IntroMinPet Lecture Red

Introduction to Mineralogy and Petrology for PE, 620.010 and 620.002 1

INTRODUCTION TO COSMOCHEMISTRY Planet Earth

Why ist the Earth so special? How did Earth form? Why is water on Earth? Why and since when does life exist on Earth?

Origin of the elements

• „Big Bang“ theory -‐ ~15x109 years ago Formation of the primordial elements (H, D, He) by primordial nucleosynthesis Formation of the heavy elements by stellar nucleosynthesis within star nebula

• Hydrogen fusion (e.g. sun) and Helium fusion H + D -‐> 3He + γ + 5,493 MeV 3He + 3He -‐> 4He + 2H + 12,859 MeV

• Stellar Fusion; Formation of the heavier elements 4He + 4He -‐> 8Be 8Be + 4He -‐> 12C + γ 12C + 4He -‐> 16O

• Neutron capture Formation of the heavy elements; Supernova-‐explosion threw out high mass elements into space 62Ni + 1ν -‐> 63Ni + γ 63Ni -‐> 63Cu + β-‐ + ν + 0,0659 MeV etc.


Element abundance in our solar system

Nebula-‐hypothesis

• Planetary nebelae: Primordial gases (H, D, He); dust • Gravitation -‐> contraction -‐> rotation -‐> Proto-‐Sun • Condensation -‐> increase of temperature (106 K) -‐> core fusion; hydrogen fusion • Liberation of huge energy masses; E=m·c2 • Accretion of planets: via gravitation formation of „planetesimals“ (condensed, solid

bodies of 1-‐10 km in diameter), that rotate around the sun. Impact processes form even larger planets (planet accretion) ca. 4.56·109 years ago = Age of the Earth

• Earth´s age determination based on meteorites: Ca-‐Al-‐silicate-‐inclusions in chondrites (a specific class of meteorite) reveal an age of 4567.2±0.6 Ma

• Prograde condensation; condensation of chemical compounds depending on the distance (and temperature) from the sun;

• Close to the sun: refractory elements (Mg, Fe, Al, Si, Ca) plus O -‐> form solid compounds with oxygen -‐> terrestrial planets (inner planets)

• Further away from the sun: light, volatile elements (S, C, N, H) -‐> methane, ammonium, H2O-‐ice -‐> outer planets


from Grotzinger et al. (2008).

Mass of the Earth: 5,974·1027 g Average density of the Earth: 5,142 g cm-‐3 Age of the Earth: 4,56·109 Jahre

3 heat sources in early planets

• Kinetic energy by meteoric impacts • Compression of matter by gravitation • Radioactive decay of von haevy elements (U, Th, K)

Differentiation of the Earth

Chemical differentiation • Earth was predominantly liquid (T> FpFe) • Heavy elements Fe-‐Ni sink into the Earth´s core • O-‐Si-‐Mg form an intermediate layer -‐ > Earth´s mantle • O-‐Si-‐Al-‐Fe-‐Ca-‐K-‐Na “swim“ at the surface -‐> Earth´s crust

Chemically differentiated layers existed already ca. 4.3 Ga (?) ago Degassing from inner parts of the Earth formed an early atmosphere (H2O, HCl, NH3, N2, CH4, He etc.); formation of the oceans via gas condensation due to cooling

6371 km

5140 km

2885 km

350 km

Innerer Kern:fest

Äußerer Kern:flüssig

Mesosphäre:heiß aber stärkerwegen des hohen Drucks

Asthenosphäre:heiß, schwach, plastisch(fest+ 1% Schmelze?)

Lithosphäre:kalt, starr, spröd

Kontinental120 km dick

Ozeanisch65 km dick

Rheologischer LagenbauPhysikalische Eigenschaften

Zuna

hme

von

T, P

, Dic

hte

15 °C 1.0·105 Pa

500 °C 9.7·109 Pa~1300 °C ~3.87·109 Pa

3800 °C 1.39·1011 Pa

~5000 °C 3.24·1011 Pa

~6600 °C 3.73·1011 Pa

2.8 g·cm-3

3.3 g·cm-3

~3.4 g·cm-3

5.5 g·cm-3

9.9 g·cm-3

12.3 g·cm-3

13 g·cm-3

Kompositioneller Lagenbau

Kontinental35-80 km thick

Oceanisch7 km thick

2885 km

6371 km

Elemente Minerale

Kruste:O > Si >> Al> Fe > Ca > Na> K > Mg(total = 98.6 wt%)

SilikateOxideHydroxideKarbonate

Mantel:O > Si > Mg >>> Fe > Al >> Ca(total = 97.3 wt%)

Silikate(Olivin,Pyroxene)OxideSulfide

Kern:Fe >> Ni(total = 98.2 wt%)+ Co, S, P, C

Metallische Legierung


Evidence for the layered Earth

• Propagation of seismic P-‐ and S-‐waves • Total density of the Earth • Inclusions in volcanic rocks • Composition of volcanic rocks • Portions of the Earth´s mantle on surface • Composition of meteorites


Early development of the Earth

• Heat increase – melting – oceans of magma? • Oldest minerals: ca. 4.35 Ga

o Zircon from Jack Hills, W.A. o SiO2-‐rich continental crust existed

• Also most likely water and oceans • Oldest rocks on Earth´s surface; ca. 3.8 Ga

o Isua Greenstone Belt; SW Greenland o Erosion; deposition of sediments

The oldest minerals on Earth – Zircons from Jack Hills, W.A., ca. 4350 Ma (A. Cavosie, www). Cross-‐sections of these zircons in a cathodoluminescence image (left), numbers illustrate 207Pb/206Pb ages, Pb-‐age distribution (right) The oldest sediments on Earth, ~3.8 Ga years old; Isua Greenstone Belt, SW Greenland. Banded iron ore (left), metaconglomerate (right)


Development of life on Earth

• Most important is the “right” distance to the sun; water as liquid • Life is possible between–15°C and 150°C; also within the upper crust -‐> deep

biosphere • First organisms were thermophile microorganisms • Did life start by accident? • 3.5-‐3.4 Ga: First life forms (Archaea and Bacteria) • 2.7 Ga: Increasing production of oxygen via photosynthesis by Cyanobakteria • <2.7 Ga: Stromatoliths • 2.4 Ga: Big “Oxidation Event” (GOE); free oxygen in the atmosphere 0.54 Ga

Our solar system with the inner and outer planets (from Grotzinger et al. 2008).


WHAT BUILDS THE EARTH UP? The Earth´s crust and mantle are formed by rocks. A rock is composed of one or more minerals. A mineral is defined as a homogeneous material of clearly defined chemical composition, which occurs on Earth. A crystal is defined by a crystal lattice, which is formed by atoms and/or molecules that build up a 3-‐dimensional symmetrical structure (= crystal lattice). Furthermore, the chemical compounds of the crystal lattice are hold together by chemical bonds. The type of chemical compounds in the crystal lattice, the distance between these compounds, their symmetrical order and the type of bond determine all the physical and chemical properties of a crystal. A crystal, which occurs naturally on Earth is also characterised by its thermodynamic stability. This means the crystal exists at certain temperature and pressure conditions within a certain chemical environment. If a mineral is formed deep in the Earth and it will come to the Earth´s surface (e.g. via geotectonic processes) at lower T-‐P conditions, it will be instable. However, it will not “fall apart” immediately, because the reaction kinetics are very slow. This status is called metastable.


INTRODUCTION TO MINERALOGY AND PETROLOGY SUBJECT: CHEMICAL BOND Structure of the atom: An atom is composed of a central nucleus (protons + neutrons) and an electronic shell (electrons, maximum of 7 main orbitals, diameter ranges between 0.5 and 2.5 Å).

The number of protons in the nucleus determines the atomic number (periodic table), the number and the location (energy level) of the electrons in the shell determines the electron configuration (= “finger print” of the atom). TYPES OF CHEMICAL BONDS WITHIN A CRYSTAL LATTICE

1) Formation of electron pairs = Covalent bond


2) Assimilation of an electron = Ionic bond

3) All electrons involved in the bond are free, not attached to any nucleus (they form an “electronic gas“) = Metallic bond

4) Weak bonds = van der Waal´s bonds, occur predominantly in noble gases and in specific positions within certain crystal lattices (e.g. phyllosilicates).

Bonding force = Coulomb´s force K = 1/ε x e1 x e2/(r1 + r2)2

e = electron charge r = ionic radius ε = dielectrical constant


PRINCIPLE CRYSTAL-‐CHEMICAL LAWS

1) ISOMORPHISM (ISOMORPHOUS MIXTURE) – the exchange of different chemical compounds in the crystal lattice without changing the structure of the crystal lattice. End-‐member minerals form solid solutions.

NaCl – KCl, MgSiO4 – FeSiO4

2) ISOTYPE STRUCTURES – two distinct crystals are characterised by identical

crystal lattices NaCl – PbS

3) POPLYMORPHISM – one solid chemical substance can exist in more than one

distinct crystal structures

C -‐> graphite, diamond FeS2 -‐> pyrite, marcasite


INTRODUCTION TO MINERALOGY AND PETROLOGY SUBJECT: SYSTEMATIC MINERALOGY 90 elements are involved in the Earth´s composition. However, 8 elements only are making up around 98.66 wt% of the Earth. That is the reason why there are not “unlimited” minerals existing. We know around 3600 minerals on Earth. Oxygen is the most abundant element within the Earth´s crust, therefore most minerals have oxygen in their mineral formula. MINERAL CLASSIFICATION Mineral classification is based on the chemical composition also taking the crystal structure into account (e.g. quartz, SiO2, belongs to the silicates, it is not an oxide).

I. MINERALS WITH OXYGEN 1. Oxide (O2 as an anion) 2. Hydroxide (OH-‐ as an anion) 3. Silicate (basic compound is the SiO44-‐ tetrahedron) 4. Carbonate (basic compound is the CO32-‐ group) 5. Sulfate (basic compound is the SO42-‐ tetrahedron)

II. MINERALS WITHOUT OXYGEN 1. Sulfide (S2-‐ anion) 2. Halogenides 3. Elements

The classification of the oxides is based on the metal (X) : oxygen ratio:

1. Metal : oxygen = 1:1 (XO-‐Type) – e.g. Periklase (MgO), Wüstite (FeO) 2. Metal : oxygen = 2:1 (X2O-‐Type) – e.g. Cuprite (Cu2O) 3. Metal : oxygen = 2:3 (X2O3-‐Type) – Corundum-‐Type

Corundum (Al2O3) Hematite (Fe2O3) Ilmenite (FeTiO3) Perowskite (CaTiO3)

4. Metal : oxygen = 3:4 (XY2O4-‐Type) – Spinel-‐group Aluminum-‐spinels (Spinel MgAl2O4, Hercynite FeAl2O4) Ferrit-‐spinel (Magnetite FeFe2O4) Chromium-‐spinel (Chromite FeCr2O4)

5. Metal : oxygen = 1:2 (XO2-‐Type) TiO2-‐minerales – Rutile, Anatase, Brookite


Pyrolusite (MnO2) Cassiterite (SnO2) Uraninite (UO2)

Common properties of the OXIDES:

Ø High lattice symmetry Ø High hardness Ø High melting point Ø High chemical resistence

The reason for these characteristic properties is predominantly based on the type of chemical bond. Oxides are characterised by a mixture of atomic and ionic bonds. Oxides are further characterised by typical, symmetrical order of the oxygens in the lattice – either hexagonal, or cubical most dense package.

Hexagonal package. Order of layers = A, B, A, B, A...


Cubical package. Order of layers = A, B, C, A, B, C, A ... OXIDES of X2O3-‐TYPE Corundum group – Corundum (Al2O3), Hematite (Fe2O3) Structure: trigonal, oxygens form a hexagonal package, metals occupy octahedral spaces. Complete solid solution between corundunm and hematite. Al2O3 represents the most important raw material for the Al-‐industry. It derives from bauxite. Corundum is mainly used as grinding and polishing material, based on its hardness of 9 according to the Mohs-‐scale. Impurities of Cr, Ti, Fe etc. lead to the gem stones ruby stone and sapphire. Hematite represents an important iron ore. It occurs predominantly in banded iron formations (BIF). Hematite does not occur in magmatic deposits. Ilmenite (FeTiO3) Structure: trigonal, in comparison to corundum, ilmenite belongs to a crystal class of lower symmetry (two different metals in the lattice). Ilmenite is an accessory mineral constituent of many rocks (predominantly in magmatic rocks). It represents the most important carrier of Ti. It further occurs economically in placer deposits (hardness, chemical resistence). It is mainly used as alloy metal and Ti-‐ore.


OXIDES of X(6)Y2(4)O4-‐TYPE (SPINELS) Structure: all spinels belong to the isometrical crystal system. Oxygens form acubical package, the metals occupy octahedral and tetrahedral spaces in between. Economically important are: Magnetite Fe3+(6)Fe2+(6)Fe3+(4)O4 Chromite Cr2(6) (4)O4 Magnetite represents, apart of hematite, the most important iron ore. It occurs in magmatic deposits, however, economically most important are BIFs. Chromite ist the classical Cr-‐ore mineral and occurs almost exclusively in magmatic deposits, such as layered intrusions (e.g. Bushveld Complex, South Africa) = stratiform chromitites, and in mantle rocks = podiform chromitites. HYDROXIDES Goethite (Fe(6)O(OH) Limonite (FeO(OH) . n H2O Fe-‐hydroxides are typical weathering minerals (“rust“) and occur predominantly within oxidation horizons of primary sulfide and oxide mineral deposits. Within the oxidation zone they are the main constituents of the so-‐called “gossan”. Fe-‐enriched gossan are also used as Fe-‐ore.


SILICATES Silicates make up around 92 % of the Earth´s crust. Their classification is based on „polymerisation“ (= various combinations) of the SiO4-‐tetrahedrons.

Nesosilicates (isolated SiO4-‐tetrahedrons)

Sorosilicates (group-‐silicates Si2O7) – simple combination of 2 SiO4-‐tetrahedrons

Cyclosilicates (ringsilicates, 3, 4 and 6 SiO4-‐tetrahedrons are combined to form a ring structure; Si3O9, Si4O12, Si6O18)


Inosilicates (single chain silicates Si2O6)

Inosilicate (double chain silicates Si4O11)

Phyllosilicates (sheet silicates Si4O10)


Tectosilicates (SiO2) NESOSILICATES Olivine group Mg2SiO4 (Forsterite) – Fe2SiO4 (Fayalite) End members form complete solid solution series.

Binary system Forsterite – Fayalite, showing solid solution field of endmemebers Olivine is a major constituent of ultramafic rocks and a predominant mineral of the Earth´s mantle. It is used in the refractory industry due to ist high melting point (depending on the Mg : Fe ratio).


Garnet group Structure: all garnets belong to the isometrical crystal system. General formula: A32+B23+(SiO4)3 A = Mg, Fe2+, Mn2+, Ca B = Al, Fe, Cr Pyralspite group Pyrope Mg3Al2(SiO4)3 Almandine Fe3Al2(SiO4)3 Spessartine Mn3Al2(SiO4)3 Ugrandite group Uwarowite Ca3Cr2(SiO4)3 Grossular Ca3Al2(SiO4)3 Andradite Ca3Fe2(SiO4)3

Crystal morphology and forms of garnet


INTRODUCTION TO MINERALOGY AND PETROLOGY SUBJECT: INO-‐, PHYLLO-‐ UND TECTOSILICATES 1. INOSILICATES – PYROXENE GROUP Pyroxenes are simple chain silicates (Si2O6); general formula:

(Ca,Na,Li)0-‐1(Mg,Fe,Al)1-‐2(Si2O6) Octahedral-‐ Tetrahedral-‐ Position Position Depending on the occupancy of the octahedral position, orthopyroxenes (no occupancy of the octahedral position) and clinopyroxenes (octahedral position occupied) can be distinguished. Orthopyroxenes have orthorhombic structure, clinopyroxenes are monoclinic. Orthopyroxenes (OPX) Enstatite Mg2(Si2O6) Bronzite (Mg,Fe)2(Si2O6) Ferrosilite Fe2(Si2O6) Klinopyroxenes (CPX) Diopside CaMg(Si2O6) Hedenbergite CaFe(Si2O6) Augite Ca(Mg,Fe)(Si2O6) Jadeite NaAl(Si2O6) Ägirine (Acmite) NaFe(Si2O6) Spodumene LiAl(Si2O6) OPX members form a solid solution series, the same applies to CPX members, but there is very limited solid solution between OPX and CPX. Occurrence: Pyroxenes are rock-‐forming minerals. They represent, together with olivine, major components of the Earth´s mantle. OPX are also characteristic minerals for high-‐grade metamorphic rocks and occur within mafic volcanic rocks. CPX are characteristic for mafic plutonic and volcanic rocks and are diagnostic for metamorphic calc-‐silicate rocks. Na-‐rich pyroxenes are typical for alkali-‐rich igneous rocks.


2. INOSILICATES – AMPHIBOLE GROUP Amphiboles are double chain silicates (Si4O11).

General formula: A 0-‐1(10,12)X2(8)Y5(6)(OH,F)(Si4O11)

Ø A = Na, K Ø X = Ca, Na, Mg, Fe2+, Mn Ø Y = Mg, Fe2+,Mn, Al, Fe3+, Ti4+ Ø Z = Si, Al

Amphiboles represent a large mineral group. Similar to pyroxenes, orthoamphiboles (orthorhombic structure) and clinoamphiboles (monoclinic structure) can be distinguished. Three main series can be distinguished:

1. Orthoamphiboles, Mg-‐Fe-‐Amphiboles Cummingtonite Grunerite Anthophyllite Gedrite

2. Ca-‐Amphiboles (Clinoamphiboles)

Tremolite Actinolite Hornblende

3. Alkali-‐amphibole Na>Ca (Clinoamphiboles)

Glaukophane Riebeckite

Amphiboles are rock-‐forming minerals and are representative for the mineral composition of many metamorphic rocks (e.g. amphibolites). Rarely amphiboles can grow in form of very thin needles. In case the needles have a thickness of < 2 µm and a length of > 5 µm, they are defined as amphibole asbestos.


3. PHYLLOSILICATES (SHEET SILICATES) Phyllosilicates are basically composed of a tetrahedral sheet (Si2O5) = T-‐sheet and an octahedral sheet (Mg,Fe,Al)2-‐3(OH)4 = O-‐sheet. Based on the combination of the T-‐O-‐sheets and the occupancy within the O-‐sheets (i.e. either all octahedral spaces are occupied by Mg or Fe, or 2/3 of the spaces are occupied by Al), the following phyllosilicates can be distinguished:

(Si2O5)-‐sheet

1. 2-‐sheet phyllosilicates (T-‐O) Serpentine Mg3(OH)4(Si2O5) Kaolinite Al2(OH)4(Si2O5) Haloysite Al2(OH)4(Si2O5) . 2H2O

2. 3-‐sheet phyllosilicates (T-‐O-‐T)

Talc Mg3(OH)2(Si4O10) Pyrophyllite Al2(OH)2(Si4O10) Phlogopite (Biotite) K(Mg,Fe)3(OH,F)2(Si4O10) Muscovite KAl2(OH)2(Si3AlO10) Saponite Mg3(OH)2(Si4O10) . nH2O Montmorillonite Al2(OH)2(Si4O10) . nH2O

3. 4-‐sheet phyllosilicates (T-‐O-‐T-‐Mg(OH))

Chlorite (Mg,Fe,Al)3(OH)2(Si,Al4O10) . Mg3(OH)6

Diagnostic properties: perfect cleavage in (001), low hardness (1-‐3 according to Mohs), pearly lustre on cleavage planes, color varies from black (biotite), via green (chlorite) to colorless, tranparent (muscovite). The swelling property is a further characteristic feature of water-‐bearing phyllosilicates (=many clay minerals, e.g. smectites -‐> montmorillonite, haloysite, saponite). The water is released in cases where the phyllosilicate is dried. However, as soon as the phyllosilicate gets into contact with water again, the water molecules are getting adsorbed and the thickness of the phyllosilicate sheets are significantly increasing. Instead of water, other molecules can be adsorbed too.


Serpentine (chrysotile) may grow under certain conditions in form of tiny reels. If these reels achieve a thickness in the range of around < 3 µm and a length of > 5 µm, these chrysotiles are defined as serpentine asbestos. Serpentine asbestos occurs predominantly in altered ultramafic rocks, where the serpentine mineral forms during serpentinisation (alteration of olivine and pyroxenes under Co2-‐rich hydrous conditions into serpentine). This type of asbestos had numerous applications (i.e. isolation material, heat resistant material, facings for clutches and brakes in cars, for roof tails etc.) until around 1998. Today (i.e. since 1998) these applications are strictly prohibited. The health risk potential of serpentine asbestos is by far less than that of amphibole asbestos, because serpentine asbestos forms reel structure, whereas amphibole asbestos is represented by fine needles. Furthermore, serpentine asbestos will become dissolved after a few months if it will get into the human pulmonary tissue (i.e. serpentine asbestos is chemically almost exclusively composed of Mg and Si, apart of O, and the human body is undersaturated in these two elements. Amphibole asbestos contains, apart of Mg, always some Fe).


4. TECTOSILICATES

(SiO2) Tectosilicates are characterised by a three-‐dimensional bonding of the SiO4-‐tetrahedrons, thus the basic formula is SiO2. Part of the Si in the lattice can be substituted by Al -‐> feldspars Overview onto the classification of the tectosilicatse:

1. SiO2-‐Group Quartz Tridymite Christobalite Coesite Stishovite Opal

2. Feldspar-‐Group

Alkali-‐Feldspars K(Si3AlO8) Plagioclase-‐Group Na(Si3AlO8) Ca(Si2Al2O8)

3. Foides

Leucite K(Si2AlO6) Nepheline (Na,K)(SiAlO4)

4. Zeolite-‐Group

Natrolite Na2(Si3Al2O10) . 2H2O Phillipsite KCa(Si5Al2O16) . 6H2O Chabasite >Ca(Si4Al2O10) . 6H2O

and many more The nembers of the SiO2-‐Group are modifications of quartz (see polymorphism !), which are stable under various P-‐T-‐conditions. Opal represents a cryptocrystalline/amorphous variety of quartz (i.e. opal is not crystallised, it is gel-‐like). Other varieties of quartz form by chemical impurities (i.e. such as Fe, Al, Li, H etc.) leading to different colors of quartz: Amethyst (purple), Rose-‐quartz (pink), Smoky quartz (grey to black), Citrine (yellow), Milky quartz.


The most important industrial application of quartz is based on its very special crystal lattice. The SiO4-‐tetrahedrons are ordered like a screw. Thus we can distinguish between a left-‐handed and a right-‐handed quartz (i.e. depending on the sense of rotation of the screw). This specific crystal lattice causes the piezo-‐electrical effect. If we cut a sheet out of the quartz, perpendicular to its crystallographical c-‐axes, the ends of this platelet are charged. If this quartz platelet (= “quartz chip”) is exposed to an ac voltage, the quartz will start to “swing” (= oscillatory movement), the quartz chip gets into rhythmical extension and compression. This application of quartz represents the prerequisite for any modern communication (e.g. TV, radio computer technology etc.). The feldspar-‐group is composed of two miscibility series, the alkali-‐feldspar and the plagioclase series. Feldspars are rock-‐forming minerals and represent significant mineral constituents of many magmatic and metamorphic rocks. They are also the most common minerals within the Earth´s crust.

The minerals of the zeolite-‐group are characterised by crystal lattice with common open spaces and cavities, in which large cations (e.g. Na, K, Ca etc.) and water molecules are located. These cations and water may be absorbed and adsorbed. This is the reason for the important industrial application of zeolites as ion-‐exchanger (e.g. water softener, treatment of radioactive water -‐ adsorption of 137Cs, 90Sr, water and gas cleaning, etc.).


INTRODUCTION TO MINERALOGY AND PETROLOGY

SUBJECT: CARBONATE MINERALS • Structures are characterized by flat, triangular CO3

2- oxyanion groups in which the C–O bonds are covalent and strong. Neighbouring CO3

2- groups do not share oxygens with each other, therefore there is no polymerisation (in contrast to the silicates)

• Although the C-O bond is strong, it is not as strong as the covalent bond in CO2. Hence, all carbonates react with acids:

2H+ + CO32- → CO2 + H2O

This "fizzing" reaction with diluted HCl is used as a diagnostic test for carbonate minerals

• Charge balance is achieved by accommodating the divalent cations Ca2+, Mg2+ and Fe2+

• The compositions of the common carbonates can be expressed in terms of three end-members (CaCO3, MgCO3, FeCO3), although solid-solution between all end-members is not complete (i.e. the compositional triangle contains large miscibility gaps):


Calcite Hexagonal-rhombohedral CaCO3 (L. calx = "burnt lime") • Structure is analogous to halite (NaCl), in which the small Na+ sites are filled by

Ca2+ ions and the large Cl- sites are replaced by CO32- groups


• The triangular CO32- oxyanions lie in planes at right angles to the c-axis, which

consequently has 3-fold rotational symmetry. Thus instead of being isometric,like halite, the symmetry of calcite is reduced to the rhombohedral crystal class

• The Ca2+ ions are in 6-fold coordination with oxygens in the CO32- groups. Each

oxygen is coordinated to two Ca ions as well as to one C ion at the centre of the CO3

2- groups

• Composition is mostly pure CaCO3, but also limited low-temperature solid solution with < 3 mol% MgCO3, <9 mol% FeCO3

Properties: Calcite occurs in over 300 different combinations of crystal forms! Most important are long or short prisms, rhombs, and scalenohedrons. Also stalactitic. Colour: white, colourless (also almost any other colour); Transparent to translucent; H = 3 on cleavage planes (Mohs' index mineral!) H = 2.5 on basal crystal faces; D = 2.7 g·cm-3; Vitreous lustre; Cleavage: { } perfect at 75°; Effervesces in cold, dilute HCl.

Use: Manufacture of cement (by heating calcite to 900°C: CaCO3 → CO2 + CaO; the

CaO reacts with water to form CaO-hydrates, which harden with time); soil fertiliser; flux for ore smelting; roading gravel.

Magnesite–Siderite Hex.-rhombohedral (Mg,Fe)CO3 solid-solution series (magnes = "Mg-bearing"; sidero = "iron") • Same structure as calcite (i.e. isostructural) • Solid-solution between magnesite (MgCO3) and siderite (FeCO3) is complete.

Small amounts of Ca may be present.

Properties: Rhombohedral crystals, magnesite aggregates often massive; Magnesite colour: white, grey; Siderite colour: light to dark brown; Transparent to translucent; H = 3.5–5; D = 3.0–4.0 g·cm-3; Vitreous lustre; Cleavage: { } perfect; Effervesces in hot, dilute HCl.

Use: Magnesite used to manufacture refractory bricks etc. (e.g. Trieben Mine,

Steiermark; Austria produces ca. 700 000 tonnes of ore per year); source of MgO for chemical industry (but metallic Mg is produced from seawater, not from magnesite). Siderite is an ore of Fe (e.g. Erzberg Mine, Steiermark, produces ca. 1.4 million tonnes of ore per year).

Dolomite Hexagonal-rhombohedral CaMg(CO3)2

• Structure is similar to calcite, but Ca- and Mg-layers alternate along the c-axis • Solid-solution exists with the Fe-bearing end-member ankerite CaFe(CO3)2

Properties: Rhombohedral crystals, sometimes saddle-shaped; Colour: pink, white, grey, colourless, ankerite is usually oxidized to yellowish-brown colours; Transparent to translucent; H = 3.5–4; D = 2.9 g·cm-3; Vitreous lustre; Cleavage: { } perfect; Crystals effervesce in hot, dilute HCl, powder effervesces slowly in cold, dilute HCl.

Use: Manufacture of certain cements, refractory bricks for steel-making process, MgO

for chemical industry

1011

1011

1011


Aragonite Orthorhombic CaCO3 (Aragon = locality in Spain) • Aragonite is a polymorph of calcite (see lecture notes on polymorphism) • The radius ratio of Ca:O in calcite is 0.714, which is very close to the limiting

value (0.732) between 6- and 8-fold coordination. When the calcite structure is compressed, the oxygen ions become smaller and the Ca:O radius ratio increases, allowing aragonite to adopt a compact orthorhombic structure with 9-coordination. Thus both the density and hardness of aragonite are greater than calcite (aragonite D = 2.94 g·cm-3, H = 4; calcite: D = 2.71 g·cm-3, H = 3)

• Although aragonite is metastable at surface T-P conditions, it precipitates at low pressure from warm springs (kinetically favoured in complex solutions) and in the shells of mollusks (extra energy supplied by living organism)

Properties: Acicular and tabular habits are common, also hexagonal prisms; Colour: colourless, white, pale yellow; Transparent to translucent; H = 3.5-4; D = 2.94 g·cm-3; Vitreous lustre; Cleavage: {010} distinct, {110} poor; Effervesces in cold, dilute HCl.


SUBJECT: SULPHATE MINERALS • Structures are characterized by small, highly polarizing S6+ ions covalently

bonded to oxygen in tetrahedral SO42- oxyanion groups. As in the carbonates,

neighbouring SO42- groups do not share oxygens with each other, therefore no

polymerisation results. • Charge balance is achieved by accommodating divalent cations, such as Ca2+ and

Ba2+ • The orthorhombic Ba-sulphate, barite (BaSO4) has such a high density (4.5 g·cm-3)

that it is used as "heavy-mud" to support drilling rods in the oil- and gas-industry Gypsum Monoclinic CaSO4·2H2O Structure consists of layers parallel to {010} in which SO4

2- groups are bonded to Ca2+ ions. These layers are separated by sheets of H2O molecules, which are only weakly bound to each other via hydrogen bonds. Cleavage along these H2O sheets is therefore excellent.

Properties: Prismatic and tabular crystals, often swallow-tail twins; Colour: colourless, white, grey, yellow; Transparent to translucent; H = 2 (Mohs’ index mineral!); D = 2.3 g·cm-3; Lustre: vitreous to silky; Cleavage: {010} perfect, yielding thin sheets; conchoidal cleavage surface parallel to {100}, fibrous cleavage parallel to {011} (these 3 different cleavages are diagnostic of gypsum); Dehydrates completely upon heating to 95°C.

Use: Manufacture of "plaster of Paris" (made by heating gypsum to 75°C to drive off

75% of H2O molecules. When later mixed with water, the partially dehydrated gypsum re-absorbs H2O molecules and hardens). Also used for wallboard, paints, soil fertiliser. Austria produces ca. 900 000 tonnes of gypsum per year.


Baryte Orthorhombic BaSO4 (barys = “schwer”) • Structure consists of large bivalent cations zweiwertige Kationen (Ba) bonded to sulfate-iones. Each Ba is surrounded by 12 oxygens, the latter belonging to 7 distinct SO4

2- oxyanion groups Properties: perfect cleavage along {001}; H = 3 - 3.5; D = 4.5 g·cm3/ (pretty heavy for a non-metallic minera !); Color is colorless, white to light blue, yellow, red, transparent; tabular crystal form common, also rosette aggregates (“Wüstenrose”)

Use: as "heavy mud" in oil drillings (i.e. supports the stability of the drillings and prevents puff blowing); Color industry; Paper industry; radiation protection; contrast material in medicine.


SUBJECT: SULFIDE MINERALS

• The sulfides are the main group of ore minerals, being the major sources of many transition-elements and of sulfur

• Metal–sulfur bonding (e.g. M2S, MS, MS2) varies between ionic, covalent and metallic

• Partial metallic character makes many sulphides soft and electrically semi-conductive

• Most sulfides are opaque, and have distinctive crystal colours and streaks

Pyrite Isometric FeS2 (pyros = "fire", because it emits sparks when struck by steel) Structure is analogous to halite (NaCl), where Fe occupies the Na sites and S2 pairs occupy the large Cl sites Properties: Equidimensional cubic, "pyritohedral", and octahedral crystals. Cubic

crystals usually have striated faces. Colour: pale brass-yellow; Streak: greenish or brownish-black; H = 6-6.5; D = 5 g·cm-3; Conchoidal fracture; Brittle; Lustre: splendent metallic; Opaque; Paramagnetic.

Use: Main Fe-ore in countries where Fe-oxides are scarce; Major source of S for

production of H2SO4 and FeSO4 (dyeing, inks, wood preservative, disinfectant). Also important source of Au, which often occurs as microinclusions in pyrite


Galena Isometric PbS Structure is analogous to halite (NaCl) with Pb in place of Na and S in place of Cl. Properties: Crystals in cubic or cubic + octahedral forms. Colour: lead-grey; Streak:

lead-grey; H = 2.5; D = 7.5 g·cm-3; Cleavage {001} perfect; Lustre: bright metallic; Opaque

Use: Main ore of Pb and important source of Ag, which occurs in solid-solution.

Used in batteries, low-melting alloys, metal pipes and other products; shields against radioactivity; as oxide used to make glass, glazes, paints.

Sphalerite Isometric ZnS (sphaleros = "treachery", because it can look like galena but contains no Pb; also Zinkblende, where blende = blind or deceiving) Structure is analogous to diamond, where half the C sites are replaced by Zn and half by Fe Properties: Commonly as tetrahedral, cubic and octahedral crystal forms. Colour:

colourless, green, yellow, brown to black; Streak: white to yellow and brown; Transparent to translucent; H = 3.5-4; D = 4 g·cm-3; Cleavage {010} perfect; Lustre: non-metallic and resinous to submetallic.

Use: Main ore of Zn. Used in alloys (e.g. brass = Zn + Cu), galvanized iron, electric

batteries, paint manufacture, wood preservatives, dyeing, medicine.


Chalcopyrite Tetragonal CuFeS2 (chalkos = "copper" + pyrite) Structure is analogous to sphalerite (ZnS), where half the Zn sites are replaced by Cu and half by Fe. Properties: Usually massive habit but sometimes as tetragonal scalenohedrons.

Colour: brass-yellow, often tarnished to bronze or iridescent; Streak: greenish-black; H = 3.5-4; D = 4.2 g·cm-3; Conchoidal fracture; Brittle; Lustre: metallic; Opaque

Use: Main ore of Cu. Used in alloys, electrical cables, etc.

Crystal lattice of chalcopyrite


SUBJECT: HALIDE MINERALS

• Halogens form large, singly-charged anions in minerals, of high electronegativity. When combined with large, electropositive cations, the structures consist essentially of perfect spheres with ionic bonding

• The ionic packing which results forms crystals with high symmetry, brittle tenacity, high melting points but high aqueous solubility

Halite Isometric NaCl (halos = "salt") Each cation and each anion is surrounded by 6 closest neighbours in octahedral coordination Properties: Cubic crystals; Colour: colourless to white, yellow, red, blue, purple;

Transparent to translucent; Salty taste; H = 2.5; D = 2.2 g·cm-3; Cleavage {001} perfect; Brittle; Melting point at 1 bar = 801°C .

Use: Food additive and preservative; Chemical industry: HCl and sodium

compounds, leather treatment, fertilizer, road salting, weed killer.

Fluorite Isometric CaF2 (fluere = "to flow", because more easily melted than similar minerals) Face-centred cubic lattice in which each Ca2+ is in cubic coordination with 8 surrounding F- ions; each F- is tetrahedrally coordinated to four Ca2+ ions

!


Properties: Usually cubic crystals; Colour: light green, yellow, bluish-green, purple, colourless, white, pink, blue, brown; Transparent to translucent; fluoresces under UV light; H = 4 (Mohs’ index mineral!); D = 3.2 g·cm-3; Cleavage {111} perfect; Brittle; Lustre: vitreous

Use: Chemical industry: production of HF; fluorine compounds, flux for steels,

glass, fibreglass, pottery, enamels, optical lenses and prisms

!

!


SUBJECT: SOME IMPORTANT ELEMENTS Graphite Hexagonal C (derives from Greec: to draw) Structure: C atoms are bonded together (i.e. very strong covanlent bond) in form of hexagonal rings. Between these hexagonal ring-sheets weak Van der Waal´s bonds. Distance from sheet to sheet is 3.44 Å). Properties: perfect cleavage along {0001}; H = 1 - 2; D = 2.23 g·cm-3; metallic lustre,

sometimes dull; Color: black to black-grey; Streek: black greasy, loosing color very easily!

Use: Steel manufacture; founding technology; facing of cucibles; grease; pencils;

batteries; electrodes; reactors. Diamond Isometric C (derives from Greec: adamas = undestroyable) Struktur: Polymorph of graphite. Crystal structure is formed by tetrahedrons of C, which are bonded by strong covalent bonds; C-C distance is 1.54 Å.


Properties: perfect cleavage along {111}; H = 10 (Mohs); D = 3.52 g·cm-3; Lustre: diamond lustre, unpolished crystals look fatty; Color: colorless to sometimes pale yellow to pink („fancy diamonds“); octahedrons common crystal forms Use: gem stone, grinding, polishing and cutting material, drilling heads.


Pressure-‐temperature diagram showing stability fields of graphite and diamond

ROCK (Gestein)

Einführung in die Mineralogie und Petrologie, LV 620.073

Rock = natural formed aggregate of one or more types of minerals

(strictly: any naturally formed aggregate or mass of mineral matter, whether or not coherent, constituting an essential and appreciable part of the earth's crust)

How to classify rock? Physical conditions include: Temperature

Pressure Deformation (Differential Pressure)

Definitions: • mineral = natural formed chemical compound having a definite range in chemistry

and a characteristic crystal form. • crystal = regular polyhedral form, bounded by plane surfaces, which is the outward

expression of a regular repeating internal arrangement of atoms. • grain = particles which comprise a rock.

A B C Chemical analysis mineral composition Physical conditions of rock formation geometric properties

grainsize shape relation between grains

PLATE TECTONICS AND ROCK FORMATION (1)


SOURCES OF HEAT WITHIN THE EARTH • Earth has not completely cooled since its molten state approximately 4 · 109 years ago • In addition to residual heat (= left over from that originally caused by meteorite

impacts and gravitational compression), radioactive decay of certain elements continually adds heat to the Earth

• Most important sources of radioactive heat are K, U, Th:

40K !

40Ar + " + 1.51 MeV (mostly as heat)

40

K !40

Ca + "#+ heat

232

Th ! K !208

Pb + 6" + 4#$+ heat

238

U ! K !207

Pb + 7" + 4#$+ heat

238

U ! K !206

Pb + 8" + 6#$+ heat

• Example: Typical heat production of granite: 3.4 · 10-2 J/kg·year • High flow of heat from interior to surface of Earth drives huge convection cells in

Asthenosphere (and at deeper levels), dimension ≈ 103 km; rates of movement ≈ cm / year

CONSEQUENCES OF ASTHENOSPHERIC CONVECTION FOR LITHOSPHERE • Ascent of thermally buoyant Asthenosphere in upflow path of convection calls

increases heat-flow into Lithosphere → thinning of Lithosphere and generation of new crust by magmatism → constructive lithospheric plate boundaries (e.g. at mid-oceanic ridges)

• Descent of cooled Asthemosphere in downflow path of cell decreases heat-flow

locally → mechanical subduction of lithospheric plates → destructive lithospheric plate boundaries (e.g. at oceanic trenches)

• Thus, sinc about 4 · 109 years ago, the Lithosphere has been divided into slowing-

moving plates (movement rates ≈ cm / year) • Mechanical interaction of these plates (collision, sliding along edges, overriding, etc.)

is known as plate tectonics • Plate tectonics has lead to the migration of continental and oceanic crust across the

Earth's surface, and to the formation of Earth's topography (mountain chains, high plateaux, deep ocea basins, etc.) (see Lectures from Geology 1)

• Heat flow and gravity are thus the driving forces of plate tectonics. In this context

and on long time-scales, Earth is therefore best viewed as a dynamic system (cf. static model in lecture 1)



CYCLING OF ROCKS WITHIN THE LITHOSPHERE • Plate tectoncs drives rocks through cycles of P-T-deformation conditions. Rocks

formed by magmatism ("magmatic" or "igneous" rocks, e.g. granites) can be uplifted above sea-level, eroded and redeposited as sediments in rives, lakes and ocean basins (hardening of these sediments produces "sedimentary rocks"). Deep burial of these sediments (or magmatic rocks) in, for example, subduction zones, leads to mineralogical recrystallisation (the recrystallised rocks are termed "metamorphic"). Metamorphic rocks sometimes melt to form new magmatic rocks, or they may be uplifted and eroded, and so on through the cycle ...

• Because of plate tectonic cycling, rocks can have long complicated P-T-deformation

histories.



Approximately ranges of pressure and temperature over which the traditional recognized sedimentary, metamorphic, and igneous (or magmatic) rock-forming processes operate. The indefinite boundaries between the three reals are real. Both P and T increase with depth in the Earth, but at different rates in different geological environments. A geothermal gradient which expresses the relation between T and depth in the Earth, such as might occur in an active mountain belt, is shown by the curve line. Heavy bars on the right of the diagram indicate the ranges of depths to the base of the oceanic and continental crusts.



MINERAL REACTION KINETICS • During plate-tectonic cycling, the minerals that make up rocks usually react to the

changes in P, T and deformation conditions. E.g. with increase in P and T within a subduction zone:

calcite → aragonite

• Whether minerals react completely (i.e. equilibrate) to changes in P-T-deformation

conditions depends on 3 main factors: 1. temperature (T) 2. time (t) 3. presence of fluids T-t dependence : The rate, k, of a specific reaction is expressed by the Arrhenius

equation:

k = A !e"E

a/ RT

where A is an empirical factor (meaning is not well understood) Ea is the activation energy for the specific reaction (i.e. the height of the potential-energy barrier to reaction), R is the gas constant, and T is the temperature. Thus at low T, k is small (reaction is slow) and at high T, k is high (reaction is fast)

Fluid dependence: Diffusion of chemical components through aqueous fluids or

silicate melts is faster than through crystal lattices or along crystal boundaries, hence fluids in rock pores catalyse mineral reactions.

Depending on the rate of plate-tectonic cycling and on the thermal and fluid history, some rocks may show no mineralogical "memory" (relict features) of their past, while other rocks may contain evidence of many stages in their history. Examples:

1) Most granites (cf. lab practical 1) do not preserve mineralogical evidence of their origin (e.g. whether they derived from melted metamorphic rock or re-melted magmatic rocks). Most minerals in granites are not thermodynamically stable in contact with ground-water. "Fresh" granites are preserved preferentially in regions of cooler climates; tropical weathering often destroys the magmatic minerals, i.e. the granite re-equilibrates at surface P, T and chemical conditions.

2) Diamonds are brought to the Earth's surface from depths of > 200 km by extremely rapid ascent of magmas. Cooling is so rapid that the thermodynamically favoured reaction to graphite:

Cdiamond → Cgraphite

is not possible (low T, short time). Similarly, kinetics are too slow at the Earth's surface to permit the thermodynamically favoured reaction of diamond with O2 in the atmosphere:

Cdiamond + O2 → CO2 (gas)

PETROGENETIC INTERPRETATION


ROCK MEMORY In Lecture 1 we learned that thermodynamic equilibrium (the energetically most stable state) is usually attained deep in the Earth's interior. Due to kinetic effects, most rocks sampled in outcrops preserve minerals that are thermodynamically metastable at the Earth's surface. This common metastability or partial reequilibration of minerals constitutes a "memory" of past states of the host rock. This is fortunate, because it allows us to reconstruct the history of rocks, using detective methods known as petrogenetic interpretation. Petrogenetic interpretation are based on several sources of information:

Source of information Scale Field relations large scale (map) Whole-rock chemical composition hand-specimen Mineralogical composition hand-specimen Crystal structure sub-microscopic

Rock fabrics (= "Gefuge") hand-specimen to microscopic

Experiments small scale short term

Theory all scales EXPERIMENTAL PETROLOGY The stability of minerals with respect to P-T conditions can be investigated experimentally (P, T, bulk composition and time are all controlled)

→ data base of thermodynamic properties of minerals → reconstruction of rock history from relict mineralogical features → reconstruction of plate tectonic history of rock sample → prediction of location of natural resources (e.g. petroleum, gas,

metals), volcanic eruptions and earthquakes.

ROCK FABRIC


Rock fabric (Gefüge auf Deutsch) ≡ the set of geometric properties of a rock sample

(non-compositional) SIX BASIC ROCK FABRICS

1) Sequential crystallization 2) Glassy 3) Drusy (aggregate of crystals, commonly incrusting the walls of a cavity) 4) Granoblastic (equidimensional elements) 5) Clastic (detritical, consisting of fragments of rocks) 6) Strained (deformed)

• Many rocks are combinations of these fabric types • The fabric types do not always correspond to the classic genetic division into

magmatic, sedimentary and metamorphic rocks PETROGRAPHIC FEATURES

PETROGENETIC INTERPRETATION


SEQUENTIAL CRYSTALLIZATION


• Mechanism of formation:

Solidification of a fluid solution, caused by changes in P, T or composition (via reaction or mixing). Crystals grow in sequence, according to their relative solubility in the mother solution.

• Types of fluid solution:

1. Silicate melts or sulphide melts 2. Cool groundwater, riverwater, seawater 3. Hydrothermal solutions 4. Gases (volcanic, metamorphic, diagentic)

• Rates of solidification:

Slow solidification of melts (e.g. magma chambers) → precipitation of silicate or sulphide crystals cf. quenching → glassy fabric Slow crystallization from melts or precipitation from aqueous solutions → "perfect" crystals Rapid crystallization or precipitation → "imperfect" crystals, containing solid or fluid inclusions.

• Diagnostic features:

Perfect crystal form (euhedral) developed only by earliest crystals in sequence; later crystals or glass adopt irregular shapes (anhedral) determined by shape of remaining space.

SEQUENTIAL CRYSTALLIZATION


GLASSY FABRIC


• Mechanism of formation

Interruption of sequential crystallization of fluid solutions • Rates of solidification

Slow, sequential crystallization of silicate melt in magma chamber is interrupted by volcanic eruption Extremely rapid cooling (quenching) of melt during eruption forms glass in space between older crystals

Note: sulphide and carbonate melts do not form glass!

DRUSY FABRIC



Interruption of sequential crystallization of fluid solutions • Rates of solidification

Sequential crystallization from aqueous solution stopped by exhaustion of solutes in flowing solution or blockage of fluid flow

Escape of fluid during erosion leaves open spaces (druses, vugs) between idiomorphic crystals

GRANOBLASTIC FABRIC



Solid-state mineral growth (recrystallization of pre-existing minerals) caused by changes in P, T or composition (via reactions)

• Rates of recrystallization

Rapid for silicate, oxide, carbonate, sulphide minerals at very high temperatures (near melting temperature of rock), hence common in metamorphic rocks Extremely slow at low temperature (no solid-state recrystallization possible)

• Diagnostic feature

Crystal faces at triple-junctions ideally meet at 120 °

CLASTIC FABRIC


• Composed of broken fragments (clasts) of pre-existing rocks (lithoclasts) or shells (bioclasts), usually within a matrix of finer grains or of crystalline cement.


Disaggregation, dispersal, deposition and lithification of pre-existing rocks. 1) Disaggregation:

Surface weathering → detritus Volcanic explosion → ejecta Fault movement or slope collapse → brecciation

2 ) Dispersal:

Relatively high-density clast transported by relatively low-density fluid (water, lava, gas, air). Abrasion during transport produces rounded clasts.

3) Deposition:

Characterised by gravitational settling structures (e.g. bedding, cross-bedding, sorting of clats sizes) • Marine and terrestrial sediments • Pyroclastic (volcanic) deposits

4) Lithification:

Cementation of clastic sediments (diagenesis). Welding of pyroclastics


Clasts do not show typical crystal forms

STRAINED FABRIC



Mechanical destruction or deformation of pre-existing mineral or rock grains, by tectonic forces (brittle faulting, ductile shearing) or shock forces (meteorite impact)

• Deformation realms

Brittle processes common at low P-T conditions and at high strain-rates Ductile processes common at high P-T conditions and at low strain-rates


Bending, distortion, rupture, flattening of mineral or rock grains; reduction of original grain size

STRAINED FABRIC



INTRODUCTION TO MINERALOGY AND PETROLOGY

SUBJECT: SEDIMENTS (SEDIMENTARY ROCKS)

Importance

• Sediments represent the world´s largest reservoirs for fossil energy resources

(petroleum, coal).

• Important mineral deposits of metallic raw materials are hosted in sedimentary rock

sequences (e.g., Fe, Mn, Cu, Pb, Zn etc.).

• Economically important industrial minerals (carbonates, salt, gypsum etc.) and

building materials derive from sedimentary rocks.

• Sedimentary rocks form ¾ of the Earth´s surface, thus they are the most important

rocks regarding the fields of technical geology.

Fig. 1. Sedimentary rocks cover most of the Earth´s surface, whereas the Earth´s crust is mostly composed of magmatic and metamorphic rocks (from Press & Siever, 1995).


Sedimentary processes – a summary

Source rock magmatic

metamorphic

sedimentary

Wheathering mechanical: components (clasts)

chemical: dissolved material

Transport water

ice

wind

gravity

detrital components (clasts) suffer from:

decrease of grain-‐size

increasing roundness

detrital grain gradation

Deposition → Sediment (soft, unconsolidated)

mechanical

chemical

biochemical

Diagenesis → Sedimentary rock (solid,

consolidated)


Fig. 2. Formation of sedimentary rocks

Sedimentary environments

Fig. 3. Sedimentary environments on land and in the sea


Classification of sedimentary rocks

The classification of sedimentary rocks is based on the type of process they are formed.

Four main groups can be distinguished.

The boundaries between the 4 groups are gradational. One sedimentary rock can be

classified in more than one group.

CLASTIC SEDIMENTS

They form by mechanical deposition of rock clasts:

(1) Siliciclastic sediments

• Conglomerate, Breccia

• Sandstone

• Siltstone, Mudrock (Shale, Argillite)

(2) Volcanoclastic (pyroclastic) sediments

• Tuffs: Deposition of volcanic material (ash, lapilli etc.)

NONCLASTIC SEDIMENTS

(3) Biogene, biochemical and organic sediments

Limestone (and Dolomites)

• SiO2-‐rich Sediments (Cherts)

• Phosphorites

• Coal und Oil shales

(4) Chemical sediments

• Evaporites

• Fe-‐rich Sediments (Iron stones; Banded Iron Formations)


Classification schemes

Siliciclastic sediments -‐ grain size

Biochemical and chemical sediments


Diagenesis

After deposition the detrital components are compacted and transferred into solid

sedimentary rocks. The combination of mechanical, chemical and biological processes

leading to solid rock is defined as diagenesis.

• Compaction due to overburden

• Cementation due to precipitation of minerals from pore fluids

• Recrystallisation of minerals due to increase of temperature and change of pore fluid

chemistry

Diagenetic processes result in changes in composition and fabric. Most processes result

in lithification of unconsolidated material.

Fig. 4. Processes of diagenesis


Fabrics of sedimentary rocks

Clastic fabric dominates in typical clastic and volcanoclastic sediments; strongly variing

ratio of detrital components and matrix.

The majority of sedimentary rocks display bedding textures (i.e. in most cases visible in

the outcrop as well as in hand specimens).

Important sedimentary textures

a) Bedding Horizontal bedding Cross bedding Ripple bedding Graded bedding Growth bedding b) Synsedimentary deformation Debris flow Sedimentary folding Load casts c) Chemical textures Dissolution textures (e.g., stylolites, "bird eye structures") Diagenetic cementation d) Organic textures Fossils

Growths lamination (e.g., Stromatolites) Trace fossils

Fig. 5. Origin of horizontal bedding in sediments


Siliciclastic sediments

Derivation of the detrital components: Magmatic, metamorphic, or sedimentary rocks,

that underlie erosion.

Transport: Wind, glacier, river, waves, tides, mud flows etc.

The detrital material is transported in suspension (fine) or at/close to the bottom

(coarse). Detrital material on the bottom is transported by jumping, rolling and sliding.

Fig. 6. Transport of clastic material in flowing medium

Suspension: Important criterium is that the ascending motion of the detrital particle is

higher than the sinking rate (w):

w = Δρd2g / 18µ (Stoke`s Law)

Δρ = density balance between particle and fluid (ρclast -‐ ρfluid)

d = particle-‐diameter

g = gravitation constant

µ = dynamic viscosity of the fluids

Turbulent flow dominates over laminar flow under the following conditions:

high current velocity, low viscosity.


Current velocity: The transport media change velocity occording to the slope angle of the

river bed, or the intensity of the current (e.g., tidal current, river current, wind force).

Gradation: Reduction of the current velocity or change of the current in general (i.e.

turbulent vs. laminar) leads to selective deposition of the detrital components according

to the grain-‐size. The consequence is gradation as a function of grain-‐size and density of

the detrital material:

(1) horizontal distance from the source area of erosion:

gravel (close to source area) → sand → mud (far away from source area)

(2) vertical distance from the footwall into the hanging wall (graded bedding):

gravel, coarse sand (in the foot wall) → silt → mud (in the hanging wall)

Fig. 7. A typical fluviatile sedimentyry cycle


Carbonate sediments

Sedimentary rocks composed predominantly of carbonate minerals are defined as

carbonate sediments. Those composed dominantly of calcite are defined as limestones,

sediments composed dominantly of dolomite are defined as dolostones.

Solubility of CaCO3 in the seawater: • Sea water is thermodynamically saturated with calcite (the triagonal CaCO3

polymorph) and undersaturated regarding aragonite (the orthorhombic CaCO3 polymorph).

• Calcite is characterised by retrograde solubility (i.e. solubility decreases with increasing temperature, solubility increases with depth and decreasing temperature).

Precipitation of CaCO3 in the seawater: • Calcite precipitates as mud (i.e. micrite) or in form of ooides in warm shallow

marine environments of tropical areas. • Most marine organisms build their skeletons in aragonite. Bioenergy is

required to stabilise aragonite compared to calcite under surface conditions.

Dissolution of CaCO3 in the seawater: • CaCO3 starts to dissolve several 100 m below the sea level. However,

plankton builds CaCO3 skeletons continuously. There is a balance of CaCO3 solubility and creation of CaCO3 between ~4.5 and 5 km depth (≡ carbonate compensation depth”, CCD) → therefore, there is a complete lack of carbonate sediments below 5 km depth!

Fig. 8. The carbonate compensation depth CCD-‐Grenze


Classification of limestones

Fig. 9. Classification of limestones after Dunham (1962); from Klein and Philpotts (2013)

Fig. 10. Growth of stromatolites, one of the oldest life forms on Earth -‐ an example of a boundstone


Formation of reefs

Fig. 11. Model for the formation of reefs according to Darwin


Carbonate platforms

Fig. 12. Map and cross section through the Bahamabank (Press & Siever, 1978)


Diagenesis of carbonates and formation of dolostone

Replacement by calcite: Aragonite in bioclasts (shales, etc.) is thermodynamically

metastable and will be replaced by coarse-‐grained thermodynamically more stable

calcite.

Replacement by dolomite: Calcite (primary or diagenetic) is stable in pore fluids with

high Ca/Mg; increase of Mg in the pore fluids leads to replacement of calcite by dolomite

(CaMg(CO3)2).

Mechanisms leading to a decease of the Ca/Mg ratio:

• Input of Mg-‐rich ground water

• Precipitation of calcite and gypsum (CaSO4.2H2O) by evaporation of seawater in a Sabkha

or restricted lagoon environment

• Mixture of seawater with ground water

• Input of formation fluids, released from compacted sediments, into sedimentary

basins

Fig. 12. Models of dolomitization


SiO2-‐rich sediments (Cherts)

Chert = extremely fine-‐grained SiO2-‐rich sediment (amorphous/cryptocrystalline silica)

Genesis

(1) Banded SiO2-‐rich sediments

• Primary deposition (marine, non-‐marine) of skeletons of plankton (Radiolaria,

Diatomea, Sponges) → main source for SiO2-‐rich mud along the sea floor and in

lakes below the CCD-‐boundary. Banded texture results from annual variation in

the nutrient supply of plankton.

• Primary chemical precipitation of SiO2 from hydrothermal fluids at the sea floor

(e.g., at mid-‐oceanic ridges)

(2) Nodular SiO2-‐rich sediments: secondary diagenetic formation of SiO2-‐minerals

within pore fluids; form chert nodules within limestones (flintstone)

Fig. 12. Microfossils (Radiolaria, Foraminifers) with SiO2-‐ and carbonate skeletons from recent marine

muds; SEM image.


Evaporites

• Important source for halite, gypsum and other halides.

• Marine evaporites form by evaporation of seawater within restricted marine

basins (lagoons) in arid climates; pogressive evaporation leads to enrichment of

Na+, Cl-‐, Mg++, SO4-‐-‐ etc. in the seawater.

• The characteristic evaporite sequence (i.e. starting with carbonates, then gypsum, then halite, and finally K- and Mg-salts) is controlled by progressive saturation of these elements in the evaporating seawater.

• Repeated input of fresh water and continuous evaporitation is significant for the

generation of thick (i.e. up to several hundreds of metres thick) evaporite

sequences.

Fig. 13. Evaporiation of seawater in a logoon environment; from Press & Siever (1995)


Other important sediments

Fe-‐rich sediments

These contain more than 15 wt.% Fe in form of ironoxides (hematite) und Fe-‐hydroxides

(goethite); Fe-‐silicates and Fe-‐carbonates (siderite) are less important.

The majority of banded iron ores (banded iron formations, BIF) have their origin at

an early stage of Earth development, at times when the atmosphere was poor in

oxygen; Fe was more soluble, was transported in seawater and was precipitated there.

Volcanism as an important derivation for Fe played an important role.

BIFs are the economically most important deposits for iron ore!

Organic sediments

Organic matter ist the most important source for fossil energy ressources, such as coal,

oil and gas. Coal is a biochemical sediment, that forms by diagenesis of plant material

(peat). Oil and gas represent fluids, that form via diagenesis of fossilized organic

matter (kerogen); deep burial transfers organic matter, deposited together with

inorganic material, into a fluid. The fluid may migrate into porous sediments (e.g.,

predominantly clastic and/or carbonate sediments) and may be accumulated and

stored there to finally form an petroleum deposit. Oil and gas are composed of organic

hydrocarbon complexes.

Fig. 14. Formation and migration of hydrocarbons in sedimentary rocks. Oil and gas migrate within strata of high permeability (e.g. sandstone) and are trapped in an anticlinal structure. Blue is a cover sequence of low permeability (e.g., shale).

IntroMinPet Lecture Red

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Transcript of IntroMinPet Lecture Red