GEOS102 Lectures

Big bang expansion (bya?)

13.7 billion ya

Earth location in galaxy

within Orion Arm region of Milky Way galaxy on outskirts of Virgo Supercluster

How many galaxies in the universe

~500 billion

Average light years and stars of galaxy

• 1 galaxy ~ 100,000 light years

• contains ~100 billion stars

Our solar system light years (hours)

~ 6 light hours

Sun mass % of our solar system

99.8%

Earth shape (circumfrence? mass? average density?)

• Circumference ~40,000 km

• Mass ~ 5.97 × 10²⁴kg

• Average density 5400kg m³

Shear waves (S-waves)

• ground motion is perpendicular to wave direction

• do not propagate in fluids

Longitudinal waves (P-waves)

• ground motion is perpendicular to wave direction

• faster than S-waves

Igneous rocks form from . . .

Form from the cooling and solidifying of molten magma

• Intrusive crystallize at depth

• Extrusive erupt at surface

Sedientary rocks from . . .

• Sediments are eroded, transported, deposited and lithified to become clastic rock.

• Precipitation of minerals out of solution forms chemical rocks.

Metamorphic rocks form . ..

Rocks are changed under influence of heat, pressure, deformation or some other agent without passing through a liquid phase.

• Regional in response to increased temperature, pressure and deformation over large area

• Contact in response to increased heat of intrusive igneous body within the crust

Distribution of rock types on earth

• Igneous rocks make up vast majority of Earth’s crust by volume

• Sedimentary rocks cover most of Earth’s surface

Age of earth

4.56 Ga

Oldest minerals

Zircons in West Australia, ~4.4 Ga ago

Earliest water

~4.4 Ga ago

Earliest life on Earth

Stromalites West Australia, ~3.7 Ga ago

Plate tectonics began . . .

~3Ga ago

Mineral

Naturally occurring inorganic substance with a definite chemical composition and a regular internal crystalline structure

Crystalline structure

Orderly internal structure made up of regularly repeating arrangement of atoms, ions or molecules in 3-dimensions

Chemical Composition

• Cations and anions go together

• constituent elements must satisfy charge-balance

• Certain groups of elements have similar electrostatic properties/behaviour but different sizes

• Relative sizes of different atoms control how well they fit together in a regular internal structure (ie, unstable, stability limit, stable)

Mineral classification based on . . .

• Chemistry and structure

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Elements making up 99.9% of crust

• Oxygen, O

• Silicon, Si

• Aluminium, Al

• Iron, Fe

• Calcium, Ca

• Sodium, Na

• Magnesium, Mg

• Potassium, K

• Titanium, Ti

Silica

• Oxygen + Silicon

• ~75% of Earths crust by weight

• ~90% of Earths crust by volume

Oxides % of crust

• ~ 5%

Structural groups

• Isolated

• Single chain

• Double chain

• Silicate sheet

• Framework silicate

Silicates structure base on . . .

• SiO₄^4- tetrahedon

Isolated mineral chemistry example

Olivine

Single chain mineral chemistry example

Clinopyroxene

Double chain mineral chemistry example

Hornblende

Silicate sheet mineral chemistry example

Biotite

Framework silicate mineral chemistry example

Quartz

Lustre

The way light interacts with the surface of a mineral

Hardness

Resistance to abrasion

Measured by Mohs hardness scale

Cleavage

Tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice

Crystal form/habit

Characteristics of external shape of individual crystal or groups of crystal

Crystallography

Experimental science of determining the arrangement of atoms in crystalline solids

Steno’s law (crystals)

Angles between equivalent faces of crystals of the same mineral

X-ray diffractions of crystals

• X-ray inference patterns give direct evidence for the periodic atomic structure of crystals

• Spacing of x-ray diffractions are a function of the spacing between the atoms they impact

X-ray diffraction used on . . .

• Single crystal to determine precise structure

• Bulk powders to determine what minerals are present and in what abundances

• Protein structure, DNA

Focus on X ray diffraction in crystallography

• Crystal structures determined by how atoms are arranged

• These structures in turn control how light passes through the crystal

Crystal systems

• Defined either in terms of axes & angles, or symmetry

• Lengths of 3 (or 4) crystallographic axes

• Usually a, b, c, or (a=a=a, if all = ) and the angles between them

Cubic

• All 3 axes of equal length

• All at 90^o to each other

• a₁ = a₂ = a₃

Tetragonal

• 3 axes, 2 are equal length

• All at 90^o to each other

• a₁ = a₂ =/ c

Hexagonal

• 4 axes, 3 are equal length

• 3 of equal length are 120^o apart in a plane, the 4th is perpendicular to that plane

• a₁ = a₂ = a₃ =/c

• Subgroups are hexagonal (6 fold) and triagonal (3 fold)

Orthorhombic

• 3 axes at different lengths

• All 90^o to each other

• a =/ b =/ c

Monoclinic

• 3 axes of different lengths

• one axis is inclined ( not at 90^o)

• a =/ b =/ c

Triclinic

• 3 axes of different lengths

• None are 90^o to each other

• a=/b=/c

Crystal system vs Crystal form

not equivalant

• Crystal structure defines ‘systems’

• Crystal faces may or may not develop depending on growth conditions, if they do develop they are controlled by structure

Electromagnetic radiation and minerals

Because minerals have a regularly repeated, ordered crystalline structure, they interact with electromagnetic radiation in predictable ways

Electromagnetic spectrum

• All wavelengths have different purposes

• Properties are fundamental for nearly all spectrocopic properties of matter

Electromagnetic light

• Visible light is 1 = 7700-3900 Å, red-violet

• Monochromatic light is light of a single wavelength

• White light is a mixture of all visible colours

Speed of light in a vacuum

• constant

• c = 3 × 10⁸ m/2 (MAXIMUM)

Speed of light through any other (than a vacuum) medium

• Is slower

• c_m = V^λ

• c = velocity (changes in different media)

• m = other medium

• λ = wavelength (changes in different media)

• V = frequency (DOESNT CHANGE_

Refractive index (what? symbols?)

Speed of light in a medium

• Higher is denoted as N

• Lower is denoted as n

Faster light in a mineral =

lower refractive index

Refractive index equation

n = velocity in air/velocity in mineral

Light changes in substances (behaviour according to Snells law)

• refracts or changes direction when passing from one substance to another

• Light is refracted toward a line drawn perpendicular to the interface if the light is passing into a medium with higher RI and away if entering medium with lower RI

Snells law equation

n₁ sin(0₁) = n₂ sin (0₂)

Polarized light

• can filter, or polarize, a beam of normal light by filtering out all but one plane of vibration. This is plane polarized.

• Can double filter, or polarize, a beam of normal light. This is cross polarized.

Opaque minerals optical mineralogy

• Do not transmit light under any circumstances

• Grain always appears black in PPL and black in XPL

Isotropic minerals optical mineralogy

• Affect light the same way in all directions

• Grain always apppears visible in PPL and black in XPL

Anistotropic minerlas

• Affect light differently in different directions

• Grain is visible in PPL and XPL

Observations in PPL

• shape

• cleavage

• relief

• colour/pleochroism

• alteration

Observations in XPL

• twinning

• extinction

• birefringence/interference colours

What proportion of the earth is the core

~15%

What proportion of the earth is the mantle

~84%

What proportion of the earth is the lithosphere

~1%

Lithosphere (where?)

• Where most earthquakes occur

• Where ore deposits form

• Where hydrocarbons accumulate

Lithosphere (gradient? on? recyc?)

• Highest thermal gradient

• Rafted on convecting interior (tectonic plates)

• Recycled at subduction zones

Lithosphere (partial melting?)

• Convecting solid

• Magma produced by partial melting in crust and upper mantle

• Melt is bouyant and movile, can ascend into and through lithosphere

Magma intruding into crust . . .

• Oceanic crust: thin, young and hot = magma easily reaches surface to extrude along mid ocean ridges

• Continental crust: thick, old, cold = more difficult for magma to extrude

Magma

• melt, usually silicate, produced by partial melting in interior of planet

• Molten rock which becomes lava when erupted at the surface

Classification of igneous rocks

• Chemical (mineralogical) composition - function of magma generation and differentiation process

• Textural characteristics - function of cooling/emplacement environment

Igneous compositional groups

• Felsic

• Intermediate

• Mafic

• Ultramagic

SiO₂ content changes with compositional group

• Felsic high ~ 70%

• Ultramafic low ~ 40%

Sodium and Potassium (Na and K) content changes with SiO₂ changes

Increases as SiO₂ increases

Iron, magnesium and calcium (Fe, Mg and Ca) content changes with SiO₂ changes

Decreases as SiO₂ increases

Temperature at which melting starts changes with SiO₂ changes

Decreases as SiO₂ increases

• ~700^oC Felsic

• ~1200^oC Ultramafic

Rhyolite (Texture and mineral composition)

Glassy volcanic rock, large crystals of quartz and feldspar and trace of amphibole glass

Microgranite (Texture and mineral composition)

Consists of crystals of quartz and feldspar, with some biotite and amphibole

Granite (Texture and mineral composition)

Coarse, interlocking grains of quartz and feldspar, with some biotite or muscovite

Basalt (Texture and mineral composition)

• Hyper quenched basaltic glass erupted as small fragment in submarine eruption, no crystals

• Glassy basalt lava erupted onto ocean floor and quenched. High nucleation, low growth rate, crystals in matrix

• Basalt lava flow erupted subarielly with crystals that nucleated and grew in hot deep crust prior to eruption

Gabbro

Gabbro that was injected into the crust and experienced low nucleation and high growth rate

Intrusive/Plutonic growth/nucleation

Slowly cooled, growth > nucleation

Extrusive/Volcanic growth/nucleation

Quickly cooled, nucleation > growth