Lecture 3: Earth Materials and their Properties I: Minerals. Introduction to the Earth System EAS 2200

Transcription

1 Lecture 3: Earth Materials and their Properties I: Minerals Introduction to the Earth System EAS 2200

2 Earth Materials Plan of the Why it matters Nature of the Earth/Composition The Solid Earth Mineral structures and properties Properties of rock Seismic properties of materials Water and Ice Week

3 Why it matters The physical and chemical properties of matter impact every process occurring on the Earth. We need to understand these properties to understand (and predict) natural processes. Conversely, knowing how properties relate to materials allows us to determine the nature of materials and their state (temperature and pressure) remotely.

4 Examples of how properties affect processes Carbon dioxide, water, and methane (and glass) all absorb infrared radiation much more strongly than visible. This leads to the greenhouse effect. Water has a very high latent heat of evaporation - meaning a lot of energy is released when water condenses. It is this energy that produces storms, thunderstorms and hurricanes. The solid form of water is, anomalously, less dense than the liquid. If this were not true, the oceans would freeze from the bottom up and not easily melt. The melting temperature of rock decreases as pressure decreases. Consequently, rock rising in convection currents within the Earth will melt. These magmas erupt through volcanoes, which through time have

5 Examples of how we can use properties Earthquakes generate seismic waves that travel completely through the Earth. One kind of seismic wave, the s-wave, does not travel through a liquid. Richard Dixon Oldham discovered the liquid outer iron core when he found that there is a shadow zone, on the opposite side of the Earth from the earthquake, where no s-waves are detected. We can measure the concentrations of atmospheric gases such as CO 2 and ozone by measuring the adsorption of electromagnetic radiation at specific wavelengths.

6 The Earth Earth has a layered structure, consisting of the following: Atmosphere and Hydrosphere Mainly water, nitrogen, oxygen Lower density Crust (6-35 km thick) formed by melting/remelting Aluminosilicates (rocks) Intermediate density Mantle (~3000 km thick) Magnesium-iron silicates (rocks) High density Core (~3000 km thick) Liquid outer core

7 Earth s Layers Thickness Volume Density Mass Mass (km) g/cc kg Percent cm 3 Atmosphere Hydrospher e Crust Mantle Core All

8 Composition of the Earth K Na.0014 Ca.075 Al.10 Ni.058 Fe 1.07 Mg 1.23 Si O 3.97

9 Chemistry of the Earth Just 7 elements have abundances in the Earth that exceed 1% (by weight): O, Mg, Al, Si, Ca, Fe, Ni These 7 elements make up over 98% of the Earth (by weight). Most of the Fe and Ni is in the Earth s core. For everything but the core (the silicate Earth ), just 6 elements constitute 99% of the mass: O, Mg, Al, Si, Ca, Fe Thus it is the properties of the compounds

10 States of Matter Gas No order above molecular level Atoms organized as individual molecules with translational freedom Limited interaction between molecules compressible Liquid Relatively incompressible Cohesive Translational freedom of individual atoms, ions, and molecules Intermediate range order, interaction among molecules

11 Fluids and Flow In traditional definitions of liquid and solid, it is often said that liquids are capable of flow and solids are not. As we shall see, this is not a useful distinction because solids are capable of flow. The mechanism of flow in solids and liquids differ. Liquids flow by free motion of molecules Solids flow by fracture, slip, dislocation of crystals and by recrystallization.

12 Solids Amorphous solids Like liquids these are cohesive and have intermediate-range order of atoms Atoms have no translational motion Unlike liquids they maintain shape in ambient P,T,t Can be thought of as liquids with very high viscosity Examples: glass (natural and man-made), amber, opal Crystals

13 Minerals: Definition A mineral is a naturally occurring, inorganic, crystalline substance of defined composition and structure. The and is important. Compounds of identical composition but distinct structure are different minerals: e.g., graphite and diamond (both C); calcite and aragonite (both CaCO 3 ). These are called polymorphs (only 1 polymorph of a compound will be stable at a given temperature and pressure). Crystals of similar structure but different composition are also distinct minerals. Halite (NaCl), periclase (MgO), and galena (PbS) are all share the same cubic structure.

14 Non-Minerals Some commonly occurring natural solid substances that are not crystalline and therefore not minerals include: Coal Complex organic solid Forms from (refractory*) organic matter that accumulates in shallow water such as swamps Opal Chemically identical to quartz (SiO 2 ), but atoms do not have long-range order Typically precipitates from aqueous solutions Igneous glass (e.g., obsidian) Complex solids forming from silicate liquids (magmas) Form when cooling occurs too rapidly for atoms to arrange themselves in lattices All share the property that they lack long range atomic order.

15 Solid Solutions The composition of a mineral can vary when one or more atoms are substituted for another without radical change in the lattice structure. Such minerals are examples of solid solutions. Two common examples: Olivine has the composition (Mg,Fe) 2 SiO 4. In nature, olivine is almost always a solution of its two ideal endmembers: forsterite (Mg 2 SiO 4 ) and fayalite (Fe 2 SiO 4 ). The same is true of plagioclase, (Na,Ca)(Al,Si) 4 O 8, which is a solution of anorthite (CaAl 2 Si 2 O 8 ) and albite (NaAlSi 3 O 8 ). Thus minerals such as olivine and

16 Ions An ion is a charged atom or group of atoms (radical). It is charged because it has either more or less electrons than protons. Positively charged atoms or groups are called cations. In most solid solutions, substitution involves only the cations. Negatively charged atoms or groups are called anions.

17 Bonds between atoms Atoms in crystals are bonded to each other. Both the nature of the atoms and the nature of the bonds determine the physical properties of the mineral. Types of bonds: Ionic one or more electrons exchanged, leading to positively and negatively charged ions Covalent One or more electrons are shared; i.e., occupy an orbit common to both atoms Many bonds are actually hybrid, in which an atom is shared, but not equally. Van der Waals bond Weak bond resulting from the residual charge on atoms

18 Crystal Lattices: infinitely O repeating pattern of atomic Ca Cl C Na Calcite (CaCO 3 ) Halite (NaCl)

19 How lattice structure affects mineral properties Consider graphite and diamond, both composed of only carbon One is among the hardest known minerals, the other among the softest One is black, the other clear One has hexagonal symmetry, the other cubic.

20 How lattice structure affects mineral properties These differences between diamond and graphite reflect different bonding between atoms and different lattice structures. In diamond, each carbon is bound to 4 other carbons in a 3-dimensional cubic structure. In graphite, each carbon is bound to 3 other carbon atoms in strong 2- dimensional sheets with only weak bonds between

21 Crystal Lattices, Unit Cells, and Symmetry The smallest part of a lattice containing all the chemical and structural components of the lattice as a whole is called a unit cell. Lattices can be thought of as infinitely repeating unit cells. *(how many unit cells in 1cm of a halite crystal?) Lattices possess varying degrees of symmetry and are classified based on this symmetry. Symmetry operations: Reflection Rotation Inversion These can be combined to give 32 groups and 7

22 Crystal Systems Crystals can be divided into 7 systems based on the symmetry they possess (which in turn reflects symmetry of lattice structure): Isometric (Cubic) Hexagonal Trigonal Tetragonal Orthorhombic Monoclinic Triclinic This symmetry occurs on the atomic or lattice scale, but is also reflected by the crystal s macroscopic properties, such as the orientation of its faces and how it transmits light and sound.

23 All edges (axes) equal, all angles 90 degrees Three 4-fold symmetry axes Examples: Diamond (C) Galena (PbS) Pyrite (FeS 2 ) Halite (NaCl) Cubic System

24 Hexagonal System 3 equal axes intersecting at 120 in one plane, one unequal axis One 6-fold rotational symmetry axis (60 rotational symmetry) Examples: Apatite (Ca 5 (PO 4 ) 3 (OH,F,Cl) Graphite (C)

25 Trigonal (Rhombohedral) Sometimes considered a subclass of hexagonal Like hexagonal, 3 equal axes intersecting at 120 in one plane, one unequal axis One - 3-fold rotational symmetry axis (120 ) Examples: Tourmaline (a borosilicate) System

26 Tetragonal Two edges (axes) equal all angles 90 degrees 1 four-fold axis of rotation Examples Zircon (ZrSiO 4 ) Cassiterite (SnO 2 )

27 Orthorhombic No edges equal, all angles 90 degrees Either 3 two-fold axes of rotation or 1 two-fold axis of rotation and 2 mirror planes. Examples: Olivine ((Mg,Fe) 2 SiO 4 ) Native sulfur (S) Orthopyroxene ((Mg,Fe) 2 Si 2 O 6 )

28 Monoclinic All three axes or edges unequal, two angles at 90 degrees Either 1 two-fold rotational symmetry (180 ) or 1 mirror plane Examples Micas (e.g., KAl 3 Si 3 O 10 (OH) 2 ) Gypsum (CaSO 4. 2H 2 O) Clinopyroxene (Ca (Mg,Fe)Si 2 O 6 )

29 Triclinic No edges equal, no angles 90 degrees Examples Plagioclase (CaAl 2 Si 2 O 8 )

30 Optical Properties of Minerals Light (and sound) interacts with minerals as it travels through the crystal lattice. In all minerals except cubic ones, the nature of this interaction will depend on the direction in which the light travels and the direction in which it vibrates. Different minerals have different optical properties, which greatly helps us identify them under the microscope. Before we discuss this, a few words about light.

31 The Nature of Light Light is an electromagnetic wave, which means it consists of propagating electric and magnetic fields. The magnetic field is oriented perpendicular to the electric one, and both are perpendicular to the travel direction.

32 Polarization of Light Light typically consists of waves oriented in all directions. Light that consists of waves with fields oriented in only one direction is said to be polarized.

33 Anisotropic Optical classification of Based on how light (and sound) propagates through them, we can divide minerals into several classes: Opaque these minerals absorb effectively all light passing through them (irrespective of their symmetry class). Isotropic Light transmitted equally in all directions Cubic symmetry system Uniaxial One unique direction, or axis, along which propagating light behaves differently than in other directions. Trigonal, hexagonal, and tetragonal symmetry systems Biaxial Two unique optic axes

34 Light in Crystals Light interacts with matter. Two principal effects: Absorption This will vary depending on wavelength (hence producing color) and, in anisotropic crystals, direction. In anisotropic crystals, light vibrating in one direction can be absorbed, while that vibrating in another will not be. As a consequence, these crystals will polarize the light. To view these effects, minerals are generally examined under the microscope using polarized light. (Nicols) Retardation As light interacts with electrons, it is slowed. One consequence of this is that light is refracted as it passes from one medium to another.

35 Compositional Groups Minerals are grouped compositionally based on the nature of the anion Most important and common Silicates (SiO 4 ) 4- - MgCaSi 2 O 6 diopside (clinopyroxene) Oxides - Fe 2 O 3 Carbonates - CaCO 3 Sulfides - FeS 2 Sulfates - CaSO 4. 2H 2 O (gypsum)

36 Silicate Minerals Silicates are by far the most abundant minerals Seven elements compose 98% of the mass of the Earth: Fe (32%),O (30%), Si (16%), Mg (15%), Ni (1.8%), Ca (1.7%) and Al (1.6%) Most of the Fe and Ni are in the Earth s core, so the outer part of the earth consists almost entirely of various silicates. Main structural element Mantle Composition: K Na.0014 Ca.075 Al.10 Fe.14 Mg 1.23 Si O 3.63

37 Structural Classes of Silicates Silicon -- 4 bonds -- polymerizes like carbon * Nesosilicates - isolated tetrahedra Sorosilicates - pairs of tetrahedra Cyclosilicates - rings of tetrahedra ** Inosilicates - chains of tetrahedra ***Phyllosilicates - sheets ***Tectosilicates - 3D networks

38 Nesosilicates Olivine is a good example - (Mg,Fe) 2 SiO 4 Two additional cations (either Mg 2+ or Fe 2+ ) balance the negative charge of the SiO Olivine has orthorhombic symmetry Most abundant mineral in outer part of Earth.

39 Sorosilicates Two tetrahedra sharing a single oxygen Example, epidote (Ca 2 (Al,Fe) 3 (SiO 4 ) 3 (OH)

40 Cyclosilicate s Tetrahedra arranged in rings An example is beryl (Be 3 Al 2 (SiO 3 ) 6 ) (hexagonal system) Gem quality beryls are known as? Emerald

41 Inosilicates Chain Silicates Two types Single chains - pyroxenes Double chains - amphiboles Each silicon shares 2 oxygens, binding the tetrahedra into chains, and producing the net formula SiO Two major kinds of Pyroxenes Orthopyroxene (having orthorhombic symmetry) and the general formula (Mg,Fe) 2 Si 2 O 6 Clinopyroxene (monoclinic symmetry) and the general formula (Mg,Fe)CaSi 2 O 6 Amphiboles are more complex and typically contain both Al

42 Phyllosilicate Micas and related minerals, including clay minerals Tetrahedra are arranged in planes with each silicon sharing 3 of its oxygens Bonds between atoms of different planes are particularly weak, accounting for the easy parting of micas into sheets. Example: Muscovite KAl 3 Si 3 O 10 (OH) 2

43 Tectosilicate s Tetrahedra arranged in frameworks with all oxygens shared between Si atoms Two important examples Quartz Feldspars

44 More about feldspars Feldspars are the most abundant minerals in the Earth s crust Three kinds Orthoclase KAlSi 3 O 8 Albite NaAlSi 3 O 8 Anorthite CaAl 2 Si 2 O 8 The latter two form a complete solid solution called plagioclase The first two are collectively called alkali feldspar, but form only

45 Key properties of minerals useful in identification Hardness Mohs hardness scale Color Luster Metallic, vitreous, resinous, silky, dull, etc. Streak Color in powdered form (observed when streaked across ceramic plate) Cleavage Fractures or breaks along preferential planes of weakness Some minerals have no cleavage and break randomly like glass - that s called concoidal fracture. Habit or form Needle-like, equant, cubes, rhombs, plates, etc. Density (or specific gravity)