Download Mineralogy and Crystals

Earth Sciences: 625-102 Geology
Course Presenters
Dr Stephen
Gallagher
Prof Andrew
Gleadow
Dr Malcolm
Wallace
Prof Ian
Plimer
Course
Coordinator
Room 214
Earth Materials
Earth Materials Series - Lecture Topics
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1. Mineralogy and Crystals
2. Atomic Structures of Minerals
3. Silicate Mineralogy
4. Igneous Rocks
5. Volcanoes and volcanic rocks
6. Igneous Intrusions
7. Metamorphism
8. Contact and Regional Metamorphism
9. Conditions of Metamorphism
Earth Materials
625-102 GEOLOGY
Lecture 1
Mineralogy
and Crystals
Melbourne EarthEarth
Sciences
Materials
Minerals and Rocks
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MINERALS - are naturally occurring inorganic
chemical compounds with a specific internal
structure and regular chemical composition
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ROCKS - are natural mixtures or aggregates of
minerals
1-01
Earth Materials
Minerals:
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are mostly crystalline with a regular atomic
structure, eg Quartz, Feldspar
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are the major solid constituents of the Earth
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have physical properties which reflect their
composition and atomic structure
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Silicates are the most common minerals
(95% of the earth’s crust)
1-02
Earth Materials
Ions and Atomic Bonding
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Ions are atoms with an excess positive or
negative charge due to loss or gain of electrons
An Ionic Bond is formed by the attraction of
oppositely charged ions
– eg between Na+ and Cl- in NaCl (Halite)
A Covalent Bond is formed where two atoms
share an electron
– eg between Si4+ and O2- in the silicate unit SiO4
Most minerals share both Ionic and covalent
characteristics
Earth Materials
Physical Properties 1
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Crystal form
1-03
– the characteristic shape of a crystal
– reflects the internal arrangement of its atoms
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Cleavage
– the tendency for a mineral to split along a particular
plane
– cleavage occurs along layers of weak bonds
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1-04
1-05
1-06
Fracture
– appearance of broken surface when no cleavage is
present
– eg Quartz has a Conchoidal fracture (‘
(‘shell-like’
shell-like’)
1-07
Earth Materials
Physical Properties 2
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Hardness
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the resistance of a mineral to abrasion
reflects overall strength of the atomic bonds
Estimated by scratching one against another
Mohs Hardness Scale
Density
– the mass per unit volume of a mineral
– controlled both by the kinds of atoms present and
especially how closely packed they are
Earth Materials
Physical Properties 3
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Colour
1-08
– an obvious but variable property of a mineral
– influenced by small variations in the composition
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Streak
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colour of a powdered sample of a mineral
seen by scratching on an unglazed porcelain plate
usually more diagnostic than colour
Lustre
1-09
– intensity and quality of light reflected from a mineral surface
– Described as metallic, glassy, earthy, pearly etc
Earth Materials
Crystals
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Crystals are solid materials naturally
bounded by flat faces meeting at sharp edges
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Crystalline shapes reflect the regular
arrangement of their constituent atoms
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This arrangement is called the Crystal Lattice
Earth Materials
Crystal Symmetry
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Crystal symmetry
– Crystals are usually described in terms of their
Symmetry
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1-10
Symmetry elements
– Symmetry is expressed by reference to:
– rotation axes
– mirror planes
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Rotation axes
– Axes about which a crystal can be rotated so that it
occupies an exactly similar position more than
once every rotation. These may be:
– Two-fold (Diad
), Three-fold (Triad), Four-fold
(Diad),
(Tetrad) or six-fold (Hexad)
Earth Materials
Axes of Symmetry
4-fold axes (3)
2-fold axes (6)
3-fold axes (4)
Earth Materials
Mirror Planes
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Planes of Symmetry (mirror planes)
– Divide a crystal into two parts which are exact mirror
images of each other
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eg a Cube has 9 planes
Type 2 (3)
Type 1 (6)
Earth Materials
Crystal Systems
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There are just seven different ways in which
symmetry elements can be combined
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These define seven different Crystal Systems
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All crystals belong to one of these crystal systems
Earth Materials
Seven Crystal Systems
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Cubic System
Tetragonal
Hexagonal
Trigonal
Orthorhombic
- four Triad axes
- a single Tetrad axis
- a single Hexad axis
- a single Triad axis
or
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Monoclinic
and/or
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Triclinic
- three mutually perpendicular diad axes
- two perpendicular mirror planes and
one diad
- a single diad axis
- a single plane of symmetry
- no planes or axes of symmetry
Earth Materials
REFERENCES
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Hamblin & Christiansen, Chapter 3, p.48-61
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Skinner and Porter, Chapter 3, p. 47-53
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Clark and Cook, Chapter 6a, 6b, 6c
Earth Materials