50427699 mineral geology

EARTH MATERIALS 2.07

Professor Peter Doyle




Course aims
• Aims to understand:
– the basic nature of the principal rock forming
minerals
– Igneous, sedimentary and metamorphic rocks

• Provides:
– the means of identifying & differentiating minerals
and rocks in hand specimen

• Practicals:
– Provide experience of handing and describing
geological materials


Reading and Assessment
Reading:
• Press, F. & Siever, R. 1994. Understanding Earth,
Freeman

Assessment:
• Practical file (worth 20% overall, to include all
practicals)
• Examination (80%), with pass mark of 40% overall.

Course outline
Part 1: Minerals
• Session 1: Introduction to minerals & crystal structures
• Session 2: Use of physical properties in the identification of
minerals in hand specimens
• Session 3: Silicate minerals: their nature and general properties
• Session 4: Non-silicate minerals their nature and general
properties

Part 2: Rocks
• Session 5: Introduction to rocks and the rock cycle: igneous
rocks
• Session 6: Sedimentary rocks – Clastics
• Session 7: Sedimentary rocks – Carbonates
• Session 8: Metamorphic rocks – general characteristics USE IN
THE PRACTICALS


MINERALS
•Minerals are naturally occurring, homogeneous,
crystalline solid with a definite chemical
composition
Halite – NaCl

Biotite – K2(Mg,Fe2+)4 (Fe3+,Al,)2(Si6 Al2 O20)
(OH,F)4

•Minerals usually form by inorganic processes but

some may be biogenic in origin


Quartz: primary rock forming mineral, simple silicate


Olivine: primary rock forming mineral, complex silicate


Other Minerals: Pyrite,
Pyrite an iron sulphide


Biogenic minerals: Modern shell debris composed of
carbonate & silica minerals


ROCKS
• A rock is:
• a naturally formed aggregate of mineral matter
constituting a significant part of the Earth's crust.
• Rocks can be consolidated or non-consolidated
• Rocks can be monomineralic or an aggregate
of mineral species
• Rocks usually form by inorganic processes but
some may be biogenic in origin


Granite: polished section showing aggregate of four
interlocking and different coloured minerals

Crushed rocks

Cement matrix

Concrete: non-natural aggregate of other rocks set in a
matrix


Coal - biogenic

Evaporite - inorganic

Limestone - biogenic

Biogenic and
inorganic
sedimentary rocks


Part 1: Minerals
• A mineral is:
• A naturally occurring, crystalline solid with a
definite chemical composition
• Structurally homogeneous

– Atomic structure is continuous and
constant throughout the mineral structure
– Mineral structure expressed as the Unit

Cell


The Unit Cell
• Unit Cell is the smallest 3D repeating unit of a
crystal structure representative of its:
• atomic structure
• chemical composition
• crystal symmetry
• Unit Cell is a regularly ordered arrangement
of atoms with a fixed geometry relative to one
another
• The atoms are arranged in a ‘box’ with
parallel sides, the unit cell, which is repeated
by simple translations to make up the crystal
• Unit cell dimensions measured in angstroms
with 1A = 10-10m


TEM image of Cordierite
(Mg2Al4Si5O18) showing
ordered structure typical of
crystalline structures

Micro

20A

Macrocrystals of Cordierite showing
well developed flat crystal faces that

characterise crystals in their macro
form

Macro


Crystal Systems: common groups of symmetry
Crystal symmetry is defined
by:
• Planes of symmetry
• Axes of rotation
• Axes of inversion

All properties of a crystal
substance conform to
symmetry
Symmetry is the defining
property of a crystal


Unit cell dimensions of the seven crystal systems
The crystal systems can be defined by their stacked unit cells
•
•
•
•
•
•
•

•

CUBIC
a = b = c; α = β = γ = 90
TETRAGONAL
a = b ≠ c; α = β = γ = 90
ORTHORHOMBIC
a ≠ b ≠ c; α = β = γ = 90
MONOCLINIC
a ≠ b ≠ c; α = γ = 90 β > 90
TRICLINIC
a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90
HEXAGONAL
a = b ≠ c; α = β = 90; γ =120
TRIGONAL – Hexagonal
a = b ≠ c; α = β = 90; γ =120
TRIGONAL – Rhombohedral
a = b = c; α = β = γ ≠ 90 < 120

Where a, b, and c are the unit cell axes
dimensions and α, β, and γ are the
inclination angles of the axes in the unit
cell.


Crystal Structures
All crystal structures can be envisaged as:
• the packing together of spherical ions/atoms
• bonded by ionic and/or covalent and/or metallic bonds
Ionic bond:

bond
• Electrical attraction between
ions of opposite charge (Na+, Cl-)
•90% minerals are ionic
compounds
Covalent bond:
• shared electrons where
electrons not readily lost/gained
•E.g. Diamond
Metallic bond:
• free-electron sharing in metallic
atoms (loose electrons)

e.g. NaCl - Salt


Crystal Growth
• Crystal growth can be envisaged as addition of unit
cells in three dimensions
•

If this occurs at the same rate in all directions the
shape of the unit cell will be retained in the macro
crystals

•

If the rate of addition of unit cells is not the same
in all directions the shape of the macro crystals
need not reflect the unit cell shape

•

The symmetry of the macro crystals ALWAYS
reflects at least the minimum symmetry of the
crystal system of the unit cell.


Calcite structure

Ionic bond

Calcite rhomb

Unit cell

Calcite crystal habit

Calcite

Crystal


Unit Cell of
pyrite

Pyrite
Unit cell

Pyrite

crystals: ideal
growth

Pyrite nodular
aggregate:
confined growth