterite + quartz); (labradorite + quartz); (orthoclase +
bytownite)
35
BOWEN’S REACTION SERIES
Discontinuous series
Continuous series
(Mafic Minerals)
(Plagioclases)
Olivine
42% SiO2
Calcic plagioclase
Mg pyroxene
58% SiO2
increasing
Fe/Mg
Calci-sodic plagioclase
Sodic-calcic plagioclase
Mg-Ca pyroxene
Amphibole
Sodic plagioclase
Biotites
K-Feldspar
Muscovite
Quartz
Zeolites
Hydrothermal
Solutions
36
PHASE DIAGRAMS
Phase diagrams are used to describe the equilibrium conditions of different
components and phases in a system.
Components: Chemically different components of a system
Phases: Chemically similar components with different physical characteristics
Ice – Water – Vapour system
P
R
E
S
S
U
R
E
WATER
ICE
4.8mm
P
VAPOUR
0.008°C
TEMPERATURE
This system has only one component (H2O) with three different phases. All three
phases can co-exist at 0.008°C and a pressure of 4.8mm. At higher
temperatures, water can be turned directly into vapour just by changing the
pressure. At lower pressures, water can be turned into ice just by changing the
temperature.
37
Two–component system with solid solutions
Solid solutions are true, homogeneous solid solutions of one substance into
another. They involve two isomorphous members with the same basic
crystallographic structure and forming a series in which physical and chemical
properties change continuously from one member to the other.
T
E
M
P
E
R
A
T
U
R
E
1
Liquidus
D
T1
C
F
T2
Liquid
E
H
T3
G
Solidus
K
Solid solutions of A and B
A
COMPOSITION
B
Consider a liquid made up of two components, A and B. The composition of this
liquid is shown in the Figure by point 1. If the liquid of composition 1 cools, no
change occurs until the Liquidus curve is reached at temperature T1. At this
temperature, crystals appear with a composition D (where the T1 temperature
intersects the Solidus curve). These crystals in fact represent a solid solution of A
and B, and are richer in component A than the starting liquid.
With further cooling, the composition of liquid changes along the Liquidus curve
towards E and the composition of the crystals changes along the Solidus curve
towards F. During slow cooling, the crystals change their composition by
continuous reaction with the liquid. At temperature T2, a solid solution of
composition F is in complete equilibrium with liquid of composition E.
At temperature T3, the last drop of the liquid is consumed and this liquid is in
equilibrium with crystals of composition H. Crystals of H have the same
composition as the starting liquid.
If the early formed crystals of composition D or F are removed from the system
(say by crystal settling), then the composition of the liquid will move beyond G
and towards K. This will give rise to later crystals of composition in which
proportion of B is higher than the starting liquid.
An example of the above system is Forsterite-Fayalite.
38
Two-component system without solid solution
The components A and B in the system shown below do not form solid solutions.
T1 and T3 are the melting temperatures of A and B respectively.
1
2
T3
T
E
M
P
E
R
A
T
U
R
E
Liquid
T1
T2
T4
Crystals of B
and Liquid in
equilibrium
Liquidus
Crystals of A
and Liquid in
equilibrium
F
Te
E
C
D
G
Te
Crystals of A and B in equilibrium
A
COMPOSITION
B
Consider the liquid of composition 1. As the liquid cools and reaches temperature
T2, crystals of A appear. With the crystallisation of A, the composition of the
liquid moves along the liquidus towards E. Until the composition of liquid reaches
the point E, crystals of A keep on forming. At temperature Te, crystals of A and B
form simultaneously until all the liquid is used up. Point E at which A and B
crystallise simultaneously is called the eutectic point and Te is the eutectic
temperature. Crystals of A and B are in the proportion of FC to CG.
Consider a cooling liquid of composition 2. At temperature T4, crystals of B
appear and the composition of the liquid moves along the liquidus towards E. At
point E, crystals of B and A appear simultaneously. The final proportion of A and
B is in the ratio of FD to DG.
System Diopside-Anorthite is an example of the above type of system with a
eutectic at 1270°C and a composition of Di58An42.
Another system of above type is Forsterite-Diopside with a eutectic at 1400°C
and a composition of Fo12Di88.
39
Phase chemistry in PT Inco furnaces
Smelting at PT Inco is carried out in furnaces with a diameter of 18m and a
power requirement of 45 MVA.
Initial expectation was to process the higher-grade West Block ores that also
have high silica to magnesia ratios of 2.3 to 2.5 in the slag. Initial smelting
showed that:
•
The liquidus temperature of the slag was relatively low (1430 – 1475°C)
•
S/M of 2.2 to 2.5 was too acidic for the magnesia refractory in the furnace
•
A higher superheat was required to continually dissolve the coarse
unaltered peridotite in the feed. This superheat made it difficult to maintain
a protective slag layer on the refractory walls
To correct the problem, PT Inco started blending West Block ores with lower S/M
East Block ores. A S/M ratio of 1.9 was targeted in the slag (during the 1980s).
The blending had the following benefits:
•
The amount of coarse unaltered peridotite was reduced
•
The blending also reduced the S/M ratio, making the slag less acidic
•
The smelting temperature (liquidus) climbed by 50°C, which made it
unnecessary to use superheat in the furnace
•
The slag skimming temperature stabilised at 1550°C.
•
Matte tapping temperature also climbed to 1360°C that saved fuel
consumption in the converters.
40
WEATHERING
Processes of change
There are four major processes under which rocks change their physical or
chemical properties:
1. Melting (takes place at very high temperatures)
2. Metamorphism (high temperature / pressure / addition of chemicals)
3. Hydrothermal alteration (through fluids at high temperature)
4. Weathering (at ordinary temperature and pressure
The term “weathering” applies to those superficial changes in rocks that are
brought about by atmospheric agencies and result in a complete destruction of
the original structure or composition. Deep-seated processes, such as melting,
metamorphism and hydrothermal alteration are excluded.
Weathering can take place in two ways:
•
Physical weathering: mechanical breakdown of rocks (erosion,
thermal expansion and contraction, expansion by freezing water,
disintegration by plants and animals)
•
Chemical weathering: breakdown of rocks through chemical
processes (contact with water, oxygen, and carbon dioxide)
Chemical weathering
“The process in which rocks react to atmospheric, hydrospheric and biologic
agencies to produce mineral phases that are more stable.”
Chemical weathering works in four ways:
1. Hydrolysis: Oxygen, carbon dioxide, ground water, and dissolved acids
attack the minerals in the rock and break down their crystal structure.
2. Oxidation: Elements released by chemical weathering are oxidised.
3. Hydration: Reaction with water adds the hydroxyl ion to many newly
formed minerals.
4. Solution: The more soluble products of the break down of minerals are
dissolved and carried away in ground water.
41
Hydrolysis
Hydrolysis is the chemical process by which minerals breakdown into more
stable components under the influence of chemical weathering. Jenny (1950) has
proposed the following explanation for the breakdown of minerals:
•
According to Pauling’s rule, the sum of negative and positive charges
must be equal within a crystal
•
However, exposed atoms and ions on the surfaces of crystals possess
unsaturated valencies and are thus charge
•
Contact with water causes hydration of the surface through the attraction
of water molecules to the charged surfaces
•
Attractive forces are strong enough to cause polarisation of water and its
dissociation into hydrogen and hydroxyl ions
•
Hydroxyl ions then bond to exposed cations
•
Hydrogen ions then bond to exposed oxygens and other negative ions
•
Hydrogen ions may also bond to exposed cations, thereby releasing them
Oxidation
•
Ferrous ion exists under reducing conditions. At pH of 8 – 8.5, ferrous ion
is slightly soluble.
•
Ferric ion exists under oxidising conditions. It is not soluble until pH is
reduced to 2.5.
•
Much of the ferrous ions in the weathering profile are converted to ferric
state under highly oxidising conditions. This ferric ion state of iron is quite
insoluble under normal pH conditions found in ground water.
•
Oxidising conditions exist only above the water table. Below the water
table, conditions are generally reducing. However, organic matter is a
powerful reducing agent and may create reducing environment above the
water table, near the upper soil horizon.
•
Hot, well-drained environment favours oxidation through the rapid
destruction of organic matter and lowering of water table.
42
•
Cool, poorly-drained environment promotes accumulation of organic
matter and reducing conditions.
Hydration
In the presence of hydroxyl ion (OH), many freshly created oxides from the
breakdown of minerals are converted to hydroxides. The more common
hydroxides that are found in the lateritic soils include:
Hydrated oxides of iron:
Hydrated oxides of aluminium:
Hydrated oxide of magnesium:
Goethite
Limonite
Boehmite
Gibbsite
Brucite
Fe2O3.H2O
2Fe2O3.3H2O
Al2O3.H2O
Al2O3.3H2O
MgO.H2O
Many new mafic minerals are formed due to hydration:
Serpentine (Mg)
Serpentine (Fe) Greenalite
Tremolite
Talc (Steatite or soapstone)
Kerolite (more hydrous form of talc)
Chlorite (Penninite / Clinochlore / Prochlorite)
Sepiolite
Mg3Si2O5(OH)4
Fe3Si2O5(OH)4
Mg5Ca2Si8O22(OH)2
Mg3Si4O10(OH)2
Mg3Si4O10(OH)2.nH2O
Mg5Al2Si3O10(OH)8
Mg4Si6O15(OH)2.4H2O
Hydration also results in the formation of clay minerals that all contain significant
quantities of the hydroxyl ion:
Kayolinite / Nacrite / Dickite
Hallyosite (endellite)
Illite
Smectite
Montmorillonite / Saponite / Pyrophyllite
Nontronite
Saponite
Al2Si2O5(OH)4
Al2Si2O5(OH)4.2H2O
KAl3Si3O10(OH)2
Al2Si4O10(OH)2
Fe2Si4O10(OH)2
Mg3Si4O10(OH)2
Solution
For chemical weathering to continue, it is important that all constituents that are
broken down from the primary minerals are removed from the environment
through the process of solution. Such a process exposes new mineral surfaces to
chemical attack.
43
•
Dissolved constituents are removed through percolating ground waters
•
Ground waters generally travel from top to bottom in a weathering profile
•
The ground waters, with dissolved constituents, eventually drain out to
rivers, lakes, and the ocean
•
The relative proportions of dissolved constituents in ground waters confirm
the relative solubilities of various oxides determined in the laboratory
Factors that influence chemical weathering
Several factors influence the speed as well as the direction in which chemical
weathering takes place. These factors include:
1. Stability of minerals (crystal structure, melting points)
2. Acidity/basicity (pH) conditions
3. Redox (reduction/oxidation) potential of the environment
4. Rate of removal of dissolved constituents
5. Climate (temperature, rainfall, fluctuation of water table)
6. Topography
1. Stability of minerals
•
Godlich (1938) determined the following sequence of decreasing
weathering susceptibilities for the common rock-forming minerals:
Olivine
Ca-plagioclase
Augite
Hornblende
Na-plagioclase
Biotite
K-feldspar
Muscovite
Quartz
44
The above order coincides exactly with that of Bowen (1928) for the
crystallisation sequence in a silicate melt.
Some minerals with otherwise highly mobile alkali elements can exhibit
remarkable resistance to chemical weathering (muscovite, K- and Na- feldspars).
In general, the crystal structure of the mafic silicates dictates the ease with which
they break down under chemical weathering:
o Olivine, with its independent silicon tetrahedra, is the most unstable
mineral and thus most susceptible to chemical weathering
o Pyroxenes, with their polymerised chains, are relatively more stable
and consequently less susceptible to chemical weathering
compared to olivines
o Amphiboles, with their ring structures, are still more stable and
more resistant to chemical weathering
o Clays and micas with their sheet-like structure are the most stable
minerals and the least susceptible to chemical weathering
Reiche (1943) devised a “weathering potential index” based on the formula:
100 x moles (Na2O + K2O + CaO + MgO – H2O)
WPI =
100 x moles (Na2O + K2O + CaO + MgO + SiO2 + Al2O3 + Fe2O3)
Calculated Weathering Potential Indices
WPI calculated for common minerals using the above formula are given below:
Mineral
Forsterite
Enstatite
Anthophyllite
Augite
Hornblende
Talc
Biotite
Orthoclase
Quartz
Muscovite
Kayolinite
Gibbsite
Formula
Weathering Potential Index
Mg2SiO4
66
MgSiO3
55
Mg7(Si4O11)2(OH)2
40
39
36
Mg3Si4O10(OH)2
29
22
12
0
- 10.7
- 67
- 300
45
2. Acidity/basicity (pH) conditions
•
pH of the ground water has a strong influence on the solution of
different materials
•
pH values of natural waters normally lie between 4 and 9
•
Most oxides show some solubility in natural waters
•
Oxides of calcium, magnesium, sodium and potassium are
completely soluble in natural waters
•
Oxides of titanium, aluminium and ferric iron are completely insoluble
in natural waters
•
Solubilities of many oxides are pH dependent and change at higher
or lower pH levels (oxides of titanium, calcium and ferrous iron)
•
Alumina is not soluble in the normal range of groundwater pH.
However, at pH values below 4 and above 10, alumina is soluble.
•
Where abundant organic matter is available, pH values may drop
below 4. Plant roots carry very low pH values, commonly 4 but down
to 2.
•
Where abundant basic minerals are being weathered (olivine,
pyroxene, nepheline), pH conditions may climb to beyond 9.
3. Role of Redox (Eh) potential
•
Redox (or reduction/oxidation) potential of a system is a measure of the
ability of that system to bring about reduction or oxidation reactions.
•
Reduction is defined as the decrease in the positive valency of an element
(Fe+++ to Fe++) or an increase in the negative valency of an element.
•
Oxidation is defined as the increase in the positive valency of an element
(Fe++ to Fe+++) or a decrease in the negative valency of an element.
•
The neutral value for redox potential is zero. At lower values (-), the redox
potential represents reducing conditions. At higher values (+), the redox
potential represents oxidising conditions.
46
•
Certain oxides are capable of existing in multiple valency states: Ti, Fe, Mn.
Laboratory tests have demonstrated that the solubility of such oxides is
highly variable under different valency states.
•
Thus, under reducing conditions, iron can be dissolved from the weathered
profile in the ferrous state. However, under oxidising conditions, iron is
stabilised in the weathering profile in the ferric state.
•
There are two factors that control the redox potential in most weathering
environments:
•
•
•
Accessibility of atmospheric oxygen (creates oxidising conditions)
Availability of organic matter (creates reducing conditions)
In most systems, Redox potentials are dependent on the pH of the
system. This is illustrated in the figure given on the following page for the
change from ferrous to ferric state.
4. Rate of removal of dissolved material
For chemical weathering to proceed at a fast pace, it is necessary to remove the
dissolved constituents from the crystal faces and to expose new surfaces to
chemical attack.
The rate of removal of dissolved constituents in turn depends on several
conditions that include:
•
•
•
The relative solubility of oxides
The amount of water moving through the system
The presence of crystal fractures, cleavages, and porosity of rock
Opposed to the concept of removal of dissolved constituents is the concept of
fixation of some elements. While average igneous rocks contain nearly equal
quantities of sodium and potassium (3.13% K2O vs. 3.89% Na2O), the
concentration of potassium in seawater is only one-tenth of sodium. This is
because much of the potassium leached from igneous rocks gets tied up in clay
minerals such as illites and does not really leave the systems.
5. Role of Climate
Climate has a great influence on the rate of chemical weathering. Climatic factors
include the following:
47
•
Rainfall: This controls the supply of moisture for chemical reactions and the
supply of water for the removal of dissolved constituents. Gentle and
persistent rainfall is more effective compared to sudden heavy rains.
•
Temperature: It influences the rate at which break down of minerals takes
place. According to Van’t Hoff’s rule, each 10°C change in the temperature
increases the speed of chemical reaction from 2 to 3. It is estimated that
chemical weathering and leaching in temperate climates amounts to only
1/20 to 1/40 of the one in tropical areas.
•
Vegetation: Extensive accumulation of organic material in tropical
environment leads to low pH values in the percolating waters.
•
Equatorial Humid Climate: It results in good and constant flushing of
magnesia and silica from the laterite system and does not allow the
formation of smectite/nontronite clays.
•
Tropical Wet-Dry Climate: It leads to poor flushing of the silica and
magnesia from the system. The presence of silica and magnesia in the
system leads to the development of smectite/nontronite clays.
6. Role of Topography
Topography exerts a powerful influence on the rate of weathering, through the
following factors:
•
Run-off of rain water vs. absorption of rain water into the profile
[on steep slopes, much of the rain water runs off and little penetrates the
rock. This promotes physical weathering instead of chemical weathering]
•
Rate of sub-surface drainage and removal of dissolved material
[higher areas afford better drainage than low-lying and flat areas]
•
Rate of erosion of the weathered product
[High erosion rates keep exposing new surfaces to chemical weathering.
Slopes of < 20° are necessary to retain the laterite ahead of erosion].
Ideal conditions for chemical weathering are attained on rolling to gently sloping
lands that are elevated and where surface run-off is not excessive and the subsurface drainage is good.
48
WEATHERING OF ULTRAMAFIC ROCKS
(Dunite, peridotite and serpentinite)
•
Dominant minerals include: Olivine, Pyroxene (orthopyroxene in
Harzburgites), Serpentine
•
Olivine (being a high-temperature mineral) is highly unstable under
atmospheric conditions
•
Much of the olivine in the ultramafics is magnesia type which is more
unstable than the iron-olivine
•
Crystal fractures in the olivine assist the access of moisture and water
into the lattice structure
•
Silica tetrahedra are bonded together by magnesian ions. Magnesium is
highly mobile and its removal causes release of individual tetrahedra units.
•
Since the solubility of silica is lower than that of magnesia, the released
silica cannot be removed from the weathering profile fast enough.
Consequently, some of the silica remains behind (temporarily) to form
serpentine, clays or even silica boxwork.
•
The ferrous iron (if released above the water table) is oxidised into ferric
iron. The ferric iron can form: Hematite (or maghemite), Goethite (iron
hydroxide), or Limonite (iron hydroxide)
•
With increased weathering, serpentine begins to decompose. Magnesia is
preferentially leached out of serpentine, leaving behind a silica-enriched
phase or montmorillonite (saponite) and chlorite.
•
Ni++ and Fe+++ replace the magnesium being leached out of serpentines.
This results in the formation of iron-rich nickeliferous serpentine.
•
Since nickel is less soluble and mobile than magnesium, the formation of
nickeliferous serpentine (and nickeliferous talc, chlorite and smectite),
results in more stable minerals that are resistant to further leaching.
•
Eventually, montmorillonite (saponite) and chlorite begin to decompose,
releasing remaining magnesia and silica and leaving behind oxides and
hydroxides of iron.
49
•
Some of the alumina present in the mafic minerals is temporarily fixed in
the chlorites. But upon the loss of magnesia and silica from this mineral,
alumina is finally fixed in gibbsite.
•
Magnetite and ilmenite are common accessory minerals in ultramafic
rocks. These minerals oxidise readily during weathering to yield hematite,
maghemite, and goethite.
•
In the limonite zone, nickel occurs largely in goethite, presumably in solid
solution. The nickel grade of goethite is about 0.5 to 1.5% Ni.
•
Towards the base of the limonite zone, some 10 – 50% of total nickel
present is in the asbolite mineral (manganese wad) which ranges in grade
from 2 to 14% Ni. Asbolite also carried significant cobalt grades and any
zinc or copper present in the profile.
•
Nickel has a much higher solubility in acid waters as compared to alkaline
waters. Thus, supergene enrichment of nickel requires its solution in the
upper acidic portion of the laterite profile, its movement downward towards
a low water table, and its ultimate precipitation in the lower saprolite profile
where pH conditions are more alkaline and where more soluble magnesia
and silica are abundant.
In flat topography with a high water table, nickel is not likely to show
supergene enrichment in the saprolite zone.
•
Some of the nickel precipitates as garnierite (hydrated silicate of nickel
and magnesium). However, much of the nickel commonly found in the
saprolite zone is in the form of a nickel-serpentine of secondary origin.
This nickel serpentine can carry appreciable quantities of nickel.
•
Laterites that are formed in humid environments with considerable annual
rainfall, undergo thorough leaching of the profile. In such laterites, the
silica and magnesia is flushed out of the system too quickly to form
smectite/nontronite clays.
•
Laterites that are formed in tropical wet-dry climate, are poorly flushed and
generally give rise to the formation of a distinct smectite/nontronite
intermediate zone.
•
Alumina and chrome in the laterite profile are largely derived from the
pyroxene and spinel in the parent rock. Along with iron, alumina and
chrome represent truly residual components and their relative proportions
in the laterite profile reflect their original ratios in the ultramafic.
50
CLASSIFICATION OF SOILS
The most commonly accepted classification of soils is the Great Soil Group.
Under this classification, soil groups have a wide distribution and a number of
common fundamental characteristics.
The Great Soil Group classification is based on temperature and humidity (or
rainfall). Two major divisions of the soils are based on humidity (or rainfall). As a
general rule, annual rainfall of 25” (635 mm) separates the two divisions:
Pedocals ⎯ soils with calcium as carbonate) and aluminium, in arid
regions)
Pedalfers ⎯ soils with aluminium and iron, in humid regions)
T
E
M
P
E
R
A
T
U
R
E
HUMIDITY / RAINFALL
Diagrammatic representation of the distribution of the Great Soil Groups
according to climate (after Millar, Turk, and Forth, 1958)
51
LATERITE SOILS
•
The term laterite is derived from the Latin word “later” which means brick.
•
Buchanan Hamilton first introduced the term in 1807 for the earthy iron
crusts that were being cut into bricks for building purpose by the people of
south-central India.
•
Currently, the term laterite is used for soils that are rich in sesquioxides of
iron and aluminium, formed under the influence of chemical weathering
with special ground-water conditions.
•
Development of laterites require:
Availability of rocks that contain iron and aluminium
Relatively high temperature (to aid in chemical attack)
High rainfall (to aid chemical weathering)
Intense leaching (to remove mobile elements)
Strongly oxidising environment (to convert Fe, Al to sesquioxides)
Gentle topography (to preserve the laterite soil after development)
Laterite Profile
Limonite zone: (also called “laterite” zone or oxide zone)
•
•
•
•
•
•
The upper zone is rich is goethite.
The limonite may be remobilised in near surface acid conditions
and recrystallised to ferricrete (iron cap, canga, cuirasse de fer).
Extremely insoluble minerals may persists in this zone (spinel,
magnetite, maghemite, and primary talc).
The base of the limonite zone is enriched in manganese, cobalt and
nickel in the form of asbolite or manganese wad. This manganese
wad usually occurs as extremely thin surface coatings on joint and
fracture planes.
The limonite zone represents laterite that has collapsed under its
own weight. Thus, dry bulk density in the limonite zone is generally
higher than in the transition zone.
Due to collapse, the original structure and texture of the rock is
completely obliterated.
52
Smectite or Nontronite zone: (also called intermediate or transition zone)
•
This is an intermediate zone between lower limonite and upper
saprolite
•
This zone contains soft smectite clays (usually Nontronite) and hard
crystalline quartz.
•
Relict texture and structure of the original rock are well preserved in
this zone that has not completely collapsed as yet.
•
The development of a distinct intermediate zone depends on
climatic peculiarities and its occurrence is limited in the world [in
areas of heavy rains throughout the year, the silica and magnesia
are completely flushed out of the system to give smectite/nontronite
clays. In areas that have a tropical wet-dry climate,
smectite/nontronite are quite likely to form and persist].
•
When the intermediate zone is developed, the occurrence of
manganese wad is more prominent in the upper part of the
intermediate zone rather than in the lower part of the limonite zone.
Saprolite zone: (Serpentine ore)
•
This is the zone of altered bedrock where the processes of
chemical weathering are proceeding most actively.
•
Chemical attack and weathering is proceeding along joints and
fractures in the rock and cleavages and micro-fractures in the
crystals.
•
Saprolisation along joint surfaces leads to the formation of
“boulders” within the saprolite zone.
•
The boulders can have a significant saprolised crust which can
carry good nickel grades.
•
All original rock textures and structures are well preserved.
•
Most of the parent rock minerals are also preserved.
•
The zone consists of bedrock fragments, saprolised rims of
boulders, and precipitated quartz and garnierite.
•
In unserpentinised peridotite, saprolisation is limited to boulder
surfaces since the fresh rock is extremely hard for the water
solutions to penetrate.
•
In unserpentinised peridotite, unsaprolised boulders remain free of
nickel.
•
In serpentinised peridotite, saprolisation proceeds through much of
the rock mass at the same time since rock is soft enough to permit
access to water solutions.
•
In serpentinised peridotite, unsaprolised boulders may contain
significant quantities of supergene nickel.
53
Rates of laterisation
Based on the solubility of various components in an ultramafic rock, some
investigators have arrived at a rate of dissolution of 1mm per 100 years. This
translates to:
1 metre / 100,000 years
10 metres / million years
50 metres / 5 million years
According to Golightly (1979), water emerging from well-drained ultramafics
typically contains:
25 ppm Mg
10 – 20 ppm Silica
pH = 7 – 8
The above numbers correspond to a maximum rock dissolution rate of 1.4mm
per 100 years and translate to:
1.4 metre / 100,000 years
14 metres / million years
70 metres / 5 million years
On an average, one metre of ultramafic rock produces only 0.4m of laterite. Thus
the rate of laterite formation would be about 4 – 6 metres per million years.
54