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Introduction to Geochemistry

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Lectures for Undergraduate Students

Introduction to Geochemistry

Dr. Osama Shaltami

Department of Earth Sciences
Faculty of Science, Benghazi University, Libya
Introduction
Geochemistry = chemistry of the Earth (uses the tools of chemistry to
understand processes on Earth)

The main focus of geochemistry is to understand the principles

governing the distribution and re-distribution 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.
Historical Review
The name ―geochemistry‖ was first introduced by Schonbein since more
than 160 years. Clark, who was a chief chemist of the US. Geological
Survey from 1884 to 1925, has contributed very much to the science
geochemistry. The modern science of geochemistry can be dated back to
Clark who published a very large number of chemical analyses of the
various rocks in the earth's crust. Goldschmidt (1888-1947) contributed
significantly to the roles of ionic size, coordination and atomic
substitution in crystal lattices. The contributions of the USSR
geochemists are remarkable, especially after the improvement of the
analytical techniques.
Modern geochemistry
Geochemistry has both pure and applied components:
Pure Geochemistry: concerned with answering how and why the earth and
solar system reached their current chemical state. Current hot topics include:
- Chemical cycles (evolution of earth and atmosphere)
- Paleoclimate
-Astrobiology

Applied Geochemistry: benefits humanity in many ways, including:

- Geochemical prospecting
-Environmental geochemistry
- Chemostratigraphy
Modern geochemists take several different approaches to answering
these questions:

Analytical Geochemistry: use analytical methods to chemically analyze

geological samples.

Experimental Geochemistry: simulate earth processes in the laboratory.

Theoretical Geochemistry: apply basic chemical principles to make

predictions.
Geochemical Engineering
Geochemists study the properties of minerals, soils, rocks, waters and
natural chemical processes. A geochemical engineer applies fundamental
understanding of geochemistry by designing methods that aim at the
most efficient transformation of an undesirable into a desirable chemical
environment. Although geochemical engineering is closely related to
chemical and civil engineering, it is distinguished by its use of natural
minerals in addition to industrial chemicals and by the development of
large-scale, long-term processes in the natural environment, in addition
to the small-scale, short-term industrial processes.
Sub-disciplines of Modern Geochemistry
The wide field of geochemistry includes:
- Trace element geochemistry
- Isotope geochemistry
- Petrochemistry
- Soil geochemistry
- Sediment geochemistry
- Marine geochemistry
- Atmospheric geochemistry
- Cosmochemistry
- Geochemical thermodynamics and kinetics
- Aquatic chemistry
- Inorganic geochemistry
- Organic geochemistry
- Biogeochemistry
- Environmental geochemistry
- Chemostratigraphy
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.
Ionic radius
Cations have smaller radii than anions. Ionic radius decreases with
increasing charge. Ionic radius is important for geochemical reactions
such as substitution in crystal lattices, solubility, and diffusion rates.
Origin of Elements
Subatomic particles are particles smaller than atoms. There are two
types of subatomic particles: elementary particles and composite
particles.

Elementary particle or fundamental particle: It is a particle whose

substructure is unknown (e.g., fermions, quarks, leptons, photons and
bosons).

Composite particle: A subatomic particle that is composed of two or

more elementary particles. The protons and neutrons in the nucleus of an
atom are composite particles (e.g., hadrons, baryons and mesons).
Origin of the elements
1) Approximately 15 billion years ago the universe began as an extremely
hot and dense region of radiant energy, the Big Bang.
2) matter + antimatter
3) matter has the advantage
4) baryons  quarks, leptons, electrons, photons (no protons or neutrons)
5) hadrons  protons, neutrons
6) H:He = 1:10 (Approximately 73% of the mass of the visible universe is
in the form of hydrogen. Helium makes up about 25% of the mass, and
everything else represents only 2%).
Note:
Antimatter is material composed of antiparticles, which have the same mass
as particles of ordinary matter but have opposite charge.
Fig. 1: The evolution of the universe
Elemental abundances
The Oddo-Harkin’s rule: Even atomic numbered elements are more
dominant than the adjacent odd numbered ones.
Crustal Element Distribution
The abundance of elements in the Earth's crust is much different from the
abundance of elements that are to be found on the other planets and our
Sun. The continental crust of the Earth also differs radically from the overall
composition of the Earth. Our Earth as a whole and its crust, in particular, have
extraordinary concentrations of elements, all associated with silicate minerals
like olivine, pyroxene, amphibole, plagioclase, micas, and quartz. Although
there are a vast number of silicate minerals, most silicate minerals are made
from just eight elements. The two most common elements in the Earth's crust,
oxygen and silicon, combine to form the "backbone" of the silicate minerals,
along with, occasionally, aluminum and iron. These four elements alone
account for about 87% of the Earth's crust. This silicate or alumina-silicate
"backbone" carries excess negative charge, however. Positive charge in the
form of cations has to be brought in to balance this negative charge. The four
most important elements that fit in the mineralogical structures of the silicates
are calcium, sodium, potassium and magnesium. Taken all together, constituting
nearly 99% of crustal elements, leaves little room for all of the other elements.
As a consequence, all other elements are either nearly absent from the Earth's
crust or are found primarily in non-silicate rocks.
Atomic Substitution
Atomic Theory
Ionic radius (IR) and Valence
Cations (+) —> generally smaller IR
Anions (-) —> generally larger IR
Coordination Number (CN) and Radius Ratio
In an ionic structure each cation tends to surround itself with anions; the
number that can be grouped around it will depend on the relative size of
the cations and anions.

The Coordination Number (CN) is defined as the number of anions that

can fit around a cation. This number increases as the radius ratio
increases. The number of anions that can ‗fit‘ around a cation is related to
the relative size difference between the ions, and this size difference can
be described using the radius ratio, which is given by rC / rA

When this number is small, then only a few anions can fit around a
cation. When this number is large, then more anions can fit around a
cation. When CN is 4, it is known as tetrahedral coordination; when it is
6, it is octahedral; and when it is 8, it is known as cubic coordination.
See the following table.
* Oxygen (O2-) is the most common anion in coordination polyhedron

* O is more tightly bonded to central, highly charged cation than to

other cations

Examples:
C4+ in triangular coordination (CN = 3) produces (CO3)2-
S6+ in tetrahedral coordination (CN = 4) produces (SO4)2-
Si4+ in tetrahedral coordination (CN = 4) produces (SiO4)4-
Atomic Substitution/Solid Solution
According to Goldschmidt's Rules atomic substitution is controlled by:
The size (i.e., radii) of the ions
1) Free substitution can occur if size difference is less than ~15%
2) Limited substitution can occur if size difference is 15 - 30%
3) Little to no substitution can occur if size difference is greater than
30%
The charge of the ions --> cannot differ by more than 1

Examples:
1) Ni2+ = 0.69Å and Mg2+ = 0.72Å
2) Pb2+ = 1.19Å and K+ = 1.38Å
*Atomic substitution is greater at higher temperature and higher pressure
* When the chemical composition of a mineral varies because of atomic
substitution, the mineral is said to exhibit solid solution.
* Solid Solution is defined as a mineral structure in which specific
atomic site(s) are occupied in variable proportions by two or more
different elements.
Example:
The Olivine group represents a complete solid solution series.
Compositions range from a 100% Mg-rich "end member" (forsterite) to a
100% Fe-rich "end member" (fayalite), with all mixtures of these two
elements possible (e.g., 90% Mg and 10% Fe).
* Complete solid solution series because Fe and Mg have same charge
and similar ionic radius (Fe2+ = 0.65Å and Mg2+ = 0.72Å).
Coupled atomic substitutions

A3+ + X5+ ↔ B4+ + Y4+

Examples:
1) Can Th substitute for Ce in monazite (CePO4)
Th4+ + Si4+  Ce3+ + P5+

2) Plagioclase: NaAlSi3O8 - CaAl2Si2O8

Na+ + Si4+  Ca2+ + Al3+

3) Gold and arsenic in pyrite (FeS2):

Au+ + As3+  2Fe2+

4) REE and Na in apatite (Ca5(PO4)3F):

REE3+ + Na+  2Ca2+
Trace Element Substitutions
There are three types of trace element substitutions
1) Camouflage: Occurs when the minor element has the same charge and
similar ionic radius as the major element (same ionic potential; no
preference (For example, Zr4+ (0.72Å); Hf4+ (0.71Å). Hf usually does not
form its own mineral; it is camouflaged in zircon (ZrSiO 4)).

2) Capture: Occurs when a minor element enters a crystal preferentially to

the major element because it has a higher ionic potential than the major
element (For example, K-feldspar captures Ba2+ (1.35Å) in place of K+
(1.38Å). This requires coupled substitution to balance charge:
K+ + Si4+  Ba2+ + Al3+

3) Admission: Involves entry of a foreign ion with an ionic potential less

than that of the major ion (For example, Rb+(1.52Å) for K+ (1.38Å) in K-
feldspar).
Meteorites
Introduction
A meteorite is a natural object originating in outer space that survives
impact with the Earth's surface. A meteorite's size can range from small
to extremely large. Most meteorites derive from small astronomical
objects called meteoroids, but they are also sometimes produced by
impacts of asteroids. When a meteoroid enters the atmosphere, frictional,
pressure and chemical interactions with the atmospheric gasses cause the
body to heat up and emit light, thus forming a fireball, also known as
a meteor or shooting/falling star.
Meteorites that are recovered after being observed as they transited the
atmosphere or impacted the Earth are called falls. All other meteorites
are known as finds.
Naming
Meteorites are always named for the places they were found usually a
nearby town or geographic feature. In cases where many meteorites were
found in one place, the name may be followed by a number or letter.
Some meteorites have informal nicknames.
Meteorite classification
Modern classification schemes divide meteorites into groups according
to their structure, chemical composition and mineralogy. Modern
classification of meteorites is complex. There are four meteorite types;
aerolites, siderites, siderolites and tektites.

1) Stony meteorites (Aerolites)

- Consisting of 90% silicate minerals and 10% Fe-Ni alloy.
- Stony meteorites can be classified as chondrites and achondrites.
a) Chondrites
-Consisting of 40% olivine, 30% pyroxene, 10% plagioclase, 10% Fe-Ni
alloy, 2% native copper and 2% chromite.
- Containing mineral spheres called chondrules.
- Similar in composition to the mantle and crust of Earth.
-Chondrites are believed to be among the oldest rocks in the solar
system.

b) Achondrites
-Consisting of 80% pyroxene and plagioclase, 10% Fe-Ni alloy, 2%
magnetite and 2% chromite.
- Lack chondrules.
- Resemble igneous rocks on Earth.
2) Iron meteorites (Siderites)
- Consisting of 90% Fe-Ni alloy and 10% silicate minerals.
- Similar in composition to the outer core of Earth.
-Iron meteorites can be classified as hexahedrites, octahedrites and
ataxites.
a) Hexahedrites
- The Fe-Ni alloy is composed of chamosite
-The concentration of Ni is about 6%
b) Octahedrites
- The Fe-Ni alloy is composed of chamosite and titanite (sphene)
- The concentration of Ni is about 10%
c) Ataxites
- The Fe-Ni alloy is composed of chamosite and titanite
- The concentration of Ni is about 14%
3) Stony iron meteorites (Siderolites)
- Consisting of 50% silicate minerals and 50% Fe-Ni alloy.
- Resemble rocks at the boundary between Earth's crust and mantle.
- Stony iron meteorites can be classified as pallasites and mesosiderites.
a) Pallasites
- The Fe-Ni alloy is composed of chamosite and titanite
- Olivine is the mainly detected silicate mineral
b) Mesosiderites
- The Fe-Ni alloy is composed of chamosite and titanite
- Feldspars are the mainly detected silicate minerals with small amounts
of olivine
4) Tektites
- They are pieces of natural black, green, brown or gray glass.
- They are similar to some terrestrial volcanic glasses (obsidians).
Hydrosphere
Atmosphere Geosphere
or
Lithosphere

Hydrosphere Biosphere
Hydrosphere
The hydrosphere describes the combined mass of water found on, under,
and over the surface of a planet.

Approximately 75% of the Earth's surface, an area of some 361 million

square kilometers, is covered by ocean.

The average salinity of the Earth's oceans is about 35 grams of salt per
kilogram of sea water (3.5 ‰)
Chemical composition of water
The chemical composition of freshwater are varied under different
conditions, but an average percentage composition can be shown:

Average percentage composition of fresh water

Cations % Anions %
Ca 60.9 CO3 72.4
Mg 19.0 SO4 16.1
Na 16.6 CI 11.5
K 3.5
The chemical composition (percentage) of sea water is quite distinct
from that of freshwater, as shown :

Cations % Anions %
Ca 3.3 CO3 0.3
Mg 10.3 SO4 12.2
Na 83.5 CI 87.2
K 3.0 Br 0.3

The salinity of water can be defined as the concentration of all cations,

significantly Na, K, Mg and Ca and of the anions CO3 and SO4 and
halides, all HCO3 being converted to CO3. The average composition of
freshwater given above is that of average river water. In soft waters Ca
and CO3 might be much reduced. In acid waters sulfate may be
dominant.
Chemical composition of water can be represented by ionic diagrams.
Thus the dominant ions can characterize the types of natural waters, e.g.,
a. Na+ and Cl- in sea water
b. Ca2+ and HCO3- in calcareous (calcium bicarbonate) water
c. SO32- and HCO3- in sulfate-carbonate water
d. Na+, Mg2+ and HCO3- in sodium - magnesium bicarbonate water

Fig: Ionic Diagrams - Arrows indicate dominant ions

Water molecule
Water is acid and base simultaneously, H2O:
H+ + OH-. An acid neutral solution has equal
amounts of H+ and OH-, as is the case for pure
water.

In fresh water and marine water the pH is

about 6.8 and 8.2, respectively

pH
pH is a function of the ratio of conjugate base/acid
What is buffering capacity?
It is the ability of a solution to withstand acid or base addition and
remain at or near the same pH. It occurs when the concentrations of acid
and conjugate base are very similar.

What is Anti-bufferingness?
Aqueous acid solutions are least buffered at the endpoints because the
concentrations of acid and conjugate base are most different, so that their
ratio is sensitive to slight changes in pH.
Solubility refers to the equilibrium quantity of a substance that can be
dissolved in a solution.

Saturation = maximum solute concentration in solution.

Concentrations are given in units of molarity (mole/l), molarity

(mole/kg), ppm by weight (or mg/kg)

Dissolution or solvation refers to the process of dissolving a solid

substance into a solvent to make a solution. Two types of dissolution
reactions exist:
1. Congruent – all of a material goes into solution, leaving nothing
behind when it is dissolved
2. Incongruent – parts of a material go into solution, leaving a new,
modified material behind.
There are different types of solutes in solution:
1. Ionically bounded solids, which dissociate upon dissolution to form
ions

2. Covalently bonded material which go into solution essentially

unchanged, such as glucose

3. Covalently or ionically bonded materials which undergo a reaction

with the solvent.
Solubility summary
Seawater – sediment interactions
Behavior of ions in water (elements in green are largely insoluble)
Horizontal and Vertical Distributions of Elements in the Ocean
Ocean chemists are concerned with the spatial and temporal distributions of
chemicals in the ocean and the interaction of these distributions with physical
and biological processes that may change chemical concentrations. In turn,
these chemical variations can have a profound impact on rates of biological
processes. The distributions of dissolved elements in the ocean are generally
controlled by the interplay of three processes:
1) The uptake and release of dissolved elements on sinking particles (primarily
sinking plankton) transports chemicals vertically within the ocean.
2) The major flow of ocean currents, called the conveyor, transports ocean
waters horizontally from the major sources of deep water in the Atlantic Ocean
to the deep waters of the Pacific Ocean. As a result of this general circulation
pattern, deep waters in the Atlantic are much younger (relative to the time since
they left the surface) than deep waters in the Indian or Pacific Oceans.
3) The distributions of some elements may reflect external sources that act as a
source of elements to the ocean or a sink that removes elements.
Elemental distributions fall into five major categories. These are :
1) Conservative elements: These elements have nearly the same
concentration vertically and horizontally in the ocean (e.g., H, O, Na, K,
Rb, Cs, Mg, Mo, W, Re, Tl, B, F, Cl, S, U and Br)

2) Nutrient-like elements: These elements are depleted at the ocean

surface due to uptake from the water by plankton and incorporation into
the biomass of the plankton (e.g., Fe, P, Ca, Si, Cu, Zn, Se, C, As and V).

Note: Concentrations of Sr are only slightly depleted near the sea

surface.
3) Scavenged elements: These elements are rapidly sorbed onto
sinking particles and removed to the sediments. The concentrations of
scavenged elements generally decrease with time, therefore. This means
that concentrations are lower in the mid-depths of the oceans than at the
surface (e.g., Al, Mn, Cr, Sn, Te, Hg, Pb, Bi, Ce and Th)

4) Radioactive elements: Seawater has many natural radioactive

elements within it. The radioactive elements, particularly the actinides,
have complicated distributions in the ocean due to production from decay
of their parent isotopes and scavenging removal by particles.
5) Stable gases: The stable gases pass from the atmosphere into the
surface waters of the ocean, where they reach saturation (the pressure of
the gas in seawater equals the pressure of the gas in the
atmosphere). The concentration at saturation of a gas (the solubility)
depends on temperature. Cold water holds more gas than does warm
water. Stable gas concentrations are higher in cold surface waters and
when these cold waters sink into the ocean depths, they carry these gases
along with them. These gases may be completely unreactive, such as the
noble gases (He, Ne, Ar, Kr, Xe and Rn) or only very slightly reactive,
such as nitrogen (N). The deep waters of the ocean are cold and have
higher concentrations than the warm surface waters.
Biosphere
The biosphere is the global sum of all ecosystems .It can also be called
the zone of life on Earth. The biosphere is the global ecological system
integrating all living beings and their relationships, including their
interaction with the elements of the lithosphere, hydrosphere and
atmosphere. The biosphere is postulated to have evolved, beginning
through a process of biogenesis or biopoesis, at least some 3.5 billion
years ago.

Biogenesis is the production of new living organisms.

Biopoesis is the natural process by which life arose from non-living

matter such as simple organic compounds.
Narrow definition
Some life scientists and earth scientists use biosphere in a more limited
sense. For example, geochemists define the biosphere as being the total
sum of living organisms. The narrow meaning used by geochemists is
one of the consequences of specialization in modern science. Some
might prefer the word ecosphere.

Specific biospheres
When the word is followed by a number, it is usually referring to a
specific system or number. Thus :
- Biosphere 1, the planet Earth
- Biosphere 2, a laboratory in Arizona
- Biosphere 3, a closed ecosystem at the Institute of Biophysics in Siberia
- Biosphere J, an experiment in Japan
Metabolism
Metabolism is the set of life-sustaining chemical transformations within
the cells of living organisms.

Metabolism is usually divided into two categories:

1) Catabolism, that breaks down organic matter and harvests energy by

way of cellular respiration.

2) Anabolism that uses energy to construct components of cells such

as proteins and nucleic acids.
Atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth
that is retained by Earth's gravity. The atmosphere protects Life on Earth
by absorbing ultraviolet solar radiation ,warming the surface through
heat retention (greenhouse effect) and reducing temperature extremes
between day and night the (diurnal temperature variation).

A greenhouse gas is a gas in an atmosphere that absorbs and emits

radiation within the thermal infrared range. This process is the
fundamental cause of the greenhouse effect. The primary greenhouse
gases in the Earth's atmosphere are water vapor, carbon dioxide,
methane, nitrous oxide, and ozone. Greenhouse gases greatly affect the
temperature of the Earth; without them, Earth's surface would average
about 33 °C colder, which is about 59 °F below the present average of
14 °C.
Composition of Earth's atmosphere or Atmospheric chemistry
Air is mainly composed of nitrogen, oxygen, and argon, which together
constitute the major gases of the atmosphere. The remaining gases are
often referred to as trace gases, among which are the greenhouse gases
such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone.
Filtered air includes trace amounts of many other chemical compounds.
Many natural substances may be present in tiny amounts in an unfiltered
air sample, including dust, sea spray, volcanic ash, pollen and spores.
Various industrial pollutants also may be present, such as chlorine
(elementary or in compounds), fluorine compounds, elemental mercury,
and sulfur compounds such as sulfur dioxide.
Atmospheric stratification or
Structure of the atmosphere
In general, air pressure and Thermopause

density decrease in the

atmosphere as height increases.
However, temperature has a
more complicated profile with Mesopause

altitude. Temperature provides a

useful metric to distinguish
between atmospheric layers. In Stratopause

this way, Earth's atmosphere

can be divided into five main
layers. From highest to lowest, Tropopause

these layers are:

1) Exosphere
The outermost layer of Earth's atmosphere. It is mainly composed of
hydrogen and helium. The particles are so far apart that they can travel
hundreds of kilometers without colliding with one another. Since the
particles rarely collide, the atmosphere no longer behaves like a fluid.
2) Thermosphere
Temperature increases with height in the thermosphere. Unlike in the
stratosphere, where the inversion is caused by absorption of radiation by
ozone, in the thermosphere the inversion is a result of the extremely low
density of molecules. The temperature of this layer can rise to 1500 °C.
3) Mesosphere
It is the layer where most meteors (shooting star) burn up upon entering the
atmosphere. Temperature decreases with height in the mesosphere. The top
of the mesosphere, is the coldest place on Earth and has an average
temperature around -85 °C. Due to the cold temperature of the mesosphere,
water vapor is frozen, forming ice clouds.
4) Stratosphere
The ozone layer is contained within the stratosphere. In this layer ozone
concentrations are about 2 to 8 parts per million. In the stratosphere,
temperature increases with height due to increased absorption of ultraviolet
radiation by the ozone layer.
5) Troposphere
The troposphere begins at the surface and extends to between 9 km at the
poles and 17 km at the equator with some variation due to weather. The
troposphere is mostly heated by transfer of energy from the surface, so on
average the lowest part of the troposphere is warmest and temperature
decreases with altitude. The troposphere contains roughly 80% of the mass
of the atmosphere.
Notes:
Ionosphere  the part of the atmosphere that is ionized by solar
radiation, stretches from 50 to 1000 km and typically overlaps both the
exosphere and the thermosphere.

Homosphere  includes the troposphere, stratosphere, and mesosphere.

Heterosphere  includes the thermosphere and exosphere.

Lithosphere
On the Earth the lithosphere includes the crust and the uppermost mantle
which is joined to the crust across the Mohorovicic discontinuity.
Lithosphere is underlain by asthenosphere, the weaker, hotter, and deeper
part of the upper mantle. The base of the lithosphere-asthenosphere
boundary corresponds approximately to the depth of the melting
temperature in the mantle. As the conductively cooling surface layer of the
Earth's convection system, the lithosphere thickens over time. It is
fragmented into tectonic plates, which move independently relative to one
another. This movement of lithospheric plates is described as plate tectonics.

There are two types of lithosphere:

1) Oceanic lithosphere, which is associated with oceanic crust
2) Continental lithosphere, which is associated with continental crust
Oceanic crust
It is the part of Earth's lithosphere that surfaces in the ocean basins.
Oceanic crust is primarily composed of mafic rocks, or sima ,which is
rich in iron and magnesium. It is thinner than continental crust or sial ,
generally less than 10 kilometers thick. Generally, oceanic crust can be
divided in three layers:
Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or
semi-consolidated sediments.
Layer 2 could be divided into two parts:
layer 2A: 0.5 km thick uppermost volcanic layer of glassy to finely
crystalline basalt.
layer 2B: 1.5 km thick layer composed of diabase dikes.
Layer 3 is on an average 5 km thick. It is formed by slow cooling of
magma beneath the surface and consists of coarse grained gabbros and
ultramafic rocks.
Continental crust
Continental crust has a density of about 2.7 g/cm3 and is less dense than
the material of the Earth's mantle (density of about 3.3g/cm3), which
consists of ultramafic rock. Continental crust is also less dense than
oceanic crust (density of about 2.9 g/cm3), though it is considerably
thicker; mostly 25 to 70 km versus the average oceanic thickness of
around 7 to 10 km. Continental crust makes up about 70% of the volume
of Earth's crust.
Earth Zones
Plate Tectonics - Igneous Genesis

1. Mid-ocean Ridges 2. Intracontinental Rifts

3. Island Arcs 4. Active Continental Margins
5. Back-arc Basins 6. Ocean Island Basalts
7. Miscellaneous Intra Continental Activity kimberlites,
carbonatites, anorthosites, etc.
Table: Mantle versus crust
Igneous Rocks
Magma is a mixture of molten or semi-molten rock, volatiles and
solids that is found beneath the surface of the Earth. Besides molten
rock, magma may also contain suspended crystals and dissolved gas and
sometimes also gas bubbles. Magma often collects in magma chambers
that may feed a volcano or turn into a pluton. Magma is capable of
intrusion into adjacent rocks, extrusion onto the surface as lava, and
explosive ejection as tephra.

Igneous rock is one of the three main rock types, the others being
sedimentary and metamorphic rocks. Igneous rock is formed through the
cooling and solidification of magma or lava.

Igneous facies is a part of an igneous rock differing in structure, texture,

or composition from the main mass.

Migmatite is a rock at the frontier between igneous and metamorphic

rocks. They can also be known as diatexite.
 Common types of magma

Mafic Intermediate Felsic

Bowen's reaction series
Chemical Composition of igneous rocks

1- Major elements  ˃ 1%; SiO2, TiO2, Al2O3, Fe2O3, FeO, MgO, CaO,
Na2O and K2O.

2- Minor elements  0.1-1%; MnO and P2O5.

3- Trace elements  < 0.1%; large ion lithophile elements, rare earth
elements, high field strength elements, heavy metals, etc.

Silica (SiO2)
- Silica represents the most abundant content in igneous rocks.

Classification of igneous rocks (depending on silica content):

1. Ultramafic or ultrabasic igneous rocks (silica < 45%)
2. Mafic or basic igneous rocks (silica between 45 and 52%)
3. Intermediate igneous rocks (silica between 52 and 63%)
4. Acidic or felsic igneous rocks (silica ˃ 63%)
Alumina (Al2O3)
- It ranks second content in abundance after silica.

Classification of igneous rocks (depending on alumina content):

1) Peraluminous rocks: Al2O3 > (Na2O + K2O + CaO) or Al2O3 /(Na2O
+ K2O + CaO) > 1

2) Metaluminous rocks: Al2O3 < (Na2O + K2O + CaO) but Al2O3 >
(Na2O + K2O) or Al2O3 /(Na2O + K2O + CaO)  1

3) Subaluminous rocks: Al2O3 < (Na2O + K2O + CaO) but Al2O3 =

(Na2O + K2O); Al2O3 /(Na2O + K2O + CaO) < 1

4) Peralkaline rocks: Al2O3 < (Na2O + K2O) or Al2O3 /(Na2O + K2O +

CaO) < 1
Alkalis (Soda (Na2O) and Potash (K2O))
- Generally, in igneous rocks, soda ranges from 2 to 5%, while potash
ranges from 0.1 to 6%.
- Ultrapotassic rocks: K2O/Na2O ˃ 3
- Potassic rocks: (K2O ˃ Na2O – 2)
-Sodic rocks: (Na2O ˃ K2O – 2)

Iron (Fe)
-In igneous rocks, iron occurs in both rock forming and accessory
minerals.

Lime (CaO)
- In igneous rocks, lime content increases with silica, soda and potash
contents decreasing.
- The basic igneous rocks tend to contain higher concentrations of lime
than the acidic rocks.
Titania (TiO2)
-In igneous rocks, titania content increases with FeO content increasing.

Manganese (Mn) and Phosphor (P)

- In igneous rocks, manganese replaced Fe+2 in iron minerals, while
phosphor occurs mainly in apatite.
Trace elements
Compatibility is a measure of how readily a particular trace element
substitutes for a major element within a mineral. Trace elements could be
divided into two groups:

1) Incompatible trace elements: Elements those are too large and/or too
highly charged to fit easily into common rock-forming minerals that
crystallize from melts. These elements become concentrated in melts.
There are two groups of incompatible elements:
a) Large-ion lithophile elements or low field strength elements (LIL
or LFSE): Incompatible owing to large size and low charge (e.g., Rb+,
Cs+, Sr2+, Ba2+).

b) High-field strength elements (HFSE): Incompatible owing to large

size and high charge (e.g., Zr4+, Hf 4+, Ta4+, Nb5+, Th4+ and U4+).
2) Compatible trace elements: Elements that fit easily into rock
forming minerals, and may in fact be preferred (e.g., heavy metals).
Granite
Introduction
Granite is a common widely occurring type of intrusive felsic igneous
rock. This rock consists mainly of quartz, mica and feldspar.

Granitoid is a general, descriptive field term for light-colored, coarse-

grained igneous rocks.

Origin of granite
1) Granite is an igneous rock and is formed from felsic magma (igneous
processes)

2) Granitization  granite is formed in place by extreme metasomatism

(metamorphic processes)
Chemical composition of granite
A worldwide average of the chemical composition of granite, by weight
percent :
Oxides Concentrattion
SiO2 72.00
Al2O3 14.42
K2O 4.12
Na2O 3.69
CaO 1.82
FeO 1.68
Fe2O3 1.22
MgO 0.71
TiO2 0.30
P2O5 0.12
MnO 0.05
Classification of granite
First classification (Chappell and White classification)
1) I-type granite or igneous protolith granite  formed by melting of
high grade metamorphic rocks, other granite or intrusive mafic rocks.

2) S-type or sedimentary protolith granite  formed by melting of

buried sediments.

3) M-type or mantle derived granite  sourced from crystallized mafic

magmas, generally sourced from the mantle. These are rare, because it is
difficult to turn basalt into granite via fractional crystallization.

4) A-type or anorogenic granite  formed above volcanic hot spot

activity.
Second classification (based on tectonic setting)
1) Volcanic Arc Granite (VAG)
2) Oceanic Ridge Granite (ORG)
3) Syn-Convergent Granite (SCG)
4) Post-Convergent Granite (PCG)
Basalt
Introduction
Basalt is a common widely occurring type of extrusive mafic igneous
rock. This rock consists mainly of plagioclase, quartz and feldspatoids.

Origin of basalt
Basalt is an igneous rock and is formed from mafic lava (igneous
processes)
Chemical composition of basalt
A worldwide average of the chemical composition of basalt, by weight
percent :

Oxides Concentrattion
SiO2 50.00
Al2O3 15.11
K2O 2.90
Na2O 2.90
CaO 10.00
FeO 9.50
MgO 8.13
TiO2 1.25
Classification of basalt
First classification (chemical classification)
1) Tholeiitic basalt is relatively rich in silica and poor in soda.

2) Mid Oceanic Ridge Basalt is relatively poor in incompatible elements.

3) Alkali basalt is relatively poor in silica and rich in soda. It is silica

undersaturated.

4) High alumina basalt may be silica-undersaturated or oversaturated. It

has greater than 17% alumina and is intermediate in composition
between tholeiite and alkali basalt.

5) Boninite is relatively rich in magnesium and poor in titanium and trace

elements.
Second classification (based on tectonic setting)
1) Mid Oceanic Ridge Basalt (MORB)
2) Oceanic Island Basalt (OIB)
3) Oceanic Floor Basalt (OFB)
4) Continental Flood Basalt (CFB)
5) Volcanic Arc Basalt (VAB)
Ultramafic Rocks
Introduction
Ultramafic rocks are igneous and meta-igneous rocks with very low silica
content (less than 45%), generally >18% MgO, high FeO, low potassium,
and are composed of usually greater than 90% mafic minerals (dark colored,
high magnesium and iron content). The Earth's mantle is composed of
ultramafic rocks.

Classification of ultramafic rocks

First classification (depending on modes of occurrence)
1) Intrusive ultramafic rocks  formed from magma that cools and
solidifies within the crust (e.g., dunite, peridotite and pyroxenite)
2) Extrusive ultramafic rocks  formed at the crust's surface as a result of
the partial melting of rocks within the mantle and crust (e.g., komatiite).
Second classification (chemical classification)
1) Ultrapotassic rocks or melilitic rocks
(e.g., Kimberlite)
2) Highly silica undersaturated rocks
(e.g., Hornblendite)
Metamorphic Rocks
Metamorphism is the change of minerals or geologic texture in pre-
existing rocks. The change occurs primarily due to heat, pressure and the
introduction of chemically active fluids. Changes at or just beneath
Earth's surface due to weathering and/or diagenesis are not classified as
metamorphism. Metamorphism typically occurs between diagenesis
200°C and melting 850°C.

Metamorphic rocks arise from the transformation of existing rock types

in a process called metamorphism which means change in form. The
original rock (protolith) is subjected to heat (200°C) and pressure (1500
bars) causing profound physical and/or chemical change. The protolith
may be sedimentary rock ,igneous rock or another older metamorphic
rock.
Metamorphic minerals are those that form only at the high
temperatures and pressures associated with the process of
metamorphism. These minerals, known as index minerals, include
laumontite, lawsonite, glaucophane, paragonite, pyrophyllite, sillimanite,
kyanite, staurolite, andalusite and some garnet.

Metasomatism is the chemical alteration of a rock by hydrothermal and

other fluids.

* Metasomatism is open system behavior which is different from

classical metamorphism which is the in-situ mineralogical change of a
rock without appreciable change in the chemistry of the rock.
Metasomatism and metamorphism nearly always occur together.
From diagenesis to metamorphism
Diagenesis comprises all changes taking place in a sediment between
sedimentation and the onset of metamorphism, except those caused by
weathering.

* Metamorphism has begun and diagenesis has ended when a mineral

assemblage is formed which cannot originate in a sedimentary
environment.

* Minerals such as feldspar or quartz form under metamorphic as well as

under diagenetic conditions.

* The first appearance of metamorphic minerals such as laumontite,

lawsonite, glaucophane, paragonite or pyrophyllite indicates the
beginning of metamorphism.
Metamorphic reactions
1)Solid-solid reactions, i.e. reactions among solid phases, involving no
liberation of volatiles.

Example:
kyanite ↔ sillimanite
andalusite ↔ sillimanite

Fig: Phase diagram

between the three
polymorphs of
Al2SiO5
2) Dehydration reactions, i.e. reactions involving liberation of H2O
with rise in temperature. The majority of metamorphic reactions belong
to this class.

Example:
Al2Si4O10(OH)2 = Al2SiO5 + 3 SiO2 + H2O
pyrophyllite kyanite quartz

3) Decarbonation reactions, i.e. reactions involving liberation of CO2

with rise in temperature.

Example:
CaCO3 + SiO2 = CaSiO3 + CO2
calcite quartz wollastonite
2) Oxidation-reduction reactions: Since many rock forming minerals
contain iron, this class of reactions is also important.

Example:
6 Fe2O3 = 4 Fe3O4 + O2
hematite magnetite

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