EARTH MATERIALS 2.07EARTH MATERIALS 2.07

Professor Peter DoyleP.doyle@imperial.ac.uk

Profdoyle@btinternet.com

Course aimsCourse 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 AssessmentReading 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.

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 outlineCourse outline

Part 1: MineralsPart 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: RocksPart 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

MINERALSMINERALS

•MineralsMinerals are naturally occurringnaturally occurring,, homogeneoushomogeneous,, crystalline solidcrystalline solid with a definite chemical definite chemical compositioncomposition

•Minerals usually form by inorganic processes but some may be biogenic in origin

Halite – NaClHalite – NaCl

Biotite – KBiotite – K22(Mg,Fe(Mg,Fe2+2+))44 (Fe (Fe3+3+,Al,),Al,)22(Si(Si66 Al Al22 O O2020))

(OH,F)(OH,F)44

Quartz:Quartz: primary rock forming mineral, simple silicate

Olivine:Olivine: primary rock forming mineral, complex silicate

Other Minerals:Other Minerals: PyritePyrite, an iron sulphide

Biogenic minerals:Biogenic minerals: Modern shell debris composed of carbonatecarbonate & silicasilica minerals

ROCKSROCKS

• A rock is: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:Granite: polished section showing aggregate of four interlocking and different coloured minerals

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

Cement matrix

Crushed rocks

Limestone - biogenic

Evaporite - inorganic

Biogenic and Biogenic and inorganic inorganic sedimentary rockssedimentary rocks

Coal - biogenic

Part 1: MineralsPart 1: Minerals

• A mineral is: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 CellThe Unit Cell

• Unit CellUnit Cell is the smallest 3D repeating unit of acrystal 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,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 Cordierite (Mg2Al4Si5O18) showing ordered structure typical of

crystalline structures

20A

Macrocrystals of CordieriteCordierite showingwell developed flat crystal faces thatcharacterise crystals in their macroform

Micro Macro

Crystal Systems: common groups of symmetryCrystal Systems: common groups of symmetry

Crystal symmetry is defined Crystal symmetry is defined by: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 systemsUnit cell dimensions of the seven crystal systems

• CUBIC a b c; αβγ• TETRAGONAL a b c; αβγ• ORTHORHOMBIC a b c; αβγ• MONOCLINIC a b c; αγβ• TRICLINIC a b c; αβγ• HEXAGONAL a b c; αβγ• TRIGONAL – Hexagonal

a b c; αβγ • TRIGONAL – Rhombohedral

a b c; αβγ

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.

The crystal systems can be defined by their stacked unit cellsThe crystal systems can be defined by their stacked unit cells

Crystal StructuresCrystal Structures

All crystal structures can be envisaged as:• the packing together of spherical ions/atomspacking together of spherical ions/atoms • bonded by ionicionic and/or covalentcovalent and/or metallicmetallic bonds

e.g. NaCl - Salt

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

Crystal GrowthCrystal Growth

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

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

• If the rate of addition of unit cells is not the sameis 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 ALWAYSALWAYS reflects at least the minimum symmetry of the crystal system of the unit cell.

Calcite structure Calcite rhomb

CalciteCalcite

Unit cell Crystal

Calcite crystal habit

Ionic bond

Unit Cell ofpyrite

Pyrite crystals: ideal growth

Pyrite nodular aggregate:

confined growth

PyritePyrite

Unit cell