50427699 mineral geology

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EARTH MATERIALS 2.07 Professor Peter Doyle P.doyle@imperial.ac.uk Profdoyle@btinternet.com 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 [...]...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... 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... 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 -
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