Geo-Chemistry
By
Engr. Asadullah Memon
B.E. (Petroleum & Natural Gas)
Introduction to Geochemistry
The field of geochemistry involves:
1. Study of the chemical composition of the Earth and other planets,
2. Chemical processes and reactions that govern the composition of rocks, water, and soils,
3. The cycles of matter and energy that transport the Earth's chemical components in time and
space, and their interaction with the hydrosphere and the atmosphere.
Some subsets of geochemistry are:
a) Isotope geochemistry: Determination of the relative and absolute concentrations of the
elements and their isotopes in the earth and on earth's surface.
b) Examination of the distribution and movements of elements in different parts of the earth
(crust, mantle, hydrosphere etc.) and in minerals with the goal to determine the underlying
system of distribution and movement.
c) Cosmochemistry: Analysis of the distribution of elements and their isotopes in the cosmos.
d) Biogeochemistry: Field of study focusing on the effect of life on the chemistry of the earth.
e) Organic geochemistry: A study of the role of processes and compounds that are derived
from living or once-living organisms.
f) Water Geochemistry: Understanding the role of various elements in watersheds.
g) Regional, environmental and exploration geochemistry: Applications to environmental,
hydrological and mineral exploration studies.
Geochemistry = chemistry of the Earth
(i.e., of earth materials — minerals and rocks)
The main focus of geochemistry is to:
 Understand the principles governing the distribution and redistribution of elements, ionic species and isotope ratios in earth
materials, so that we can interpret the formation of mineral
assemblages: conditions (P, T, etc.), processes (magmatic
crystallization, weathering, chemical precipitation, metamorphism,
etc.), and even the age.
 Predict changes in mineral assemblages (minerals, concentrations of
elements, isotopic ratios) if a given mineral assemblage is subjected
to different conditions (T, P, interaction with a fluid, etc.)
 Geochemistry plays an important role in forecasting the quality of
crude oil in the accumulation.
THE EARTH'S CHEMISTRY
The bulk of the Earth is made from iron, oxygen, magnesium and silicon.
More than 80 chemical elements occur naturally in the Earth and its atmosphere.
Mostly Earth is composed of three parts
1. Crust
2. Mantle (Upper & Lower)
3. Core
The Earth's crust is a thin layer of rock that floats on the mantle. The crust is made
mostly from oxygen and silicon (silicate minerals such as quartz), with aluminium, iron,
calcium, magnesium, sodium, potassium, titanium and traces of 64 other elements.
The upper mantle is made up of iron and magnesium silicates; the lower is silicon and
magnesium sulphides and oxides.
The core is mostly iron, with little nickel and traces of sulphur, carbon, oxygen and
potassium.
Fig.- This diagram shows the percentages of the chemical elements that make up the Earth.
Fig.- This diagram shows the Earth interior.
EARTH'S INTERIOR
The Earth's crust is a thin hard outer shell of rock. Under the crust, there is a
deep layer of hot soft rock called the mantle.
The crust and upper mantle can be divided into three layers according to their
rigidity:
1.
2.
The lithosphere (The lithosphere is the upper, rigid layer of the Earth. It consists of the crust and
the top of the mantle and it is about 100 km thick ),
The asthenosphere (Below the lithosphere, in the Earth's mantle, is the hot, soft rock of the
asthenosphere. The boundary between the lithosphere and the asthenosphere occurs at the point where
temperatures climb above 1300°C),
3.
The mesosphere.
Beneath the mantle is a core of hot iron and nickel. The outer core is so hot
(4500°C - 6000°C) that it is always molten. The inner core is even hotter (up to
7000°C) but it stays solid because the pressure is 6000 times greater than on the
surface.
The inner core contains 1.7% of the Earth's mass, the outer core 30.8%; the
core - mantle boundary 3%; the lower mantle 49%; the upper mantle 15%; the
ocean crust 0.099% and the continental crust 0.374%.
Fig.- The main layers that form the Earth.
 Our knowledge of the Earth's interior comes mainly from studying how
earthquake waves move through different kinds of rock.
 Analysis of how earthquake waves are deflected reveals where different materials
occur in the interior. S (secondary) waves pass only through the mantle. P (primary)
waves pass through the core as well. P waves passing through the core are deflected,
leaving a shadow zone where no waves reach the far side of the earth.
 The speed of earthquake waves reveals how dense the rocky materials are. Cold,
hard rock transmits waves more quickly than hot, soft rock.
Geo Chemical Classification of Elements
 There are several trials to classify elements on geochemical basis.
 Names such as siderophile, chalcophile, lithophile, hydrophile,
thalassophile, atmophile are commonly used to denote particular
geochemical affinity of elements.
 chemical affinity is the electronic property by which dissimilar chemical
species are capable of forming chemical compounds or Affinity is the
tendency of a molecule to associate with another. .
 Chemical affinity can also refer to the tendency of an atom or compound to
combine by chemical reaction with atoms or compounds of unlike
composition.
Geochemical Affinity
In the classification scheme of Goldschmidt, elements are divided
according to how they partition between coexisting silicate liquid, sulfide
liquid, metallic liquid, and gas phase…defined by examining ore smelting
slags and meteorites
• Melting a chondrite gives 3 immiscible liquids plus
vapor:
Gas Phase
Atmophile
H, He, N, Noble gases
Silicate Liquid
Sulfide Liquid
Metallic Liquid
Lithophile
Chalcophile
Siderophile
Alkalis, Alkaline Earths,
Halogens, B, O, Al, Si, Sc,
Ti, V, Cr, Mn, Y, Zr, Nb,
Lanthanides, Hf, Ta, Th,
U
Cu, Zn, Ga, Ag, Cd, In, Hg,
Tl, As, S, Sb, Se, Pb, Bi,
Te
Fe, Co, Ni, Ru, Rh, Pd, Os,
Ir, Pt, Mo, Re, Au, C, P, Ge,
Sn
To first order, the distribution of elements between core and
mantle resembles equilibrium partitioning between metal liquid and
silicates…confirmed by iron and achondrite meteorites (but at high
What makes an element siderophile or lithophile?
Notably, the Goldschmidt categories are wellgrouped in the periodic table of the elements:
IA
1
2
Li
11
3
4
37
5
5
Be
20
13
IIIB
22
39
40
Y
VIIIB
VIB VIIB
24
V
41
VIA VIIA VIIIA
25
26
27
IB
28
29
IIB
30
Al
31
44
43
74
45
47
46
75
76
77
78
Ta W Re Os Ir
105
C
N
Si
32
O
16
15
10
9
P
F
34
51
52
18
17
S
33
Ne
Cl Ar
35
36
53
54
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br
42
73
8
7
14
48
49
50
Zr Nb Mo Tc Ru Rh Pd Ag Cd In
104
Fr Ra
106
108
107
79
80
82
81
Pt Au Hg
Sn Sb Te
Tl
83
84
Kr
I
Xe
85
86
Pb Bi Po At
Rn
109
Rf Db Sg Bh Hs Mt
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
89
Actinides
Ti
Hf
88
VB
23
72
56
Lanthanides
IVB
21
Ca Sc
38
6
B
Chalcophile
Cs Ba
87
7
VA
He
12
K
IVA
Artificial
4
Rb Sr
55
6
Lithophile
Na Mg
19
IIIA
Siderophile
2
H
1
3
Atmophile
IIA
90
91
Ac Th Pa
92
93
94
95
96
97
98
99
100
101
102
U Np Pu Am Cm Bk Cf Es Fm Md No
103
Lr
Chemical Weathering
Chemical Weathering
Weathering is a process of the disintegration and degeneration of rocks minerals or soils
as a result of direct contact with the atmosphere of the Earth OR The disintegration, or
breakdown of rock material is called Weathering.
Three types of weathering:
1) Chemical weathering: breakdown as a result of chemical reactions
(CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-)
2) Mechanical Weathering: No change in chemical composition, just disintegration into
smaller pieces
3) Biological Weathering: Can be both chemical and mechanical in nature
(For Example: Tree throw).
The rate of weathering differs with variation in the chemical composition
and structure.
Chemical Weathering
Definition: Transformation/decomposition of one mineral into another
Mineral breakdown
• carbonate dissolves
• primary minerals --> secondary
minerals (mostly clays)
 Water is the main operator:
 Dissolution
Many ionic and organic compounds dissolve in water
Silica, K, Na, Mg, Ca, Cl, CO3, SO4
water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3 Acid Reactions
Water + carbon dioxide <---> carbonic acid
Water + sulfur <--> sulfuric acid
H+ effective at breaking down minerals
Chemical Weathering
 Oxidation
Oxygen dissolved in water promotes oxidation of sulfides, ferrous
oxides, native metals
 Organic Activity
Plant material makes H+ ions available
Hydration
Attachment of water molecules to crystalline structure of a rock, causing
expansion and weakness
Hydrolysis
combination of hydrogen and oxygen in water with rock to form new
substances
Factors that Influence Chemical Weathering
The factors that influence Chemical weathering are,
 The climate of the place
(Temperature and moisture characteristics),
 The vegetation
(Most effective in areas of warm, moist climates – decaying vegetation
creates acids that enhance weathering)
 The physical nature of the rock.
In case of Chemical weathering water plays a major role as is
evident from the description of methods, therefore in the
absence of water, chemical weathering is nearly impossible.
TYPES OF WEATHERING REACTIONS