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 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 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 Mineralsnaturally occurring,homogeneous, crystalline soliddefinite chemical compositionMinerals are naturally occurring, homogeneous, crystalline solid with a definite chemical composition Minerals usually form by inorganic processes but some may be biogenic in origin Halite – NaCl Biotite – K 2 (Mg,Fe 2+ ) 4 (Fe 3+,Al,) 2 (Si 6 Al 2 O 20 )(OH,F) 4

Quartz: Quartz: primary rock forming mineral, simple silicate

Olivine: Olivine: primary rock forming mineral, complex silicate

Other Minerals:Pyrite Other Minerals: Pyrite, an iron sulphide

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

ROCKS 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 inorganic sedimentary rocks Coal - biogenic

Part 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 Cell 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 unit cell, 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 = m

Cordierite TEM image of Cordierite (Mg 2 Al 4 Si 5 O 18 ) showing ordered structure typical of crystalline structures 20A Cordierite Macrocrystals of Cordierite showing well developed flat crystal faces that characterise crystals in their macro form MicroMacro

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 CUBIC a  b  c; α  β  γ  TETRAGONAL a  b  c; α  β  γ  ORTHORHOMBIC a  b  c; α  β  γ  MONOCLINIC a  b  c; α  γ  β  TRICLINIC a  b  c; α  β  γ  HEXAGONAL a  b  c; α  β  γ  TRIGONAL – Hexagonal a  b  c; α  β  γ  TRIGONAL – Rhombohedral a  b  c; α  β  γ  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 cells

Crystal Structures All crystal structures can be envisaged as: packing together of spherical ions/atoms the packing together of spherical ions/atoms ioniccovalentmetallic bonded by ionic and/or covalent and/or metallic bonds e.g. NaCl - Salt Ionic bond Ionic 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)

Crystal Growth Crystal growthCrystal growth can be envisaged as addition of unit cells in three dimensions same rate shape of the unit cell will be retained in the macro crystals If this occurs at the same rate in all directions the shape of the unit cell will be retained in the macro crystals is not the same 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 ALWAYS The symmetry of the macro crystals ALWAYS reflects at least the minimum symmetry of the crystal system of the unit cell.

Calcite structureCalcite rhomb Calcite Unit cellCrystal Calcite crystal habit Ionic bond

Unit Cell of pyrite Pyrite crystals: ideal growth Pyrite nodular aggregate: confined growth Pyrite Unit cell