Chapter
3
Minerals and Rocks
October 2012
Introduction to rock-forming Minerals
• The Earth is composed of rocks.
• Rocks are aggregates of minerals.
• Minerals are composed of atoms.
• In order to understand rocks, we must first have an
understanding of minerals.
• In order to understand minerals we must have some
basic understanding of atoms - what they are and how
they interact with one another to form minerals.
Definition of a Mineral
• Naturally formed; it forms in nature on its own,
• Solid (it cannot be a liquid or a gas),
• With a definite chemical composition (every time
we see the same mineral it has the same
chemical composition that can be expressed by a
chemical formula), and
• Characteristic crystalline structure (atoms are
arranged within the mineral in a specific ordered
manner).
Cont’d
Examples
• Glass - can be naturally formed (volcanic glass called
obsidian), is a solid, its chemical composition, however,
is not always the same, and it does not have a
crystalline structure. Thus, glass is not a mineral.
• Ice - is naturally formed, is solid, and does have a
definite chemical composition that can be expressed by
the formula H2O. Thus, ice is a mineral, but liquid water
is not (since it is not solid).
• Halite (salt) - is naturally formed, is solid, does have a
definite chemical composition that can be expressed by
the formula NaCl, and does have a definite crystalline
structure. Thus halite is a mineral.
•Therefore, a mineral is a naturally occurring, inorganic, solid with a definite
composition and a regular internal crystal structure.
Atomic Chemistry and Bonding
• All matter is made up of atoms, and all atoms are made up of
three main particles known as protons, neutrons and electrons.
• As summarized in the following table, protons are positively
charged, neutrons are uncharged and electrons are negatively
charged. The negative charge of one electron balances the
positive charge of one proton. Both protons and neutrons have a
mass of 1, while electrons have almost no mass.
Elementary
particle
Charge
Mass
Electron
-1
~0
Proton
+1
1
Neutron
0
1
Cont’d
• The simplest atom is that of hydrogen, which has one proton and one
electron.
• The proton forms the nucleus of hydrogen, while the electron orbits
around it.
• All other elements have neutrons as well as protons in their nucleus.
• The positively-charged protons tend to repel each other, and the
neutrons help to hold the nucleus together.
• For most of the 16 lightest elements (up to oxygen) the number of
neutrons is equal to the number of protons.
• For most of the remaining elements there are more neutrons than
protons, because with increasing numbers of protons concentrated in
a very small space, more and more extra neutrons are needed to
overcome the mutual repulsion of the protons in order to keep the
nucleus together.
• The number of protons is the atomic number; the number of protons
plus neutrons is the atomic weight.
• For example, silicon has 14 protons, 14 neutrons and 14 electrons. Its
atomic number is 14 and its atomic weight is 28.
Structure of Atoms
•
Electrons orbit around the nucleus in different shells, labeled from the innermost shell
as K, L, M, N, etc. Each shell can have a certain number of electrons. The K-shell can
have 2 Electrons, the L-shell, 8, the M-shell 18, N-shell 32.
#electrons = 2N2, where N=1 for the K shell, N=2 for the L shell, N=3 for the M shell, etc.
• A Stable electronic configuration for an atom is one 8 electrons in outer shell (except in
the K shell, which is completely filled with only 2 electrons). Thus, atoms often loose
electrons or gain electrons to obtain stable configuration.
• Noble gases have completely filled outer shells, so they are stable. Examples He, Ne, Ar,
Kr, Xe, Rn.
• Others like Na, K loose an electron. This causes the charge balance to become unequal.
In fact to become + (positive) charged atoms called ions.
• Positively charged atoms = cations. Elements like F, Cl, O gain electrons to become (-)
charged. (-) charged ions are called anions.
Types of bonding:
Ionic bonding- caused by the force of attraction between ions of opposite
charge. Example: Na+1 and Cl-1. Bond to form NaCl (halite or salt).
Covalent bonding - Electrons are shared between two or more atoms so that each
atom has a stable electronic configuration (completely filled outermost shell) part of
the time. Example: H has one electron, needs 2 to be stable. O has 6 electrons in its
outer shell, needs 2 to be stable. So, 2 H atoms bond to 1 O to form H2O, with all
atoms sharing electrons, and each atom having a stable electronic configuration part
of the time.
Cont’d
• Metallic bonding -- Similar to covalent bonding, except
innermost electrons are also shared. In materials that bond
this way, electrons move freely from atom to atom and are
constantly being shared. Materials bonded with metallic bonds
are excellent conductors of electricity because the electrons
can move freely through the material.
• Van der Waals bonding -- a weak type of bond that does not
share or transfer electrons. Usually results in a zone along
which the material breaks easily (cleavage). A good example is
graphite. Several different bond types can be present in a
mineral, and these determine the physical properties of the
mineral.
Crystal Structure
• Solids having a regular, orderly arrangement of their internal
atoms are said to have crystalline structure and are known as
crystals. Most solid substances, including rocks and minerals,
are made up of aggregates of many small crystals.
• Crystals are characteristically bounded by flat surfaces, which
are large-scale reflection of the internal arrangement of the
atoms in the crystal.
• Study of the arrangement of faces in natural crystals showed
that there are six basic groups, each with a characteristic
symmetry of the faces.
• Packing of atoms in a crystal structure requires an orderly and
repeated atomic arrangement. Such an orderly arrangement
needs to fill space efficiently and keep a charge balance.
• Since the size of atoms depends largely on the number of
electrons, atoms of different elements have different sizes.
• Crystal structure depends on the conditions under which the
mineral forms.
The Six basic Systems of Crystal symmetry
1. Isometric: Three equal-length axes at right angles to
each other. Example; Garnet Magnetite, Halite, Pyrite
2. Tetragonal: Two equal axes and a third either longer
or shorter, all at right angles. Example; Zircon
3. Orthorhombic: Three unequal axes, all at right angles.
Example; Olivine, Aragonite, Anhydrite, Goethite
4. Monoclinic: Three unequal axes, two at right angles
and a third perpendicular to one but oblique to the
other. Example; Pyroxene, Amphibole, Micas,
Orthoclase, Gypsum
5. Triclinic: Three unequal axes meeting at oblique
angles. Example; Plagioclase, Aluminosilcate
6. Hexagonal: Three equal axes in the same plane
intersecting at 600 and a fourth perpendicular to the
plane of the other three. Example; Quartz, Calcite,
Hematite
Composition of Minerals
The variety of minerals we see depend on the chemical elements
available to form them. In the Earth's crust the most abundant
elements are as follows:
• 1. O, Oxygen 45.2% by weight
• 2. Si, Silicon 27.2%
• 3. Al, Aluminum 8.0%
• 4. Fe, Iron 5.8%
• 5. Ca, Calcium 5.1%
• 6. Mg, Magnesium 2.8%
• 7. Na, Sodium 2.3%
• 8. K, Potassium 1.7%
• 9. Ti, Titanium 0.9%
• 10. H, Hydrogen 0.14%
• 11. Mn, Manganese 0.1%
• 12. P, Phosphorous 0.1%
The most common minerals are those based on Si and O: the Silicates.
Silicates are based on SiO4 tetrahedron. 4 Oxygens covalently bonded to one
silicon atom.
ionic radii
Ionic radii of ions commonly found in rock-forming minerals.
• Polymorphs are minerals with the same chemical composition but
different crystal structures. The conditions are such things as
temperature (T) and pressure (P), because these affect ionic radii. At
high T atoms vibrate more, and thus distances between them get larger.
Crystal structure changes to accommodate the larger atoms. At even
higher T substances changes to liquid and eventually to gas. Liquids and
gases do not have an ordered crystal structure and are not minerals.
Increase in P pushes atoms closer together. This makes for a more
densely packed crystal structure.
Examples:
 Carbon
(C) has two different
polymorphs. At low T and P pure carbon is
the mineral graphite, (pencil lead), a very
soft mineral.
 At higher T and P the stable form is
diamond, the hardest natural substance
known.
Ionic Substitution (Solid Solution)
 Ionic substitution - (also called solid solution), occurs because
some elements (ions) have the same size and charge, and can
thus substitute for one another in a crystal structure.
Examples:
• Olivines Fe2SiO4 and Mg2SiO4. Fe+2 and Mg+2 are about the same
size, thus they can substitute for one another in the crystal
structure and olivine thus can have a range of compositions
expressed as the formula (Mg, Fe)2SiO4.
• Alkali Feldspars: KAlSi3O8 (orthoclase) and NaAlSi3O8, (albite) K+1
can substitute for Na+1
• Plagioclase Feldspars: NaAlSi3O8 (albite) and CaAl2Si2O8
(anorthite) NaSi+5 can substitutes for CaAl+5 (a complex solid
solution).
Properties of Minerals
Physical properties of minerals allow us to distinguish between minerals and thus
identify them, as you will learn in lab. Among the common properties used are:
• Habit - shape
• Color
• Streak (color of fine powder of the mineral)
• Luster -- metallic, vitreous, pearly, resinous (reflection of light)
• Cleavage (planes along which the mineral breaks easily)
• Density (mass/volume)
• Hardness: based on Mohs hardness scale as follows:
1. Talc
2. Gypsum (fingernail)
3. Calcite (penny)
4. Fluorite
5. Apatite (knife blade)
6. Feldspar (Orthoclase)
7. Quartz
8. Topaz
9. Corundum
10. Diamond
Two German Captains Fought A French Queen To Cause Death
Formation of Minerals
Minerals are formed in nature by a variety of processes.
Among them are:
 Crystallization from melt (igneous rocks).
 Precipitation from water (chemical sedimentary rocks,
hydrothermal ore deposits).
 Biological activity (biochemical sedimentary rocks).
 Change to more stable state - (the processes of weathering,
metamorphism, and diagenesis).
 Precipitation from vapor. (not common, but sometimes
does occur around volcanic vents).
Group of Minerals
• We group minerals into classes on the basis of
their predominant anion or anion group.
These include oxides, sulphides, carbonates,
silicates, and others.
• Silicates are by far the predominant group in
terms of their abundance within the crust and
mantle.
GROUP
EXAMPLES
Oxides
hematite (iron-oxide – Fe2O3), corundum (aluminum-oxide
Al2O3), water-ice (H2O)
Sulphides
galena (lead-sulphide - PbS), pyrite (iron-sulphide – FeS2),
chalcopyrite (copper-iron-sulphide – CuFeS2)
Carbonates
calcite (calcium-carbonate – CaCO3), dolomite (calciummagnesium-carbonate – (Ca,Mg)CO3
Silicates
quartz (SiO2)*, feldspar (sodium-aluminum-silicate –
NaAlSi3O8), olivine (iron or magnesium-silicate - FeSiO4)
Halides
fluorite (calcium-fluoride – CaF2), halite (sodium-chloride NaCl)
gypsum (calcium-sulphate – CaSO4·H2O), barite (bariumsulphate - BaSO4)
Sulphates
Phosphate
Native elements
The most important phosphate mineral is apatite
(Ca5(PO4)3(OH))
gold (Au), diamond (C), graphite (C), sulphur (S), copper (Cu)
Silicate Minerals
• The vast majority of the minerals that make up the rocks of the
earth's crust are silicate minerals.
• These include minerals such as quartz, feldspar, mica,
amphibole, pyroxene, olivine, and a great variety of clay
minerals.
• The building block of all of these minerals is the Silica
tetrahedron, a combination of four oxygen atoms and one
silicon atom.
• These are arranged such that planes drawn through the oxygen
atoms describe a tetrahedron (a four-faced object)—which is a
pyramid with a triangular base.
Silicon-oxygen tetrahedron
Cont’d
• The bonds in a silica tetrahedron have some of the properties of
covalent bonds and some of the properties of ionic bonds.
• As a result of the ionic character, silicon becomes a cation (with a
charge of +4) and oxygen becomes an anion (with a charge of -2),
hence the net charge of a silica tetrahedron Si04 is -4.
• Silica tetrahedra are linked together in a variety of ways to form
most of the common minerals of the crust.
• Most minerals are characterized by ionic or covalent bonds or a
combination of the two, but one other type of bond which is
geologically important is the metallic bond.
• Elements that behave as metals have outer electrons that are
relatively loosely held. When bonds between such atoms are formed
these electrons can move freely from one atom to another.
• A metal can thus be thought of as an array of positively charged
nuclei immersed in a sea of mobile electrons. This characteristic
accounts for two very important properties of metals: their electrical
conductivity and their malleability.
Silicon-oxygen tetrahedron Structure
The silica tetrahedron and
structure of silicate minerals.
the
a. The silica tetrahedron consists of a
central silicon atom bound to 4
oxygens.
b. In orthosilicates such as olivine, the
tetrahedra are separate
and each oxygen is also bound to other
metal ions that occupy interstitial
sites between the tetrahedra.
c. In pyroxenes, the tetrahedra each
share two oxygen and are bound
together into chains. Metal ions are
located between the chains.
d. In sheet silicates, such as talc, mica,
and
clays, the tetrahedra each share 3
oxygens and are bound together into
sheets.
Silicon-oxygen tetrahedron
Name
Structural Group
Nesosilicates
Independent tetrahedra
Sorosilicates
Two tetrahedra
one oxygen
Unit
SiO4
sharing Si2O7
Example
Typical Formula
Olivine
(Fe,Mg)2SiO4
Melilite
Ca2MgSi2O7
Cyclosilicates Closed rings of tetrahedra
(SiO3)n
n=3,4,6
Beryl (6-fold)
Axinite (4-fold)
Benitoite (3-fold)
Be3Al2(SiO3)6
Ca2(Mn,Fe2)Al2BO3(SiO3)4(OH)
BaTi(SiO3)3
(a) Continuous single
chains of tetrahedra, each
sharing two oxygens
(b) Continuous double
chains
of
tetrahedra
alternately sharing two
and three oxygens
Phyllosilicates Continuous sheets of
tetrahedra sharing three
oxygens
Tektosilicates Three-dimensional
framework of tetrahedra
with all four oxygen atoms
shared
(SiO3)
Si4O11
Pyroxenes
Pyroxenoids
Amphiboles
MgSiO3
CaSiO3
Mg7Si8O22(OH)2
Si4O10
Micas
Talc
KAl2(Si3Al)O10(OH,F)2
Mg3Si4O10(OH)2
Quartz
Feldspars
SiO2
KAlSi3O8
each sharing two oxygens
Inosilicates
SiO2
IGNEOUS ROCKS
Origin of Igneous rocks
• An igneous rock is any crystalline or glassy rock that forms
from cooling of magma.
• Magma consists mostly of liquid rock matter, but may contain
crystals of various minerals, and may contain a gas phase that
may be dissolved in the liquid or may be present as a separate
gas phase.
Cont’d
• Magma
can cool to form an igneous rock either on the surface of the
Earth - in which case it produces a volcanic or extrusive igneous rock,
or beneath the surface of the Earth, in which case it produces a plutonic
or intrusive igneous rock.
• Atliquid,
depth in the Earth nearly all magmas contain gas dissolved in the
but the gas forms a separate vapor phase when pressure is
decreased as magma rises toward the surface.
• This
is similar to carbonated beverages which are bottled at high
pressure. The high pressure keeps the gas in solution in the liquid, but
when pressure is decreased, like when you open the can or bottle, the
gas comes out of solution and forms a separate gas phase that you see
as bubbles.
• Gas
gives magmas their explosive character, because volume of gas
expands as pressure is reduced.
Cont’d
The composition of the gases in magma is:
• Mostly H2O (water vapor) with some CO2 (carbon dioxide)
• Minor amounts of Sulfur, Chlorine, and Fluorine gases
• The amount of gas in magma is also related to the chemical
composition of the magma.
• Rhyolitic magmas usually have higher dissolved gas contents than
basaltic magmas.
Types of Magma
Types of magma are determined by chemical composition of the magma. Three
general types are recognized, but we will look at other types later in the course:
1. Basaltic magma (1000-1200oC) -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
2. Andesitic magma (800-1000oC) -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na,
K
3. Rhyolitic or Granitic magma (650-800oC) -- SiO2 65-75%, low in Fe, Mg, Ca, high in
K, Na
Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Depends on
composition, temperature, & gas content. Higher SiO2 content magmas have
higher viscosity than lower SiO2 content magmas. Lower Temperature
magmas have higher viscosity than higher temperature magmas.
Summary
Summary Table
Magma
Type
Basaltic
Solidified Solidified
Volcanic Plutonic
Rock
Rock
Basalt
Gabbro
Chemical
Composition
45-55 SiO2 %,
high in Fe, Mg,
Ca, low in K, Na
Temperature
1000 - 1200 oC Low
Andesitic Andesite Diorite
55-65 SiO2 %,
800 - 1000 oC
intermediate in
Fe, Mg, Ca, Na, K
Rhyolitic Rhyolite
65-75 SiO2 %,
low in Fe, Mg,
Ca, high in K, Na
Granite
Viscosity
650 - 800 oC
Gas
Content
Low
Intermed Intermed
iate
iate
High
High
Origin of Magma
• In order for magmas to form, some part of the Earth must
get hot enough to melt the rocks present.
• Since rocks are mixtures of minerals, they behave
somewhat differently. Unlike minerals, rocks do not melt at
a single temperature, but instead melt over a range of
temperatures.
• Thus, it is possible to have partial melts from which the
liquid portion might be extracted to form magma.
• The two general cases are:
– Melting of dry rocks is similar to melting of dry minerals,
melting temperatures increase with increasing pressure, except
there is a range of temperature over which there exists a partial
melt. The degree of partial melting can range from 0 to 100%
– Melting of rocks containing water or carbon dioxide is similar to
melting of wet minerals; melting temperatures initially decrease
with increasing pressure, except there is a range of temperature
over which there exists a partial melt.
Origin of Basaltic Magma
 Much evidence suggests that Basaltic magmas result from dry partial melting
of mantle.
 Basalts contain minerals like olivine, pyroxene and plagioclase, none of which
contain water.
 Basalts erupt non-explosively, indicating a low gas content and therefore low
water content.
 The Mantle is made of garnet peridotite (a rock made up of olivine, pyroxene,
and garnet).
 Under normal conditions the temperature in the Earth, shown by the
geothermal gradient, is lower than the beginning of melting of the mantle.
 Thus in order for the mantle to melt there has to be a mechanism to raise the
geothermal gradient.
 Once such mechanism is convection, wherein hot mantle material rises to
lower pressure or depth, carrying its heat with it.
 If the raised geothermal gradient becomes higher than the initial melting
temperature at any pressure, then a partial melt will form.
 Liquid from this partial melt can be separated from the remaining crystals
because, in general, liquids have a lower density than solids.
 Basaltic or gabbroic magmas appear to originate in this way.
Origin of Granitic or Rhyolitic Magma
Most Granitic or Rhyolitic magma appears to result
from wet melting of continental crust. The evidence
for this is:
• Most granites and rhyolites are found in areas of
continental crust.
• When granitic magma erupts from volcanoes it does
so very explosively, indicating high gas content.
• Solidified granite or rhyolite contains quartz,
feldspar, hornblende, biotite, and muscovite. The
latter minerals contain water, indicating high water
content.
Origin of Andesitic Magma
• Average composition of continental crust is
andesitic, but if andesite magma is produced by
melting of continental crust then it requires
complete melting of crust. Temperatures in crust
unlikely to get high enough.
• Andesitic magmas erupt in areas above
subduction zones, suggests relation between
production of andesite and subduction.
• One theory involves wet partial melting of
subducted oceanic crust. But, newer theories
suggest wet partial melting of mantle.
Magmatic Differentiation
• When magma solidifies to form a rock it does so over a range of
temperature. Each mineral begins to crystallize at a different
temperature, and if these minerals are somehow removed from
the liquid, the liquid composition will change.
• Depending on how many minerals are lost in this fashion, a wide
range of compositions can be made. The process is called
magmatic differentiation.
Over the years, various processes have been suggested to explain the
variation of magma compositions observed within small regions. Among
the processes are:
1. Distinct melting events from distinct sources.
2. Various degrees of partial melting from the same source.
3. Crystal fractionation.
4. Mixing of 2 or more magmas.
5. Assimilation/contamination of magmas by crustal rocks.
6. Liquid Immiscibility.
7. Combined process (a combination of one of these)
1. Distinct Melting Events
• One possibility that always exists is that the magmas are not related
except by some heating event that caused melting. In such a case each
magma might represent melting of a different source rock at different
times during the heating event. The possibility of distinct melting
events is not easy to prove or disprove.
2. Various Degrees of Partial Melting
• When a multicomponent rock system melts, unless it has the
composition of the eutectic, it melts over a range of temperatures at any
given pressure, and during this melting, the liquid composition changes.
Thus, a wide variety of liquid compositions could be made by various
degrees of partial melting of the same source rock.
3. Crystal Fractionation
Liquid compositions can change as a result of removing crystals from the liquid as
they form. In all cases, crystallization results in a change in the composition of the
liquid, and if the crystals are removed by some process, then different magma
compositions can be generated from the initial parent liquid. If minerals that later
react to form a new mineral or solid solution minerals are removed, then crystal
fractionation can produce liquid compositions that would not otherwise have been
attained by normal crystallization of the parent liquid.
Bowen's Reaction Series
• Norman L. Bowen, an experimental Petrologist in the early 1900s, realized this
from his determinations of simple 2- and 3-component phase diagrams, and
proposed that if an initial basaltic magma had crystals removed before they
could react with the liquid, that the common suite of rocks from basalt to
rhyolite could be produced.
• This is summarized as Bowen's Reaction Series.
Bowen's Reaction…cont’d
Bowen suggested that the common minerals that crystallize from magmas could be
divided into a continuous reaction series and a discontinuous reaction series.
• The continuous reaction series is composed of the plagioclase feldspar solid
solution series. A basaltic magma would initially crystallize a Ca- rich plagioclase
and upon cooling continually react with the liquid to produce more Na-rich
plagioclase. If the early forming plagioclase were removed, then liquid
compositions could eventually evolve to those that would crystallize a Na-rich
plagioclase, such as a rhyolite liquid.
• The discontinuous reaction series consists of minerals that upon cooling
eventually react with the liquid to produce a new phase. Thus, as we have seen,
crystallization of olivine from a basaltic liquid would eventually reach a point
where olivine would react with the liquid to produce orthopyroxene. Bowen
postulated that with further cooling pyroxene would react with the liquid, which
by this time had become more enriched in H2O, to produce hornblende. The
hornblende would eventually react with the liquid to produce biotite. If the earlier
crystallizing phases are removed before the reaction can take place, then
increasingly more siliceous liquids would be produced.
This generalized idea is consistent with the temperatures observed in magmas and
with the mineral assemblages we find in the various rocks. We would expect that with
increasing SiO2 oxides like MgO, and CaO should decrease with higher degrees of
crystal fractionation because they enter early crystallizing phases, like olivines and
pyroxenes. Oxides like H2O, K2O and Na2O should increase with increasing crystal
fractionation because they do not enter early crystallizing phases.
Mechanisms of Crystal Fractionation
Crystal Settling/Floating - In general, crystals forming from magma will have
different densities than the liquid.
Inward Crystallization - Because a magma body is hot and the
country rock which surrounds it is expected to be much cooler,
heat will move outward away from the magma. Thus, the walls of
the magma body will be coolest, and crystallization would be
expected to take place first in this cooler portion of the magma
near the walls. The magma would then be expected to crystallize
from the walls inward.
Filter pressing - this mechanism has been proposed as a way to separate a liquid from a
crystal-liquid mush. In such a situation where there is a high concentration of crystals the liquid
could be forced out of the spaces between crystals by some kind of tectonic squeezing that
moves the liquid into a fracture or other free space, leaving the crystals behind. It would be kind
of like squeezing the water out of a sponge.
4. Magma Mixing
• If two or more magmas with different chemical compositions come in
contact with one another beneath the surface of the Earth, then it is
possible that they could mix with each other to produce compositions
intermediate between the end members. If the compositions of the
magmas are greatly different (i.e. basalt and rhyolite), there are several
factors that would tend to inhibit mixing.
– Temperature contrast - basaltic and rhyolitic magmas have very different
temperatures. If they come in contact with one another the basaltic
magma would tend to cool or even crystallize and the rhyolitic magma
would tend to heat up and begin to dissolve any crystals that it had
precipitated.
– Density Contrast- basaltic magmas have densities on the order of 2600 to
2700 kg/m3, whereas rhyolitic magmas have densities of 2300 to 2500
kg/m3. This contrast in density would mean that the lighter rhyolitic
magmas would tend to float on the heavier basaltic magma and inhibit
mixing.
– Viscosity Contrast- basaltic magmas and rhyolitic magmas would have very
different viscosities. Thus, some kind of vigorous stirring would be
necessary to get the magmas to mix.
5. Crustal Assimilation/Contamination
• Because the composition of the crust is generally different from the
composition of magmas which must pass through the crust to reach the
surface, there is always the possibility that reactions between the crust
and the magma could take place.
• If crustal rocks are picked up, incorporated into the magma, and
dissolved to become part of the magma, we say that the crustal rocks
have been assimilated by the magma.
• If the magma absorbs part of the rock through which it passes we say
that the magma has become contaminated by the crust.
• Either of these processes would produce a change in the chemical
composition of the magma unless the material being added has the same
chemical composition as the magma.
• In a sense, bulk assimilation would produce some of the same effects as
mixing, but it is more complicated than mixing because of the heat
balance involved.
• In order to assimilate the country rock enough heat must be provided to
first raise the country rock to its solidus temperature where it will begin
to melt and then further heat must be added to change from the solid
state to the liquid state. The only source of this heat, of course, is the
magma itself.
6. Liquid Immiscibility
•
Liquid immiscibility is where liquids do not mix with each other. We are all familiar with
this phenomenon in the case of oil and water/vinegar in salad dressing. We have also
discussed immiscibility in solids, for example in the alkali feldspar system. Just like in the
alkali feldspar system, immiscibility is temperature dependent.
Two important properties of immiscible liquids.
– 1. If immiscible liquids are in equilibrium with solids, both liquids must be in equilibrium with the
same solid compositions.
– 2. Extreme compositions of the two the liquids will exist at the same temperature.
•
Liquid immiscibility was once thought to be a mechanism to explain all magmatic
differentiation. If so, requirement 2, above, would require that siliceous liquids and mafic
liquids should form at the same temperature. Since basaltic magmas are generally much
hotter than rhyolitic magmas, liquid immiscibility is not looked upon favorably as an
explanation for wide diversity of magmatic compositions. Still, liquid immiscibility is
observed in experiments conducted on simple rock systems.
There are however, three exceptions where liquid immiscibility may play a role.
• 1. Sulfide liquids may separate from mafic silicate magmas.
• 2. Highly alkaline magmas rich in CO2 may separate into two liquids, one rich in carbonate, and the
other rich in silica and alkalies. This process may be responsible for forming the rare carbonatite
magmas.
• 3. Very Fe-rich basaltic magmas may form two separate liquids - one felsic and rich in SiO2, and the
other mafic and rich in FeO.
7. Combined Processes
• As pointed out previously, if any of these processes are possible,
then a combination of the process could act to produce chemical
change in magmas.
• Thus, although crystal fractionation seems to be the dominant
process affecting magmatic differentiation, it may not be the
only processes.
• As we have seen, assimilation is likely to accompany by
crystallization of magmas in order to provide the heat necessary
for assimilation. If this occurs then a combination of crystal
fraction and assimilation could occur.
• Similarly, magmas could mix and crystallize at the same time
resulting in a combination of magma mixing and crystal
fractionation. In nature, things could be quite complicated.
Mode of occurrence of Igneous bodies
Eruption of Magma
• When magmas reach the surface of the Earth they
erupt from a vent.
• They may erupt explosively or non-explosively.
• Non-explosive eruptions are favored by low gas
content and low viscosity magmas (basaltic to
andesitic magmas). Usually begin with fire fountains
due to release of dissolved gases Produce lava flows
on surface and produce Pillow lavas if erupted
beneath water.
Type of volcanic flows
Ropy surface of a Pahoehoe flow
Tephra that falls from the eruption
column produces a tephra fall deposit
AA flow, the left side on the photo is a pahoehoe flow
If eruption column collapses a pyroclastic flow may occur,
wherein
gas and tephra rush down the flanks of the volcano at high
speed. This is the most dangerous type of volcanic
eruption. The deposits that are
produced are called ignimbrites.
Structures and Field relationships
VOLCANOES
• Shield volcano – volcanoes that erupt low viscosity magma
(usually basaltic) that flows long distances from the vent.
• Pyroclastic cone or cinder cone – a volcano built mainly of
tephra fall deposits located immediately around the vent.
Volcanoes…cont’d
• Stratovolcano (composite volcano) – a volcano built of
interbedded lava flows and pyroclastic material.
• Crater - a depression caused by explosive ejection of magma or gas.
Volcanoes…cont’d
• Caldera - a depression caused by collapse of a volcano into the
cavity once occupied by magma.
• Lava Dome - a steep sided volcanic
structure resulting from the eruption of
high viscosity, low gas content magma.
• Fissure Eruptions - An eruption that occurs
along a narrow crack or fissure in the Earth's
surface.
• Pillow Lava - Lavas formed by eruption
beneath the surface of the ocean or a lake.
PLUTONS
• Igneous rocks cooled at depth.
• Name comes from Greek god of the underworld - Pluto.
 Dikes are small (<20 m wide) shallow intrusions that show a discordant
relationship to the rocks in which they intrude. Discordant means that they cut
across preexisting structures. They may occur as isolated bodies or may occur as
swarms of dikes emanating from a large intrusive body at depth.
 Sills are also small (<50 m thick) shallow intrusions that show a concordant
relationship with the rocks that they intrude. Sills usually are fed by dikes, but
these may not be exposed in the field.
Plutons…cont’d
• Laccoliths are somewhat large intrusions that result in uplift and folding of the
preexisting rocks above the intrusion. They are also concordant types of
intrusions.
• Batholiths are very large intrusive bodies, usually so large that there bottoms are
rarely exposed. Sometimes they are composed of several smaller intrusions.
• Stocks are smaller bodies that are likely fed from deeper level batholiths. Stocks
may have been feeders for volcanic eruptions, but because large amounts of erosion
are required to expose a stock or batholith, the associated volcanic rocks are rarely
exposed.
RELATIONSHIPS TO PLATE TECTONICS
Diverging Plate Boundaries
 Diverging plate boundaries are mostly beneath the oceans and occur at oceanic
ridges.
 Here, basaltic magma is erupted at the oceanic ridge and is intruded beneath the
ridge where it forms new oceanic crust.
 Only rarely does the oceanic ridge build itself above the oceans surface. One
example of where this occurs is the island of Iceland in the northern Atlantic
Ocean. Eruptions of magma in Iceland are mostly basaltic.
Converging Plate Boundaries
• Where lithospheric plates converge, oceanic lithosphere subducts
beneath either another plate composed of oceanic lithosphere or
another plate composed of continental lithosphere.
 If an oceanic lithospheric plate subducts beneath another oceanic lithospheric plate, we
find island arcs on the surface above the subduction zone. These are volcanoes built of
mostly andesitic lavas pyroclastic material, although some basalts and rhyolites also occur.
 If an oceanic plate subducts beneath a plate composed of continental lithosphere, we
find continental margin arcs. Again, the volcanoes found here are composed mostly of
andesitic lavas and pyroclastics. It is likely that some magmas cool beneath the volcanic arc
to form dioritic and granitic plutons.
Reflection
• Why basaltic magmas are not explosive and
rhyolitic magma is explosive?
• Are Volcanic or Plutonic igneous rocks are most
common in Ethiopia?
• How plutonic rocks are exposed to the surface?
Textures of Igneous Rocks

The main factor that determines the texture of an igneous rock is the
cooling rate (dT/dt)
 Other factors involved are:
– The diffusion rate - the rate at which atoms or molecules can move (diffuse)
through the liquid.
– The rate of nucleation of new crystals - the rate at which enough of the
chemical constituents of a crystal can come together in one place without
dissolving.
– The rate of growth of crystals - the rate at which new constituents can arrive
at the surface of the growing crystal. This depends largely on the diffusion
rate of the molecules of concern.
•
•
•
In order for a crystal to form in magma enough of the chemical constituents that
will make up the crystal must be at the same place at the same time to form a
nucleus of the crystal.
Once a nucleus forms, the chemical constituents must diffuse through the liquid
to arrive at the surface of the growing crystal.
The crystal can then grow until it runs into other crystals or the supply of
chemical constituents is cut off.
• Nucleation and growth cannot occur until temperatures are below the
temperature at which equilibrium crystallization begins.
• The rate of crystal growth and nucleation depends on how long the magma
resides at a specified degree of undercooling (ΔT = Tm - T), and thus the rate
at which temperature is lowered below the crystallization temperature.
1. For small degrees of undercooling (region A in the
figure) the nucleation rate will be low and the growth rate
moderate. A few crystals will form and grow at a moderate
rate until they run into each other. Because there are few
nuclei, the crystals will be able to grow to relatively large
size, and a coarse grained texture will result. This would be
called a phaneritic texture.
2. At larger degrees of undercooling, the nucleation rate will be
high and the growth rate also high. This will result in many crystals
all growing rapidly, but because there are so many crystals, they
will run into each other before they have time to grow and the
resulting texture will be a fine grained texture. If the sizes of the
grains are so small that crystals cannot be distinguished with a
handlens, the texture is said to be aphanitic.
3. At high degrees of undercooling, both the growth rate and nucleation rate
will be low. Thus few crystals will form and they will not grow to any large
size. The resulting texture will be glassy, with a few tiny crystals called
microlites. A completely glassy texture is called holohyaline texture.
4. Two stages of cooling, i.e. slow cooling to grow a few large crystals,
followed by rapid cooling to grow many smaller crystals could result in a
porphyritic texture, a texture with two or more distinct sizes of grains. Single
stage cooling can also produce a porphyritic texture. In a porphyritic texture,
the larger grains are called phenocrysts and the material surrounding the
phenocrysts is called groundmass or matrix
Grain Size
• In a rock with a phaneritic texture, where all grains are
about the same size, we use the grain size ranges shown
below to describe the texture:
<1 mm
1 - 5 mm
5 - 3 cm
> 3 cm
fine grained
medium grained
coarse grained
very coarse grained
• In a rock with a porphyritic texture, we use the above table to
define the grain size of the groundmass or matrix, and this
table to describe the phenocrysts:
0.03 - 0.3 mm
0.3 - 5 mm
> 5 mm
microphenocrysts
phenocrysts
megaphenocrysts
Fabric
Fabric refers to the mutual relationship between the grains.
 Three types of fabric are commonly referred to:
• 1. If most of the grains are euhedral - that is they are bounded by wellformed crystal faces. The fabric is said to be idomorphic granular.
• 2. If most of the grains are subhedral - that is they bounded by only a
few well-formed crystal faces, the fabric is said to be hypidiomorphic
granular.
• 3. If most of the grains are anhedral - that is they are generally not
bounded by crystal faces, the fabric is said to be allotriomorphic
granular.
Shape
Some common grain shapes are:
•Tabular - a term used to describe grains with rectangular tablet shapes.
•Equant - a term used to describe grains that have all of their boundaries of
approximately equal length.
•Fibrous - a term used to describe grains that occur as long fibers.
•Acicular - a term used to describe grains that occur as long, slender
crystals.
•Prismatic - a term used to describe grains that show an abundance of
prism faces.
Some common terms to describe texture
• Vesicular - if the rock contains numerous holes that were once occupied by a gas
phase, then this term is added to the textural description of the rock.
• Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals,
then the rock should be described as glomeroporphyritic instead of porphyritic.
• Amygdular - if vesicles have been filled with material (usually calcite, chalcedony,
or quartz, then the term amygdular should be added to the textural description of
the rock. An amygdule is defined as a refilled vesicle.
• Pumiceous - if vesicles are so abundant that they make up over 50% of the rock
and the rock has a density less than 1 (i.e. it would float in water), then the rock is
pumiceous.
• Scoraceous- if vesicles are so abundant that they make up over 50% of the rock
and the rock has a density greater than 1, then the rock is said to be scoraceous.
• Graphic - a texture consisting of intergrowths of quartz and alkali feldspar
wherein the orientation of the quartz grains resembles cuneiform writing. This
texture is most commonly observed in pegmatites.
• Spherulitic - a texture commonly found in glassy rhyolites wherein spherical
intergrowths of radiating quartz and feldspar replace glass as a result of
devitrification.
• Obicular - a texture usually restricted to coarser grained rocks that consists of
concentrically banded spheres wherein the bands consist of alternating light
colored and dark colored minerals.
Textures on microscopic examination of igneous rocks
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in
plagioclase. This texture is found in granites.
Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally
surrounded by pyroxene grains. This texture is common in diabases and gabbros.
Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of
pyroxene grains.
Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another
mineral.
Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of
ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.
Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are
occupied by glass or cryptocrystalline material.
Hyaloophitic texture - a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths.
Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass and the groundmass
consists of glass which occupies the tiny interstices between plagioclase grains.
Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage and the interstices
between plagioclase grains are occupied by glass or cryptocrystalline material.
Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal
becoming unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be
noted in the textural description.
Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different
compositions as evidenced by going extinct at angles different from other zones in the crystal.
Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones
of different composition as evidenced by extinction phenomena.
Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase wherein individual plagioclase grains
show an abundance of glassy inclusions.
Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline.
Antiperthitic texture – Exsolution lamellae of orthoclase or microcline occurring in albite.
Classification of Igneous rocks
• Classification of igneous rocks is one of the most confusing aspects of geology.
This is:– partly due to historical reasons,
– partly due to the nature of magmas, and
– partly due to the various criteria that could potentially be used to classify
rocks.
There are various criteria that could be used to classify igneous rocks. Among them are:
1. Minerals Present in the Rock (the mode). The minerals present in a rock and their
relative proportions in the rock depend largely on the chemical composition of the
magma. This works well as a classification scheme if all of the minerals that could
potentially crystallize from the magma have done so - usually the case for slowly cooled
plutonic igneous rocks. But, volcanic rocks usually have their crystallization interrupted
by eruption and rapid cooling on the surface. In such rocks, there is often glass or the
minerals are too small to be readily identified.
2. Texture of the Rock. Rock texture depends to a large extent on cooling history of the
magma. Thus rocks with the same chemical composition and same minerals present could
have widely different textures. In fact we generally use textural criteria to subdivide igneous
rocks in to plutonic (usually medium to coarse grained) and volcanic (usually fine grained,
glassy, or porphyritic.) varieties.
3. Color. Color of a rock depends on the minerals present and on their grain size. Generally,
rocks that contain lots of feldspar and quartz are light colored, and rocks that contain lots of
pyroxenes, olivines, and amphiboles (ferromagnesium minerals) are dark colored.
 But color can be misleading when applied to rocks of the same composition but different
grain size.
 For example granite consists of lots of quartz and feldspar and is generally light colored.
But a rapidly cooled volcanic rock with the same composition as the granite could be entirely
glassy and black colored (i.e. an obsidian).
 Still we can divide rocks in general into felsic rocks (those with lots of feldspar and quartz)
and mafic rocks (those with lots of ferromagnesium minerals). But, this does not allow for a
very detailed classification scheme.
4. Chemical Composition. Chemical composition of igneous rocks is the most distinguishing
•
•
•
•
•
feature.
The composition usually reflects the composition of the magma, and thus provides
information on the source of the rock.
The chemical composition of the magma determines the minerals that will crystallize and
their proportions.
A set of hypothetical minerals that could crystallize from a magma with the same
chemical composition as the rock (called the Norm), can facilitate comparison between
rocks.
Still, because chemical composition can vary continuously, there are few natural breaks
to facilitate divisions between different rocks.
Chemical composition cannot be easily determined in the field, making classification
based on chemistry impractical.
IUGS system of classification:-Plutonic
Olivine
Dunite
90
Ultramafic rocks
Peridotites
Lherzolite
40
Orthopyroxenite
10
Orthopyroxene
Felsic to Intermediate rocks
Mafic rocks
Pyroxenites
Olivine Websterite
10
Websterite
Clinopyroxenite
Clinopyroxene
IUGS system of classification:-Volcanic
Q
60
60
Rhyolite
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
P
10
Tephrite
60
60
(Foid)ites
F
Classification of the pyroclastic rocks. a. Based on type of
material. After Pettijohn (1975) Sedimentary Rocks, Harper &
Row, and Schmidt (1981) Geology, 9, 40-43. b. Based on the
size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287298.
IUGS-chemical classification
• IUGS chemical classification of volcanic rocks (based on total alkalies
[Na2O + K2O] vs. SiO2
Other chemical classification
SiO2 (Silica) Content
•
•
•
•
> 66 wt. % - Acid
52-66 wt% - Intermediate
45-52 wt% - Basic
< 45 wt % - Ultrabasic
Silica Saturation
• Silica Undersaturated Rocks - In these rocks we should find minerals that, in general, do not
occur with quartz. Such minerals are:
Nepheline- NaAlSiO4 Leucite - KAlSi2O6
Forsteritic Olivine - Mg2SiO4
Sodalite - 3NaAlSiO4
Perovskite - CaTiO3
Melanite - Ca2Fe+3Si3O12
Melilite - (Ca,Na)2(Mg,Fe+2,Al,Si)3O7
• Silica Oversaturated Rocks. These rocks can be identified as possibly any rock that does not
contain one of the minerals in the above list.
• Silica Saturated Rocks. These are rocks that contain just enough silica that quartz does not
appear, and just enough silica that one of the silica undersaturated minerals does not appear.
Alumina (Al2O3) Saturation
Three possible conditions exist.
1. Peraluminous rocks containing an excess of Alumina over that required forming
feldspars. This condition is expressed chemically on a molecular basis as:
– Al2O3 > (CaO + Na2O + K2O). In peraluminous rocks we expect to find an Al2O3rich mineral present as a modal mineral - such as muscovite [KAl3Si3O10(OH)2],
corundum [Al2O3], topaz [Al2SiO4(OH,F)2], or an Al2SiO5- mineral like kyanite,
andalusite, or sillimanite. Peraluminous rocks will have corundum [Al2O3] in the
CIPW norm and no diopside in the norm.
2. Metaluminous rocks are those for which the molecular percentages are as follows:
– Al2O3 < (CaO + Na2O + K2O) and Al2O3 > (Na2O + K2O). These are the more
common types of igneous rocks. They are characterized by lack of an Al2O3-rich
mineral and lack of sodic pyroxenes and amphiboles in the mode.
3. Peralkaline rocks are those that are oversaturated with alkalies (Na2O + K2O), and
thus undersaturated with respect to Al2O3. On a molecular basis, these rocks
show:
– Al2O3 < (Na2O + K2O). Peralkaline rocks are distinguished by the presence of Narich
minerals
like
aegerine
[NaFe+3Si2O6],
riebeckite
[Na2Fe3+2Fe2+3Si8O22(OH)2], arfvedsonite[Na3Fe4+2(Al,Fe+3)Si8O22(OH)2],or
aenigmatite [Na2Fe5+2TiO2Si6O18] in the mode.
Alkaline/Subalkaline Rocks
One
last general classification scheme divides rocks that alkaline from
those that are subalkaline.
Note that this criterion is based solely on an alkali vs. silica diagram, as
shown below. Alkaline rocks should not be confused with peralkaline rocks as
discussed above.
While most peralkaline rocks are also alkaline, alkaline rocks are not
necessarily peralkaline.
On the other hand, very alkaline rocks, that are those that plot well above
the dividing line in the figure below, are also usually silica undersaturated.
Diagram showing Alkaline and
Subalkaline division
SEDIMENTARY ROCKS
Sedimentary Rocks
Deposited on or Near Surface of Earth by
Mechanical or Chemical Processes
Sedimentary Rock
Types of Sedimentary Rock
• Clastic (terrigenous or detrital)
–
–
–
–
Conglomerate or Breccia
Sandstone
Siltstone
Shale
• Chemical/biochemical
– Evaporites
– Carbonate sedimentary rocks (limestones and
dolostone)
– Siliceous sedimentary rocks
• Organic (coals)
– Other - ironstones
• Volcanoclastic
Clastic rocks
•
•
•
•
Chemical & Organic rocks
Sandstones
Conglomerates
Breccia
Shale/mudstones
Evaporites rocks
These rocks are formed
due to evaporation of saline
water (sea water)
eg. Gypsum, Halit
(rock salt)
Carbonate rocks
Organic rocks
Form basically from
CaCO3 – both by
chemical leaching and
by organic source
(biochemical)
eg.
Limestone; dolomite
Form due to
decomposition of
organic remains
under temperature
and pressure eg.
Coal/Lignite etc.
CLASTIC ROCKS
• Formed from broken rock fragments weathered and
eroded by river, glacier, wind and sea waves. These
clastic sediments are found deposited on floodplains,
beaches, in desert and on the sea floors.
Classified by:
• Grain Size
• Grain
Composition
• Texture
• Clastic rocks are classified on
the basis of the grain size:
conglomerate, sandstone, shale
etc.
The formation of a clastic sedimentary rock involves three processes:
Transportation- Sediment can be transported by sliding down slopes, being
picked up by the wind, or by being carried by running water in streams,
rivers, or ocean currents. The distance the sediment is transported and
the energy of the transporting medium all leave clues in the final
sediment of the mode of transportation.
Deposition - Sediment is deposited when the energy of the transporting
medium becomes too low to continue the transport process. In other
words, if the velocity of the transporting medium becomes to low to
transport sediment, the sediment will fall out and become deposited. The
final sediment thus reflects the energy of the transporting medium.
Diagenesis - is the process that turns sediment into rock. The first stage of
the process is compaction. Compaction occurs as the weight of the
overlying material increases. Compaction forces the grains closer
together, reducing pore space and eliminating some of the contained
water. Some of this water may carry mineral components in solution, and
these constituents may later precipitate as new minerals in the pore
spaces. This causes cementation, which will then start to bind the
individual particles together. Further compaction and burial may cause
recrystallization of the minerals to make the rock even harder
Sediment Sizes and Clastic Rock Types
Rock Type
Sediment Grain Size
Shale
Clay
less than 0.001 mm
Siltstone
Silt
.001-0.1 mm
Sandstone
Sand
.01-1 mm
Conglomerate Gravel
1mm +
Sedimentary rocks made of silt- and clay-sized
particles are collectively called mudstones, and are
the most abundant sedimentary rocks.
The Udden-Wentworth grain-size scale
Grain-size
(mm)
Sediment
64-256
Cobble
4-64
Pebble
2-4
Granule
1-2
Very Coarse Sand
0.5-1
Coarse Sand
0.25-0.5
Medium Sand
0.125-0.25
Fine Sand
0.6250.125
0.031-0.625
Coarse Silt
Medium Silt
0.0080.016
Fine Silt
Silt
Very Fine Silt
Clay
<0.004
Sand
Very Fine Sand
0.016-0.031
0.0040.008
Gravel
Clay
Clasts (larger pieces, such as sand or
gravel)
• Clasts and matrix
(labelled),
and iron oxide
cement
(reddish brown
color)
Gravel:
Grain size greater than 2 mm
Conglomerates
1. If rounded clasts =
conglomerate
Breccia
2. If angular clasts = breccia
Sandstone
• Sandstones ( > 50% sand-sized (0.062 - 2 mm) clastic grains ):
• Sandstones are classified according to the types of clastic
grains present (quartz, feldspar, & lithic fragments) and the
presence (wackes) or absence (arenites) of significant finegrained matrix material (< 0.03 mm).
• After this subdivision they are described in terms of the types
of preserved sedimentary structures, using terms like crossbedded sandstone and relative maturity using criteria such as
degree of sorting, roundness of the clasts, diversity of clast
types, etc.
Mudstone
Mudstones: ( > 50% silt (0.062 - 004 mm) and / or clay (< 0.004 mm)
• Mudstones are composed of silt-sized quartz and feldspar grains and much smaller
clay mineral particles. Depending of the relative proportions of these two types of
grains, mudstones range from siltstones to shales, and claystones.
• Siltstones can be distinguish from shales and mudstones by biting a piece between
your teeth. If it feels "gritty" then it is a siltstone, if it feels smooth or slick, then it
is a shale or claystone.
• One of the most important features of mud rocks is their colour, an indication of
their oxidation state and the paleo-environment of their deposition:
– Red shales are oxidized and typically represent sub-aerial detritus derived from the
continents. They may represent in sub-aerial deposits, but also are formed by
continental dust settling into organic-poor deep marine environments.
– Green shales are relatively reduced, and common in the shallow submarine
environments depleted in oxygen by the decay of organic matter.
– Black shales are rich in organic matter and highly reduced, typically deposited in anoxic
environments. They sometimes act as source rocks from which oil and gas are released
during burial and diagenesis.
SUMMARY: Characteristics and names of some common clastic sedimentary rocks.
Particle Size
Rock Name
Rock Characteristics
mud
(see below)
Shale
smooth feel, layered appearance
mud
(mud sized particles:
< 0.063 mm)
Mudstone
smooth feel, massive to layer
silt
(silt sized particles:
0.063 - 0.004 mm)
Siltstone
slightly gritty feel, may have
layered appearance
sand
(sand-sized particles:
0.0625-2.0 mm)
Sandstone
granules, pebbles, cobbles,
boulders (granule to
boulder
sized particles:
2 mm - > 256 mm)
Conglomerate
large rounded fragments composed of older rock
materials
granules, pebbles, cobbles,
boulders (granule to
boulder sized particles:
2 mm - > 256 mm)
Breccia
large angular fragments composed of
older rock materials
rough gritty feel, constituent grains clearly visible,
including quartz, feldspar, other minerals, and rock
fragments.
CHEMICAL ROCKS
Carbonate sediments
• Limestone: It is a non-clastic rock
formed either chemically or due to
precipitation of calcite (CaCO3) from
organisms usually (shell). These
remains will result in formation of a
limestone.
•Limestones formed by chemical precipitation are usually fine
grained, whereas, in case of organic limestone the grain size
vary depending upon the type of organism responsible for the
formation
–Fossiliferous Limestone: which medium to coarse grained, as it is
formed out of cementation of Shells.
Three classification schemes are currently used, each with a different emphasis,
but the third, that of Dunham, based on texture, is now used more widely.
A very simple but often useful scheme divides limestones on the basis of grain
size into calcirudite (most grains >2mm), calcarenite (most grains between 2mm
and 62um) and calcilutite (most grains <62um).
The classification scheme of R.L. Folk based mainly on composition,
distinguishes three components: (a) the grains (allochems), (b) matrix, chiefly
micrite and (c) cement, usually drusy sparite. An abbreviations for the grains used
as prefixes are bio- (referring skeletal components), oo- (for ooids), pel- (for
peloids), and intra- (for intraclasts) together with either sparite or micrite,
whichever is the dominant. Terms can be combined if two types of grains are
dominant in a rock, as in biopelsparite, or bio-oosparite. Terms can be modified to
give an indication of coarse grain size, as in biosparrudite, or intramicrudite.
Dunham classification of limestones divides the rocks into: grainstone, grains
without matrix (such as a bio- or oosparite); packstone, grains in contact but with
considerable matrix (e.g. biomicrite); wackstone, grains are floating in a matrix
(could also be a biomicrite); and a mudstone, chiefly micrite with few grains
(<10%). The terms can be qualified to give information on composition, e.g. oolitic
grainstone, peloidal mudstone.
Classification of Limestone, based on depositional texture (after Dunham)
Depositional Texture Recognizable
Depositional
texture not
recognizable
Original components not bound together during
deposition
Contains mud (particles of clay and
Lacks mud
fine silt size)
and is grain
supported
Mud-supported
Mudstone
(Grains<10
%)
Wackstone
(Grains>10
%)
Grainsupported Grainstone
(mudstone<1
%)
Packston
e
Original components
were bound together
during deposition as
shown by intergrown
skeletal
matter,
lamination contrary
to
gravity,
or
sedimentat-floored
cavities that are
roffed
over
by
organic
or
questionably organic
matter and are too
large
to
be
interstices.
Boundstone
Crystalline Carbonates
(subdivided according
to
classifications
designed to bear on
physical texture or
diagenesis)
EVAPORITIC ROCKS
These rocks are formed within the a depositional basin
from chemical substances dissolved in the seawater or
lake water.
Gypsum
CaSO4.2H20
Halite
(NaCl)
SILICEOUS ROCKS
 Chert is the most common chemical siliceous
sediment.
 It is a dense rock composed of one or several
forms of silica (opal, chalcedony, microcrystalline
quartz).
 It occurs either as nodular segregations, mainly
in a carbonate host rock, or as areally extensive
bedded deposits.
 It has a tough, splintery to conchoidal fracture.
 It may be white or variously colored gray, green,
blue, pink, red, yellow, brown, and black.
 Flint (feuerstein) is a term widely used both as a
synonym for chert and as a variety of chert.
Coals
• Delta, continental
environments
• Carbonized Woody Material
• Often fossilized trees, leaves
present
Coal Formation
Oil shale
 The term oil shale has been applied to any rock from
which substantial quantities of oil can be extracted by
heating.
 Lithology is diverse and may include shales, marlstones,
dolomitic limestones and siltstones.
 Normally these rocks are fine textured and laminated.
They range in color from light shades of brown, green to
dark brown, gray or black.
Volcanoclastic Sediments
• Fragmental volcanic rocks formed by any mechanism
or origin emplaced in any physiographic environment
(on land, under water, or under ice), or mixed with
any nonvolcanic fragment types in any proportion are
called Volcaniclastic sediments.
• Volcaniclastic materials exhibit all possible degrees of
sorting. Some are very well sorted and finely
laminated; others are chaotic and unsorted and
contain debris ranging from the finest ash to great
blocks of either cognate or noncognate (accidental)
rocks.
Volcaniclastic sediments (tuffaceous sandstone); Wonchi crater lake
Texture and Structure of Sedimentary rocks
Texture- refers to the size, shape, arrangement of the
grains the make up the rock.
• Clastic- composed of individual fragments that were
transported and deposited as particles.
• Crystalline- results from the in situ precipitation of solid
mineral crystals.
Texture
• Grain size- grain
diameter (boulders,
pebbles, cobbles, sand,
silt, or clay).
• Shape- is described in
terms of sphericity
• Roundness or
(angularity) refers to
the sharpness or
smoothness of their
corners.
Degree of roundness helps in knowing the
distance of transportation (method of erosion)
•Angular clasts- short distance transport from the source
•Rounded clasts- long distance transport
 Analyzing the parameters of clastic rocks, one may realize about the matrix content,
sorting, roundness, and composition of a clastic rock at hand. An important aspect dealing
with such variations is known as the Maturity of the rock. Two types of maturity; textural
and compositional maturities are devised to analyze degree of weathering and, energy level
and persistency of transporting media, during deposition.
 Texturally immature sediments are those with much matrix, poor sorting and angular
grains.
Texturally mature sediments, on the other hand, are characterized by little matrix,
moderate to good sorting and subrounded to rounded grains.
Sediments with no matrix, very good sorting and well-rounded grains are known as
super matured.
Textural maturity in sandstones is largely a reflection of the depositional process,
although it can be modified by diagenetic processes.
Where there has been minimal current activity, the sediments generally are texturally
immature; persistent current or wind activity results in more mature sandstone.
STRUCTURES
 The process of deposition usually imparts variations in layering, bed forms
or other structures that point to the environments in which deposition
occurred. Such things as water depth, current velocity and current direction
can some times be determined from sedimentary structures.
 Thus, it is important to recognize various sedimentary structures to infer
the depositional environments of ancient sediments. In the study of
stratigraphy, especially in the deformed and folded areas, sedimentary
structures are so important to understand which way is up/down, so that able
to determine the sequence of events occurred in the area.
 The structural features that tell us which way is up/down are often referred
to as top and bottom indicators.
1.Stratification and Bedding
A. Layering (bedding): One of the most obvious features of sedimentary rocks is
layering structure or stratification. The layers are evident because of differences in
mineralogy, grain size, degree of sorting, or color of the different layers. In rocks,
these differences may be made more prominent by the differences in resistance to
weathering or color changes brought out by weathering. Layering is usually,
described on the basis of layer thickness as shown in the table below. Distinctive
types of layering are described below.
Bed thicknesses (cm)
>300
100-300
30-100
10-30
3-10
1-3
0.3-1
< 0.3
Massive
Very thickly bedded
Thickly bedded
Medium bedded
Thinly Bedded
Very thinly bedded
Thickly laminated
Thinly laminated
B. Cross bedding: consists of sets of beds that are inclined with respect to one
another. The beds are inclined in the direction that the wind or water was moving at
the time of deposition. Boundaries between sets of cross beds, usually, represent an
erosional surface. Cross bedding is very common in beach deposits, sand dunes, and
river sediments. Individual beds within cross bedded strata are useful indicators of
current direction and to indicate the top and bottom part of the bed. All cross beds
have asymptotic contact in their lower contact on which they were deposited.
Cross bed sets
boundary
Bed
set
Cross
beds
C. Graded bedding: with decreasing of current velocity, larger or more dense particles are
deposited first and followed by smaller particles. If this occurred within a bed, then it
results in the formation of a bed that sows decreasing of grain size, upwards. This structure
is important in determination of tops and bottoms of beds. Commonly, reverse graded
bedding cannot be occurred, as current velocity increased. This is because, as the velocity
of the current increased, it will start to erode the surface of the bed instead of progressive
deposition of coarser materials.
Upward
direction of
the
succession
Graded bed
2. Surface Features
These sedimentary structures are developed on the surface of the beds and
tell us about water currents, wind direction and climate conditions.
A. Ripple marks: Ripple marks are characteristic of shallow water deposition. They
are caused by waves or winds piling up the sediments into long ridges. Based on
their geometry, two types of ripple marks are known: Symmetrical and
Asymmetrical. Asymmetrical ripple marks can give an indication of current
direction of water or wind direction. Symmetrical ripples formed under the
condition when the water moves back and forth. Symmetrical ripple marks typify
standing water with a steady back and forth movement, such as tidal action.
Back and forth movement
of water
Schematic draw of symmetric ripple marks
( repetition of lines is to illustrate their
appearance in 3D)
Current or wind
direction
Asymmetric ripple marks
(nearly vertical in the
windward side, and gentle
slope in the leeward side)
B. Mud cracks: These structures result from the drying out of wet sediments at the surface of
the earth. The cracks are due to shrinkage of the sediments (clays), on drying. In cross section,
the mud cracks tend to curl up, thus becoming a good top/bottom indicator. The presence of
mud cracks indicates that the sediments were exposed to the surface shortly after deposition.
Dallol salt plain
C. Casts and Molds: Any depression formed on the surface of previously deposited sediment,
at the interface of water and sediment, may become a mold for any sediment that come later.
The body of the newly sediment that takes on the shape of the mold is referred to as cast.
Load casts: These are bulbous protrusions that are formed when compaction causes
sediment to be pushed downward in to softer sediments.
Flute casts (Sole marks): Flutes are elongated depressions formed at the surface of the
formerly deposited sediment by current erosion. The flutes form an elongated mold for the
new sediment. Preservation of the overlying sediment as cast resulted in flute casts, which
are some times referred to as sole marks. Flute casts are excellent indicators of current
direction and tops/bottoms of beds.
D. Tracks and Trails: These features result from organisms moving across
the sediment as they walk, crawl, or drag their body parts through the
sediments.
E. Burrow marks: Any organism that burrows in to soft sediment can disturb
the sediment and destroy many of the structures. If burrowing is not
extensive, the holes made by such organisms can later become filled with
water that deposits new sediment in the holes. Burrow marks are also used
to indicate top and bottom parts of beds. If animals were churning up and
intensively burrowed through the sediment, bioturbation may be resulted.
Bioturbation disrupts and even destroys primary bedding and lamination. It
may produce nodularity in the sediment, with subtle grain-size differences
between burrow and surrounding sediments. A color mottling can be
produced by burrowing organisms.
F. Slump folds: formed by down slope mass movement of sediment up on
glide plane involving significant bending of sediment layers.
Depositional Environments of Sedimentary rocks
• Sediments
are formed and accumulated in different places under different
conditions.
• The sedimentary depositional environment describes the combination of physical,
chemical and biological processes took place during the deposition of a particular
type of sediment.
• Determination of depositional environment of sediments is important to
construct the paleogeography and paleoclimatic condition in the study of the
geologic history of an area, and to infer economically potential parts of the basin in
the exploration of hydrocarbons and minerals.
• In most cases, ancient depositional environments are found to be analogous to
the existing sedimentation areas.
Important parameters to determine the depositional environment of sediment
are:
lithologic composition and rock association
texture
sedimentary structures
fossil content
the geometry of the sediment
TYPES OF DEPOSITIONAL
ENVIRONMENTS
Continental Environments
•Alluvial Environment
•Aeolian Environment
•Fluvial Environment
•Lacustrine/ lake/ Environment
•Glacial Environment
Transitional Environments
•Deltaic Environment
•Tidal Environment
•Lagoonal Environment
•Beach Environment
Marine Environment
•Shallow water marine
•Deep water Marine
Rock type
Continental
environments
Marine environments
Mudstones& shale
lakes and river flood-plains
Sandstone
rivers, deserts (eolian)
deep ocean and submarine
fans
deltas, submarine fans
Conglomerate
rivers, glaciers
submarine fans
Breccia
talus slopes
coal-bearing rock
river-bank or near-shore
swamps
Clastic sediments
Chemical sediments
Reef Environment
Limestone
shallow to moderately deep
ocean
deep ocean
Chert
evaporite deposits
salt lakes and inland seas
METAMORPHIC ROCKS
1. Definitions of Metamorphism
The word "Metamorphism" comes from the Greek: meta = change, morph = form,
Therefore, Metamorphism is defined as :
The mineralogical and structural adjustment of solid rocks to physical and
chemical conditions that have been imposed at depths below the near surface
zones of weathering and diagenesis and which differ from conditions under
which the rocks in question originated.
So, Metamorphism, In geology refers to the changes in mineral assemblage and
texture that result from subjecting a rock to conditions such pressures,
temperatures, and chemical environments different from those under which the
rock originally formed.
Metamorphism is characterized by:
Phase changes: - growth of new physically discrete, separable components
(minerals), either with or without (isochemical) addition of new material;
and/or
Textural changes: - recrystallization, alignment and/or grain size, usually as a
result of unequal application of stress.
LIMITS OF METAMORPHISM
• Note that Diagenesis is also a change in form that occurs in sedimentary
rocks. In geology, however, we restrict
diagenetic processes to those which
o
occur at temperatures below 200 C and pressures below about 300 MPa
(MPa stands for Mega Pascals), this is equivalent to about 3 kilobars of
pressure (1kb = 100 MPa).
• Metamorphism
therefore occurs at temperatures and pressures higher than
o
200 C and 300 MPa. Rocks can be subjected to these higher temperatures
and pressures as they are buried deeper in the Earth. Such burial usually
takes place as a result of tectonic processes such as continental collisions or
subduction.
• The upper limit of metamorphism occurs at the pressure and temperature
where melting of the rock in question begins. Once melting begins the
process changes to an igneous process rather than a metamorphic process.
Factors that Control Metamorphism
TEMPERATURE
 Temperature increases with depth in the Earth along the Geothermal Gradient. Thus
higher temperature can occur by burial of rock. Temperature can also increase due to
igneous intrusion.
PRESSURE
 Pressure increases with depth of burial, thus, both pressure and temperature will vary
with depth in the Earth. Pressure is defined as force acting equally from all directions. It is
a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from
all directions, then the stress is called a differential stress.
FLUID PHASE
 Any existing open space between mineral grains in rocks can potentially contain a fluid.
This fluid is mostly H2O, but contains dissolved mineral matter. The fluid phase is
important because chemical reactions that involve one solid mineral changing into
another solid mineral can be greatly speeded up by having dissolved ions transported by
the fluid. Within increasing pressure of metamorphism, the pore spaces in which the fluid
resides is reduced, and thus the fluid is driven off. Thus, no fluid will be present when
pressure and temperature decrease and, as discussed earlier, retrograde metamorphism
will be inhibited.
TIME
 The chemical reactions involved in metamorphism, along with recrystallization, and
growth of new minerals are extremely slow processes. Laboratory experiments suggest
that the longer the time available for metamorphism, the larger are the sizes of the
mineral grains produced. Thus, coarse grained metamorphic rocks involve long times of
metamorphism. Experiments suggest that the time involved is millions of years.
Mineral Asseemblage/Paragenesis
Minerals those possessing the lowest chemical potential energy under the
conditions of metamorphism are said to be in equilibrium.
 These equilibrium minerals referred to simply as Mineral assemblage or Mineral
paragenesis.
 Mineral assemblages will be written as lists of mineral names separated by plus
signs, thus: A+B+C
If a rock is to be regarded as having an equilibrium mineral assemblage are
as follows:
 Each mineral in the assemblage list must have a boundary somewhere in the
rock with all the other members.
 The
texture must be of a type thought to have formed by metamorphic
recrystallization, not by fragmentation during dynamic metamorphism or igneous
crystallization from a melt.
 The minerals must not show compositional zoning.
 The minerals must not show obvious replacement textures such as reaction
rims or alteration along cracks.
Reflection!
• How do you distinguish Sedimentary process
from Metamorphic process?
• What is the critical factor in determining the
upper limit of metamorphism?
116
The difference between diagenetic (sandstone),
and metamorphic (quartzite) texture is exemplifies
By the development of grain boundaries at the latter.
Sandstone
texture
Quartzite
texture
117
The critical factor in determining the upper limit of
metamorphism is the presence of water.
 If water present (hydrous condition) the metamorphism
ceased at low temperature (600-800c), Where melting will
start.
 If water absent (anhydrous
metamorphism exceeds up to 1000c.
condition),
the
118
2. Types of Metamorphism
Contact Metamorphism
Regional Metamorphism
Cataclastic Metamorphism
Hydrothermal Metamorphism
Burial Metamorphism
Shock (impact) Metamorphism
Contact Metamorphism
•
occurs adjacent to igneous intrusions and results
from high temperatures associated with the
igneous intrusion.
•
Since only a small area surrounding the intrusion is
heated by the magma, metamorphism is restricted
to the zone surrounding the intrusion, called a
metamorphic or contact aureole.
•
The grade of metamorphism increases in all
directions toward the intrusion. Because the
temperature contrast between the surrounding
rock and the intruded magma is larger at shallow
levels in the crust where pressure is low.
•
contact metamorphism is often referred to as high
temperature, low pressure metamorphism.
•
The rock produced is often a fine-grained rock that
shows no foliation, called a hornfels.
Regional Metamorphism
 Occurs over large areas and generally does not show any relationship to igneous
bodies.
 Most regional metamorphism is accompanied by deformation under non-hydrostatic or
differential stress conditions.
 Thus, regional metamorphism usually results in forming metamorphic rocks that are
strongly foliated, such as slates, schist, and gneisses.
 Regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in
eroded mountain ranges.
 Compressive stresses result in folding of rock and thickening of the crust, which tends
to push rocks to deeper levels where they are subjected to higher temperatures and
pressures.
Cataclastic Metamorphism
 Occurs as a result of mechanical deformation, like when two bodies of
rock slide past one another along a fault zone.
 Heat is generated by the friction of sliding along such a shear zone, and
the rocks tend to be mechanically deformed, being crushed and
pulverized, due to the shearing.
 is not very common and is restricted to a narrow zone along which the
shearing occurred.
Hydrothermal Metamorphism
Rocks that are altered at high temperatures and
moderate pressures by hydrothermal fluids.
This is common in basaltic rocks that generally lack
hydrous minerals.
The hydrothermal metamorphism results in
alteration to Mg-Fe rich hydrous minerals such as
talc, chlorite, serpentine, actinolite, tremolite,
zeolites, and clay minerals.
Rich ore deposits are often formed as a result of
hydrothermal metamorphism.
Burial Metamorphism
 When sedimentary rocks are buried to depths of
several hundred meters, temperatures greater than
300oC may develop in the absence of differential
stress. New minerals grow, but the rock does not
appear to be metamorphosed.
 The main minerals produced are often the Zeolites.
 Overlaps, to some extent, with diagenesis, and
grades into regional metamorphism as temperature
and pressure increase.
Shock Metamorphism (Impact Metamorphism)
 When an extraterrestrial body, such as a meteorite or
comet impacts with the Earth or if there is a very
large volcanic explosion,
 Ultrahigh pressures can be generated in the
impacted rock.
These ultrahigh pressures can
produce minerals that are only stable at very high
pressure, such as the SiO2 polymorphs coesite and
stishovite,
 Produce textures known as shock lamellae in
mineral grains, and such textures as shatter cones in
the impacted rock.
3. Grade of Metamorphism
Metamorphic grade is a general term for describing the relative
temperature and pressure conditions under which
metamorphic rocks form.
• Low-grade metamorphism takes place at temperatures
between about 200 to 320oC, and relatively low pressure. Low
grade metamorphic rocks are generally characterized by an
abundance of hydrous minerals. With increasing grade of
metamorphism, the hydrous minerals begin to react with
other minerals and/or break down to less hydrous minerals.
• High-grade metamorphism takes place at temperatures
greater than 320oC and relatively high pressure. As grade of
metamorphism increases, hydrous minerals become less
hydrous, by losing H2O, and non-hydrous minerals become
more common.
4. Metamorphic facies
 The
mineral assemblages that are observed must be an indication of the
temperature and pressure environment that the rock was subjected to. This pressure
and temperature environment is referred to as metamorphic Facies. (This is similar to
the concept of sedimentary facies, in that a sedimentary facies is also a set of
environmental conditions present during deposition).
 The sequence of metamorphic facies observed in any metamorphic terrain,
depends on the geothermal gradient that was present during metamorphism (see fig.
below).
 A high geothermal gradient might be present around an igneous intrusion, and
would result in metamorphic rocks belonging to the hornfels facies (Low P, High T).
 Under a normal geothermal gradient, would progress from zeolite facies to
greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or
depth of burial) increased (Medium P, Medium T).
 If a low geothermal gradient was present, then rocks would progress from zeolite
facies to blueschist facies to eclogite facies (High P, Low T).
 Thus, if we know the facies of metamorphic rocks in the region, we can determine
what the geothermal gradient must have been like at the time the metamorphism
occurred.
Facies
Prehnitepumpellyite
Zeolite
Greenschist
Epidoteamphibolite
Amphibolite
Granulite
Pyroxene hornfels
Sanidinite
Glaucophane schist
Eclogite
Typical mineral assemblages in basic
igneous rocks
(with relict igneous plagioclase and
clinopyroxene)
smectite + zeolite (with relict igneous
plagioclase)
chlorite + actinolite + albite + epidote
+ quartz
hornblende + epidote albite +
almandine garnet + quartz
hornblende + andesine garnet +
quartz
clinopyroxene + labradorite +
orthopyroxene + quartz
clinopyroxene + labradorite + quartz
clinopyroxene + labradorite + Quartz
glaucophane + lawsonite + quartz
pyrope-garnet + omphacite and
clinopyroxene)
Typical mineral assemblages in pelitic
rocks
not defined
not defined
chlorite + muscovite + chloritoid +
quartz
almandine garnet + chlorite +
muscovite+ biotite + quartz
garnet + biotite + muscovite +
sillimanite + quartz
garnet + cordierite + biotite +
sillimanite + quartz
cordierite + andalusite + biotite +
quartz
sanidine + sillimanite + hypersthene +
cordierite + quartz
muscovite + chlorite + spessartine
garnet + quartz
not known
Medium
pressure and
Medium
temperature
Low pressure
and High
temperature
High pressure
and Low
temperature
Metamorphism and Plate Tectonics
 At present, the geothermal gradients observed are strongly affected by plate tectonics.
 Along zones where subduction is occurring, magmas are generated near the subduction


zone and intrude into shallow levels of the crust. Because high temperature is brought
near the surface, the geothermal gradient (region A in Fig. below), in these regions
becomes high and contact metamorphism (hornfels facies) results.
Because compression occurs along a subduction margin (the oceanic crust moves toward
the volcanic arc) rocks may be pushed down to depths along either a normal or slightly
higher than normal geothermal gradient. Actually the geothermal gradient is likely to be
slightly higher, because the passage of magma through the crust will tend to heat the crust
somewhat. In these regions (region B in Fig. below), we expect to see greenschist,
amphibolite, and granulite facies metamorphic rocks.
Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great
depths. This results in producing a low geothermal gradient (temperature increases slowly
with depth). This low geothermal gradient results in metamorphism into the blueschist and
eclogite facies (region C in Fig. below).
5. Classification of Metamorphic rocks
• Metamorphic rock names are commonly derived
utilizing any one, or a combination of the
following criterion (Yardley, 1989):
1) The nature of the parent material (bulk
composition)
2) The metamorphic mineralogy
3) The rock's texture (grain size and fabric
development)
4) Any appropriate special name
I. Classification of metamorphic rocks based on the nature of the
parent material (bulk composition)
Parent Material
Rock type
Argillaceous/clay-rich
sediments
(lutites)
Arenaceous (predominately sandsize) sediments
Pelites
Clay-sand mixtures
Quartz-sand (quartz arenite)
(lime muds)
Limestone or dolostone
Psammites
Semi-pelite
Quartzite
Calc-silicate/calcareous
Marble
Volcanics (basalt, andsite, rhyolite, Metavolcanics (metabasite
etc.)
(metandesite….etc.)
Ultramafics
Metaultramafics
Cont’d
•
•
•
•
•
•
•
Pelitic. These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks. Because
of their high concentrations of alumina they are recognized by an abundance of aluminous minerals,
like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.
Quartzo-Feldspathic. Rocks that originally contained mostly quartz and feldspar like granitic rocks and
arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since
these minerals are stable over a wide range of temperature and pressure. Those that exhibit mostly
quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.
Calcareous. Calcareous rocks are calcium rich. They are usually derivatives of carbonate rocks,
although they contain other minerals that result from reaction of the carbonates with associated
siliceous detrital minerals that were present in the rock. At low grades of metamorphism calcareous
rocks are recognized by their abundance of carbonate minerals like calcite and dolomite. With
increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich
biotite), chlorite, and tremolite. At even higher grades anhydrous minerals like diopside, forsterite,
wollastonite, grossularite, and calcic plagioclase.
Basic. Just like in igneous rocks, the general term basic refers to low silica content. Basic metamorphic
rocks are generally derivatives of basic igneous rocks like basalts and gabbros. They have an
abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like
plagioclase and epidote.
Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian. Such rocks would
contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite. In general, such rocks
usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.
Ferruginous. Rocks that are rich in Fe with little Mg are termed ferruginous. Such rocks could be
derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals
like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite,
hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine
garnet at higher grades.
Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed
manganiferrous. They are characterized by such minerals as Stilpnomelane and spessartine.
II. Textural and Fabric development
TEXTURAL CLASSIFICATION
The textures are used to differentiate the relative timing of crystal growth and deformation.
Below is a list of common textures. Note: Many of the terms for metamorphic textures contain
the suffix "-blastic ".
Terms related to crystals: shape, orientation, and content:
Porphyroblast: a mineral that is larger than its neighbors which grew in the solid
state.
Porphyroblast
– Idioblast: euhedral (well developed crystal faces) porphyroblast
– Xenoblast: anhedral (poorly developed crystal faces) porphyroblast
Granoblastic polygonal: a texture in which all grains are about the same size
and have planar boundaries intersecting at approximately 120 degrees.
Granoblastic
Poikiloblastic: a texture produced when a growing crystal face has enveloped
inclusions from its surroundings.
Poikiloblastic
134
Replacement textures:
Corona: a ring of one or more minerals around another mineral or
structure, formed by reaction with its surroundings.
Pseudomorph: produced when one or more minerals replaces
another mineral while retaining its crystal shape.
Symplecitite: intergrowth between minerals
Terms related to deformation and timing of recrystallization:
Relict: A texture of mineral (porphyroclasts) that is inherited from
unmetamorphosed rock or from another metamorphic grade (e.g.,
bedding).
Porphyroclast
Helicitic: applies to porphyroblasts or porphyroclasts possessing
internal foliations (Si) that are curved.
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METAMORPHIC FABRIC
Grain Size
Fine
Medium
Coarse
granofels
marble
quartzite
amphibolite
granofels
marble
quartzite
amphibolite
Poorly Foliated
hornfels
Well Foliated
slate, phyllite
schist
gneiss
Well Foliated and Sheared
mylonite
mylonite schist
augen gneiss
Poorly to non-foliated rocks
Quartzite
Marble
Hornfels
Hornfels: contact metamorphic rock; tough rock with a random
fabric of interlocking grains
Geological Survey of Ethiopia
137
Foliated texture
Name
Description
Slate
strongly cleaved rock
-cleavage planes pervasively developed through rock due to orientation of fine phyllosilcate grains
-individual grains too fine to be visible with naked eye
-overall dull appearance
Phyllite
similar to slate, but slightly coarser phyllosilicate grains
-grains sometimes discernible in hand specimen, giving silk appearance to cleavage surfaces
-often cleavage planes less perfectly planar than slates
Schist
-parallel alignment of moderately coarse grains (fabric=schistosity)
-grains usually cle4arly visible by eye
-grains composed of phyllosilicates and other minerals such as hornblende, actinolite, kyanite
Gneiss
coarse grained rock (grain size several millimeters) and
-foliated (planar fabric: either schistosity or compositional layering)
-tendency for different minerals to segregate into layers parallel to foliation (gneissic layering):
typically quartz and feldspar rich layers tend to separate from micaceous layers.
Varieties:
--Orthogneiss: igneous parentage
-- paragneiss: metasedimentary gneisses
Mylonite
fine grained rock produced in intense ductile deformation
-pre-existing grains are deformed and re-crystallized as finer grains
Geological Survey of Ethiopia
Slate
Schist
Phyllite
Mylonite
Gneiss
Geological Survey of Ethiopia
III. Mineralogical classification
• The most distinguishing minerals are used as a prefix to a
textural term.
• Thus, a schist containing biotite, garnet, quartz, and feldspar,
would be called biotite-garnet schist.
• A gneiss containing hornblende, pyroxene, quartz, and feldspar
would be called hornblende-pyroxene gneiss. A schist containing
porphyroblasts of garnet would be called garnet porphyroblastic
schist.
• If a rock has undergone only slight metamorphism such that its
original texture can still be observed then the rock is given a
name based on its original name, with the prefix meta- applied.
For example: metabasalt, metagraywacke, meta-andesite,
metagranite.
IV. Special metamorphic rocks
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Amphibolites: These are medium to coarse grained, dark colored rocks whose principal
minerals are hornblende and plagioclase. They result from metamorphism of basic igneous
rocks. Foliation is highly variable, but when present the term schist can be appended to
the name (i.e. amphibolite schist).
Marbles: These are rocks composed mostly of calcite, and less commonly of dolomite.
They result from metamorphism of limestones and dolostones. Some foliation may be
present if the marble contains micas.
Eclogites: These are medium to coarse grained consisting mostly of garnet and green
clinopyroxene called omphacite, that result from high grade metamorphism of basic
igneous rocks. Eclogites usually do not show foliation.
Quartzites: Quartz arenites and chert both are composed mostly of SiO2. Since quartz is
stable over a wide range of pressures and temperatures, metamorphism of quartz arenites
and cherts will result only in the recrystallization of quartz forming a hard rock with
interlocking crystals of quartz. Such a rock is called a quartzite.
Serpentinites: Serpentinites are rocks that consist mostly of serpentine. These form by
hydrothermal metamorphism of ultrabasic igneous rocks.
Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock
a greasy feel, similar to that of soap. Talc is an Mg-rich mineral, and thus soapstones from
ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by
hydrothermal alteration.
Skarns: Skarns are rocks that originate from contact metamorphism of limestones or
dolostones, and show evidence of having exchanged constituents with the intruding
magma. Thus, skarns are generally composed of minerals like calcite and dolomite, from
the original carbonate rock, but contain abundant calcium and magnesium silicate minerals
like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by
reaction of the original carbonate minerals with silica from the magma. The chemical
exchange is that takes place is called “Metasomatism”.
Cont’d
• Mylonites: Mylonites are cataclastic metamorphic rocks that
are produced along shear zones deep in the crust. They are
usually fine-grained, sometimes glassy, that are streaky or
layered, with the layers and streaks having been drawn out by
ductile shear.
• Migmatites: a mixed rock of schistose or gneissic portion
intimately mixed with veins of apparently quartzo-feldspathic
material (known as leucosomes). Migmatites and its related
terms are best reserved for regional field studies and should
not be used in hand specimen descriptions.
Agmatitic
Vein-type
Nebulitic
Philibitic
At high-temperatures, rocks undergo partial
melting, and the yielded rock is known as
Migmatites.
Stromatic
The leucocratic (granitic) materials are well
known as leucosomes, while the metamorphic
parts are known as mesosome (resistite) and
melansomes.
Schilleren
Folded
Migmatitic structures on outcrop
6. Structure of Metamorphic rocks
 If
differential stress is present during metamorphism, it can have a profound
effect on the texture of the rock.
 Rounded grains can become flattened in the direction of maximum
compressional stress. Minerals that crystallize or grow in the differential stress field
may develop a preferred orientation.
 Sheet silicates and minerals that have an elongated habit will grow with their
sheets or direction of elongation orientated perpendicular to the direction of
maximum stress.
This
is because growth of such minerals is easier along directions parallel to
sheets or along the direction of elongation and thus will grow along 3 or 2,
perpendicular to 1.
The type of structures formed during metamorphism is represented as follows:
•Slates/phyllites form at low metamorphic grade by the growth of fine grained chlorite
and clay minerals. The preferred orientation of these sheet silicates causes the rock to
easily break planes parallel to the sheet silicates, causing a slatey cleavage.
•Schist - The size of the mineral grains tends to enlarge with increasing grade of
metamorphism. Eventually the rock develops a near planar foliation caused by the
preferred orientation of sheet silicates (mainly biotite and muscovite). Quartz and
feldspar grains however show no preferred orientation. The irregular planar foliation at
this stage is called schistosity
•Gneiss As metamorphic grade increases, the sheet silicates become unstable and
dark colored minerals like hornblende and pyroxene start to grow.
These dark colored minerals tend to become segregated into distinct bands through
the rock (this process is called metamorphic differentiation), giving the rock a gneissic
banding. Because the dark colored minerals tend to form elongated crystals, rather
than sheet- like crystals, they still have a preferred orientation with their long directions
perpendicular to the maximum differential stress.
•Granulite - At the highest grades of metamorphism most of the hydrous minerals and
sheet silicates become unstable and thus there are few minerals present that would
show a preferred orientation. The resulting rock will have a granulitic texture that is
similar to a phaneritic texture in igneous rocks.
In general, the grain size of metamorphic rocks tends to increase with increasing grade
of metamorphism, as seen in the progression form fine grained shales to coarser (but
still fine) grained slates, to coarser grained schists and gneisses.
Structural development in metamorphic rocks.