BAG 101:
PHYSICAL ENVIRONMENTAL SYSTEMS
Dr. K. Chatterjea
Lecture Topic: MINERALS
Minerals are the basic ingredients of the earth’s solid crust. They have a
homogeneous constitution throughout. Several minerals together form a rock.
Minerals must:
 be naturally formed.
 solid
 have specific chemical composition
 have characteristic crystal structure.
Properties of minerals
These are determined by composition and crystal structure. The properties most often
used to identify minerals are: colour, external shape, hardness, as well as lustre,
cleavage, and specific gravity.
Crystals in minerals: form when mineral grains can grow freely in an open,
unobstructed space. Uncommon.
Cleavage: the tendency of the mineral to break in preferred directions along shiny
surfaces.
A crystal face is a growth surface and a cleavage face is a breakage surface.
Lustre: the quality and intensity of the light reflected from the mineral.
Colour: often a striking property but not a very reliable means of identification.
Streak: can be misleading, but often used.
Hardness: refers to the relative resistance of a mineral to scratching. This is governed
by crystal structure and by the strength of the bonds between atoms.
A scale called Moh’s relative hardness scale is used.
Common minerals:
Scientists have identified about 3500 minerals.
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Only 12 (referred to as the abundant elements) make up 99.23% of the crust
mass.
Silicates are the most abundant of all naturally occurring, inorganic compounds.
QUARTZ: composed exclusively of silicon and oxygen. Forms six-sided crystals.
Found in beautiful colours that come from minute amounts of iron, aluminium,
titanium etc.. Occurs in Igneous, Sedimentary, and Metamorphic rocks.
Certain specimens of quartz are formed by precipitation from cool water solutions.
Fine-grained. Common name: Chalcedony. Examples: Agate, if coloured bands are
present. Jasper, if uniformly red. Flint, if uniformly grey.
Quartz has been identified as one of the main
constituents of BARNACLE BILL!!
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FELDSPAR: 60% of all minerals on the crust. Together with Quartz constitutes
about 75% of the volume of the continental crust. Feldspar is also abundant on the
seafloor.
MICA: Muscovite, Biotite: Common in Igneous and Metamorphic rocks.
Carbonate, Phosphate, and Sulphate minerals:
CALCITE/ DOLOMITE: common minerals. Look very similar. Common in
sedimentary rocks.
APATITE: most important phosphate. Apatite is the substance from which our bones
and teeth are made. Found commonly in Igneous and Sedimentary rocks.
The Ore Minerals: Sulphides, Oxides:
PYRITE is a common sulfide mineral.
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MAGNETITE, HEMATITE, CASSITERITE are common oxides.
Minerals contain within their make-up information about how they are formed.
Therefore, they hold keys to the conditions under which they are formed.
Lecture Topic: ROCKS
Rocks are aggregates of minerals. They are also documents that record events far back
in Earth’s history. Rocks in the visible part of the Earth are divided into three classes:
Igneous rocks: formed by the cooling of magma. Examples: Granite, Basalt
Sedimentary rocks: formed through the interaction of the atmosphere, water and the
pre-existing rocks.
Metamorphic rocks: these are igneous or sedimentary rocks that have been subjected
to considerable heat and pressure so that the component minerals have been
recrystallized.
IGNEOUS ROCKS
These are formed from solidified magma.
 Magma is characterised by a range of composition: silica is predominant.
 Magma is characterised by high temperatures.
 Magma has the properties of a liquid.
 There are three types of magma: basaltic (80%), andesitic (10%), rhyolitic (10%)
 Small amounts of gas are dissolved in magma: they strongly influence the
properties of magma
 Temperature: difficult to measure!! 10000 - 12000C or more!!
 Viscosity of magma depends on temperature and composition.
 Magma could be erupted in explosive or non-explosive manner.
Extrusive and Intrusive rocks
Texture - size of individual mineral grains
Intrusive: coarse-grained
Extrusive: fine-grained
Phenocrysts: formed from partly solidified magma that moved quickly upwards.
Obsidian: cooled too fast: extremely fine-grained
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Pegmatite: intrusive rock that contains grains larger than 2cm in dia.
Mineral assemblage;
Most important minerals present:
quartz, feldspar, mica, amphibole, pyroxene, olivine.
BOWEN’S REACTION SERIES
Varieties of Igneous Rocks:
Granite and Granodiorite : Feldspar and quartz are the chief minerals.
Granite contains potassium-feldspar.
Granodiorite contains plagioclase feldspar.
Diorite: Plagioclase is the main mineral. Quartz and mica are absent.
Gabbro and Peridotite: Dark-coloured, coarse-grained.
Varieties of Extrusive Rocks:
Rhyolite: Fine-grained, close chemical kinship to granite.
Andesite: Equivalent in composition to Diorite. It is a typical rock around the world.
Basalt: Dominant rock of the oceanic crust. Very fine-grained.
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Lecture Topic: SEDIMENTARY ROCKS
Sediments are continuously being carried away and deposited on the surface of the
earth. We find sediments everywhere.
Stratification: characteristic of sedimentary rocks, results from the arrangement of
the sedimentary particles in layers.
The layered arrangement is called bedding. The top and bottom surfaces of each
bedding are called bedding planes.
Sediments are derived from broken-up rocks as well as altered rock particles. These
are called clastic sediments. They could be transported by various agents of transport:
rivers, glaciers, wind etc.
Some conspicuous features of clastic sediments:
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sorting: sediments may be sorted according to their particle sizes or they may be
all mixed up. This usually throws light on the mode of transportation.
particle shape: also provides clues to the mode of transportation.
rhythmic layering:
cross- bedding:
graded bedding:
Some common sedimentary rocks
Clastic sedimentary rocks are classified on the basis of their predominant particle
size. Four basic classes are: conglomerate, sandstone, siltstone, and shale.
Conglomerate:
Sandstone:
Shale:
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Biogenic sedimentary rocks: result from the lithification of biogenic sediment
(limestone) or sediments having a high organic component (coal).
Limestone: formed chiefly of calcite.
Coal: formed of decomposed plant matter.
Sedimentary rocks provide clues to the depositional environments.
Ripple marks, mud cracks, raindrop marks, foot prints, fossils.
Watson, do you think the
tyrannosaurus fractured its leg
in the fight and dragged its left
foot as it went into the lake for
a drink???
Lecture Topic: Metamorphic Rocks
Metamorphism refers to all changes in mineral assemblage and rock texture that
takes place in rocks within the earth’s crust as a result of changes in temperature and
pressure.
Metamorphism refers to changes only in solid rocks.
Temperature: above 2000C
Pressure: in excess of 300 Mpa
Any type of rock could be metamorphosed. Resultant rock is changed in appearance
and structure. Minerals are reconstituted, recrystallised.
Shale ----------> Slate ----------> Schist
Granite -------> Schist --------> Gneiss
Limestone ------------> Marble
Sandstone ------------> Quartzite
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Kinds of metamorphism:
Contact metamorphism:
Regional metamorphism:
Suggested readings for the Topics on Minerals and Rocks:
1.
Skinner B.J. and S.C. Porter (1995), The Dynamic Earth, an Introduction
to Physical Geography, Pages 54 - 70; 94 - 98; 114 - 130, Chapter 5 selected parts.
This book is available in the Library
2.
Strahler A.H. and A.N. Strahler (1992) Modern Physical Geography,
Chapter 12.
3.
Check in your regular books, too. Try to find out for yourself first. If you
can't --- well, you know my office, don't you?
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