Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Materials and Isotopes
Materials 1
Geochemistry – mineral exploration, cleaning pollutants and biomarkers for climate change
...
1nm-0
...
The mass of an electron is 1/1840
...

Increasing electronegativity as you go left to right, fluorine being the highest; decreasing
ionic radius
...

Factors which affect atomic radius:




Size of the + charge
Distance of the outer shell from the nucleus
Screening effect of inner shells that are completely filled

Goldschmidts rules for the ions of one element to extensively replace those of another in
ionic crystals if radius differ by less than 15%
...

Ionic Bonds
o
o
o
o
o
o

Opposite charges that attract
High melting points because of strong bonding
Easily disrupted by polarised solvents eg water
Poor electrical conductors
Non-directional so symmetrical
Strength (1/bond length) & valence

Also, if electronegativity differs by 2 or more the bond will be ionic
...
High density sea of mobile electrons few loosely held
electrons
...
Held together by weak van
der walls
Covalent crystals have high electronegativity eg diamond

Metallic crystals are highly symmetrical and dense with same size atoms with close packing
– up to 12 touching nearest neighbour
...
Cations are positively charged ions and anions are
negatively charged
...
Only occupy
types where they can fit and only enough to attain electric neutrality
...


Materials 2
The gradual synthesis heavier to light elements by a series of nuclear reactions in stars
...

Heavier nuclei are more stable as far as Iron (56)
...
More energy was required to form these
elements
...

If photons are absorbed by an electron it becomes ‘excited’ and moves to a higher
energy shell
...


Meteorites
Only 1% of meteorites are recoverable
...
Chondrites contain CHONDRULES – mm sized spheroidal
assemblages of solidified melt droplets that were produced by the melting of dust in an
early solar nebulae- possibly as a result of collisions 10Ba
...

Chlorite chondrites are RELICS of PRIMORDIAL MATTER in the solar system: from these we
can determine overall composition of primordial soups except for the most volatile
elements
...

Irons represent the Earth’s core (32% of the planet), Achondrites represent the
Earth’s mantle (67%) and chondrites represent the bulk Earth
...
Higher atomic numbers = less
abundant
...


Chondrite meteorites very similar in composition to the sun but sun is depleted in Li and
meteorites are depleted in N, C, O
...

Planetesimals combined to form planets
...


1) Separation of core from mantle
2) Separation of mantle from crust due to igneous activity

Incompatible – elements that prefer the open
and distorted nature of a melt rather than crystal
lattice
Large ion lithophile – Most crystals exclude ions
because ions are too large to fit into the
structure eg Rubidium and Barium
High field strength – Similar radius but charge is
too great ionic charge meaning it cannot replace
other elements with a similar radius

o Crust extraction has no effect on major element content of the mantle
...
High concentration of incompatible trace
elements suggest large volumes of mantle processed
...
These high Titanium
ferrosbasalts crystallised at 1150°C, 3
...
Also some
feldspathic rocks from impacts
...
They are mainly found on the near
side of the moon due to heat producing REE elements
...
It has very little metal
in its core and is mainly made up of silicates
...
5 g/cm3 where as the moon has a
density of 3
...
The collision of planetesimals gave Earth its metallic core and
the other planetesimal that became the moon took mantle
...


Materials 3
Magmas produced by partial melting of peridotite leaving a residue
...
As it
crystallises it removes elements so the liquid changes composition
...
Basalt  Residual liquid (basaltic
andesite) + cumulate Peridotite, Basaltic Andesite  Andesite residual liquid + gabbro
cumulate
...

Bowen’s
reaction series:
Becomes more
alkaline going
down

Fractional Crystallisation example
1) Most common upper mantle melt is picrite (low in SiO2)
...
The second mineral to crystallise must be orthopyroxene because it uses
less oxygen, SiO3
...
PERIDOTITE
2) As composition begins to change, plagioclase begins to crystallise
...
GABBRO AND BASALT
3) Removal of Oxygen – silicates must become more complex
...
Changing ratios of Mg Fe Ca: Na K leads to
intermediate rocks DIORITE AND LIQUID ANDESITE
4) Cooling – joined by biotite and K-Feldpsar
...

5) Plagioclase and Hornblende joined by Quartz and white mica  GRANITE/RHYOLITE
Basalt – 1400-1200C, Rhyolite 900-700C, basalt being the most abundant out of all the
magmas
...

___ + F = C +2
Invariant – equilibrium cannot move, not flexible
...
Lowest melting temperature
...
Evolving magmas converge on one or
two residual melt compositions
Equilibrium crystallization – crystals form a cooling melt in a closed system
...

Fractional crystallization – initial equilibrium but then as chemicals are removed from
contact (magma settling) there becomes a disequilibrium
...


Binary phase diagram:
Diopside (pyroxene) and
Anorthite (plag feldspar)

Isotopes 1
Half life – time taken for half the number of parent atoms to decay
...
Daughter isotope tries to escape
...
Loss of radiogenic Ar from biotite
occurs as diffusion – temperature controlled process
...
In amphibole, the temperature where all argon is lost is 500C
If a rock cools quickly (volcanic rock), age is the age of crystallisation
...

Uses: Time of denudation (reflects movement
of rock towards the surface (erosion)) and times
the cooling of rocks in orogens

Time of crystallisation
Zircon – 238U  206Pb
...
Zircon is an accessory mineral that crystallises in granites (haloes of radiation
damage)
...


Sensitive High Resolution Ion Microprobe (SHRIMP) Mass Spectrometer
...

Uses: Geological timescale, where did sediment come from as Zircon is robust to
weathering
Time of cooling to very low temperature
Apatite fission track gives time of cooling 60-120C
...
Uses: 10Be concentrations give indication
of exposure age – time spent in top 2m of crust
...
DO <<< DR
In beta decay a neutron changes into a proton plus an electron
...

Isochron is a line showing a suite of rock or mineral samples all forming at the same time
...

Upper crust has high Rb/Sr ratio
Mantle has lower Rb/Sr ratio
Therefore Upper crust develops high 87Sr/86Sr with time
whilst 87Sr/86Sr of the mantle remains low
...

Sr behaves like Calcium, so it is in Basalt
...

LOW 87/86 Sr CONTENT
...

Initial 87/86Sr ratio allows us to find information about where magmas come from
...
7, with continental crust ratio the highest at 0
...


Higher 87/86Sr means less mantle volcanism or increased river input
Decreased 87/86Sr means increased mantle volcanism or lack of river input

Isotopes 3
Evaporation sees the fractionation of stable isotopes
...
O18 falls as
precipitation first, so greater O16 content in the clouds
...

Abundance: [(Sample – standard)/standard] x 100


Ocean has more O18 during glaciation periods

In the process of fractionation the heavier mineral prefers to go into the liquid
...
Higher temperature = less
fractionation
...
the composition of the fluid,
B
...
the type of mineral
Stable isotopic fingerprinting of fluids – can distinguish between rainwater and seawater
C12 is liked by plants (organic) while C13 is liked by carbonates



---