Elements and Minerals Flashcards
What reactions are possible at 101-103 K, 107-1010 K and around 1012 K and what forms of matter are stable at these temperatures?
101-103 K - chemical reactions, molecules and solids 107-1010 K - nuclear reactions, nuclei and electrons 1012 K - elementary particle reactions, elementary particles
What do chemical synthesis and nuclear synthesis reactions require?
Chemical Synthesis requires manipulation/changes in valence electrons Nuclear synthesis requires temperatures > 102
State Coulomb’s law of electrostatics and compare the size amnd potential energy of atoms and nuclei
Coulomb’s law:
V = q1q2/4πε0r
V = potantial energy q = charge ε0 = vacuum permitivity
r = distance
Atom: r = 200 pm = 2x10-10 m
V = 1.2x10-19 J (7eV) or 700 kJmol-1
Nuclei: r = 10 fm = 10x10-14 m
V = 1 MeV or 100 GJmol-1
Explain the differences between the strong interaction and electrostatic interaction
The string interaction operates equally between protons and neutrons
The strong interaction has very short range (2 fm or less)
Which force dominates when particles are at different distances?
The stability of the nucleus is based on the balance between the strong interation *(between all nucleons) and the electrostatic interation between protons (replusive).
For a pair of nuclei, coulombioc interatiuons dominate.
Approaching a distance of 2 fm the strong interaction dominates
WHat is binding energy and what is the most stable nucleus?
The energy required to break up the nucleus into constituent nucleons
56Fe is the most stable nucleus
How can the binding energy be calculated?
Using Einstein’s theory of relativity: E = mc2
m is calculated as the difference between the mass of an element b y definition (carbon 12 has a mass of 12) and the sum of all the protons, neutrons and electrons (12C = 12.09893876)
m = 0.09893976
c = 9x1016 Jkg-1 = 931.5 MeV
E = 92.1 MeV
What affects nucleus stability?
While electrons can be seen as quantised so can nucleons in the nucleus.
Nuclei with even number of protons and neutrons are more stable than odd numbers
Certain numbers are particularly stable: 2,8,20,28,50,82,126
Sometimes referred to as magic numbers similar to closed shell configurations of nobel gases: 2,8,18,32…
What are radioactive elements?
PArticles that decay by a number of pathways with the release of energy (gamma) and particles
Describe alpha radiation
The emission of an alpha partic;le (loss of He nucleus)
All nuclei with Z > 82 are alpha emitters
Occurrs via tunnelling through the string interation/coulombic barrier
e.g 23892 U ——> 23490Th + 42α(He)
What are the three types of beta decay?
Beta (β-) emission - loss of an electron
electron capture (EC) - gain of an electron
positron (β+) emission - loss of an anti-electron
Describe beta (β-) emission
e.g 13755Cs —–> 13756Ba + e- + ~ve
~ve = an anti neutrino
This occurrs when a neutron —–> a proton and an electron
Describe electron capture (EC)
e.g 2211Na —-> 2210Ne- + ve
This releases a neutrino
This occurrs when an electron and a proton combine to form a neutron
Describe positron (β+) emission
e.g 2211Na —> 2210Ne- + e+ + ve
e+ is a positron
This occurrs when a proton breaks into a neutron and an electron
The different types opf decay can be competative with each other as a particular nucleus may decay by more than one method
What does the line of stability show in terms of β decay
The line of stability shows neutron number vs atomic number
More neutrons are required at higher atomic number to reduce the coulombic repulsion between protons .
The binding energy per nucleon reduces leading to instability.
Above the line there are too few neutrons whereas under the loine there are too many neutrons.
What are some important long lived radioactive nuclides
40K, 87Ru, 147Sm, 187Re, 232Th, 235U, 238U
All have significant earth abundances
Fuel geological processes (heat the mantle) and cause genetic mutations driving evolution
Describe what happoened in the big bang timeline at t = 10-35 s
The universe began 13.7 billion years ago
At this time it was a hit dense plasma
T = 1032 K
No matter - radiation, sea of photons and neutrinos
Radiation described as a black body (UAVE = 2.7 KT)
Describe what happened in the big bang theory time lone at
t = <0.5 s
T = 1032 K —> 1013
Photons materialise pairs of particles
2γ <—-> p+ + ~p- (antiphoton)
2γ <—-> n + ~n
2γ <—–> e- +~e- Positron/antielectron
Photons must have enough energy to form matter particles
E = 2.7kT > 2mC2
Describe what happened in the big bang theory time line at
t = >0.5-1 s
T = 1013 - 109
Photons will lack the energy to form new protons below a threshold energy
2.7kTp = 2mc2 = 1013 K
No new proton-antiproton pairs materialise and protons and antiprotons then annihilate each other:
p+ + ~p- —–> 2γ
However, a slight excess of matter over antimatter remains at equlibrium [p~+]/[γ] 10-9
Describe what happened in the big bang theory timeline at
t = 1-100 s
Protons and electrons interact to form neutrons and neutrinos:
p+ + e- —-> n + ve
Half life deoends on temperature and density of the universe
t = 1s T = 1010 K
Therefore neutrons and other matter no longer materialise
[n]/[p] =~ 1/5
neutrons then decay - half life = 11 mins
As the temperature falls neutrons react faster with protons to form stable deuterium
n + p+ —–> 2H+ + γ
All neutrons end up as deuterium which is reactive
Which reactions are involved in the cascade of reactions after protons and neutrons have reacted?
2H + 1H —–> 3He + γ
2H + n —-> 3H + γ
2H + 2H —–> 4He + γ
3He + n —–> 4He + γ
3H + 1H —–> 4He + γ
Net reaction: 21H (p+) + 2n —-> 4He + γ
4He + 3He —–> 7Be —-> 7Li + γ - minor reaction
Process blocked - no stable 5 or 8
2H is slow to be formed due to low binding energy
What are the three pieces of evidence in support of the big bang theory?
The big bang theory correctly predicts the relative concentration of the lighter elements particularly the H:He ratio of 8:1
The big bang theory predicts that between 100s and 4x105 years ago radiation and matter were at equilibrium and homogenously distributed (black body distribution) - Cosmic Background Radiation
The universe is expanding in accordance with Hubbles law:
v = Hod
v = velocity Ho = hubble’s constant d = distance
Between 100s and 4x105 years what happened with radiation and matter?
Radiation interacts with free electrons leading to an opaque universe. Radiation and matter in equilibrium and homogenously distributed. Wait 4x105 years and the universe expands and cools to the point where radiation and matter are no longer equilibrated
At T<3000 all electrons are bound in atoms - these scatter radiation weakly. MAtter and radiation become decoupled (leads of CMS) and the universe is transport.
At t>4x105 what happens to matter and radiation?
H and He atoms spread evenly over universe - too colf for nuclear reactions - gravity begins to be felt- shapes the universe - releases energy
Space continues to expand - not uniformly
Matter (mainly H and He atoms) clump together underngravity to form galaxies and eventually stars
What are stars?
Molecular clouds located in the spiral arm of a galaxy can collapse under gravity ti form protostars
Gravitational nergy converted to kinetic energy
Temp rises to T = 107 K - nuclei begin to fuse to give heavier nuclei - stars begin t shine
Essentially stars are dense sphere of gas
What are the characteristics of stars
Luminosity - product of the brightness x distance2 x mass
Spectral type (colour) - gives surface temperature
Cool stars with high luminosity must be very large where as hot stars must be very small
Where does a star spend ost of its lifetime and what does it depend on?
Many stars lie in the main sequence - majority of the active life
Lifetime depends on mass
The sun lifetim: 1010 γ, age = 4.5x109 γ
Can nucleosynthesis occur in stars?
Stars are hot enough for nucleosynthesis to begin again
During the big bang thee was only 3 minutes for reactions but in stars these conditions last for million of years - slower reactions are more important
T = 107 but no neitrons
What is the main sequance of H burning?
What makes up the star in the main sequence?
Core of 4He2+ which forms. The energy generated is balanced by escaping radiation.
The star is supported against gravity by the thermal pressure generated by fusion.
Main sequence stars are not expanding or contracting much and are in hydrostatic balance. Rate of fuel burning (lifetime) depends on luminosity (L) and the mass
How do stars evolve?
Low mass stars (<4 suns) evolve into red giants - sources of 12C + 16O. Corpse is usually a whte dwarf or if another star is close by in a so called binary system mass transfer can occur which can lead to a type 1 supernova.
High mass stars (> 4 suns) evolve into super red giants - sources of C, N, O, Si. - Burn to 56Fe.
The corpse is usually a neutron star or a black hole via a type 2 supernova
What happens in a red giant?
When 1H becomes exhausted in the core the thermalmpressure cannot support the gravity core contacts raising T to 108 K - helium starts to burn
Energy released blows out the envelope to form Red giant
After 107 years He is exhausted and the star evolves into a white dwarf
What happens in a super red giant?
Core of 12C and 16O contracts
Fusion of heavier nuclei makes envelope swell
What is neutron capture?
Slow process that absorbs energy
Protons are introduced via beta decay after neutron absorption
What is explosive nucleosynthesis?
Type 2 supernova = when the iron core develops all the fuel will become exhausted and the thermal energy from fusio will no longer be able to support the weight of the star This leads to a catastrophic gravitational collapse occuring in seconds
The core disintergrates: T = 1010
56Fe => p + n - lots of neutrons
THe outer part of the core bounces of the hard central part and shock wave leads of explosion
Does neutron capture occur in a neutron star?
Neutron capture occurs causing rapid necleosynthesis (r-process) forming all the heavier elements up to 232Th and 238U
The explosion causes matter to be spread out into space and the core is left as a black hole or neutron star
What is the difference between old and young main sequence stars?
Old main sequance stars form from the debris of the big bang - contain mainly protons and helium
PPI cycle catalysed by 2H - 41H => 4He + 2e+ + 2v
Young main sequence starts form from the debris of a super nova and these are kmown as metal rich. The sun is a young star and these are important sources of isotopes like 13C, 14N, 15N, 17O
H burning is catalysed by 12C - CNO cycle
Can nucleosynthesis occur in an insterstellar medium?
A few nucleons are not formed in stars (or the BIg Bang)
6Li, 9Be, 10B, 11B are not formed by any of the process descirbed so far and are easily destroyed in stellar interiors
Formed as fragments by collisions (spallation) of very energetic galactic cosmic rays (including neutrinos, 1H, 4He, 12C and 16O with interstellar gas
Cosmic rays are accelerated by shock waves driven by the rapidly moving ejecta of supernova - sort of naturally occurring fission
15N, 17O, 18O, 19F, 21N are spallation products
What is the solar system like?
Isolated - distnace to nearest star is 5x104 time the diameter of the sun
Formed 4.6 x 109 years ago - it didn’t exist exist for most of the history of the universe
Now agreed that the sun and the planets have a common origin
What is the nebular theory of the solar system origin?
Planets form concurrently or consecutively from the same nebular as the sun
What is the cndensationtheory of the formation of the solar system?
4.6x109 years ago a small fragment of a molecular cloud detached from the arm of the milky way probably triggered by a near by supernova.
The cloud condensed towards the centre under gravity and the dust settled into the plane of a rotating dish
The angular momentum ransferred towards.
How did the planets form?
The centreof the nebula heated by gravitational contraction at 107 K - nuclear reaction begin and surface temp rises to 4000 K
Turbulaent gas eddies caused by heating of the sun lead to aggregation of particles under gravity to give planetisimols (<1km).
Gases are not included in the planetisimols due to heating from the sun. The surviving planetisimols aggregate further into the four inner terrestrial planets
Is the composition of the earth the same as the solar system?
Composition of earth is generally similar to the solar system but some are different -
oxygen, aluminium and silicon are very abundant
What kinds of elements are found on earth?
Lithophiles: rock loving found predominantly as oxides in the crust - early condensers
Siderophiles: iron loving elements found predominantly as metals or alloys in the core - volatiles > 600 K
Atmophiles: volatiles found in the ocean or atmosphere - volatiles < 600 K and extreme volatiles
Why is it thought the elements are found in the states they are found in?
Elements condensing at different temperatures due to their enthalpies of atomisation (for sidophiles) and oxide formation (for lithophiles).
There is a strong correlation between the form the leements are found in on earth and the rate at which they are predicted to condense.
What happened in the early history of the earth?
Accretion - (coagulation of dust) 4.6x109 years ago
Impacts and radioactivity of 238U lead to heating and some melting
Fe and Ni form core and a crust forms on an ocean of magma. Giant impact era ends 3.9x109 years ago and a supercontinuent formed 2.7 x 109 and plate techtonics become operational
What is the composition of the layers of the earth?
Atmosphere - atmophiles
Ocean + crust - lithophiles
Mantle - Mg2SiO4 (s) - viscous melt
Outer core - sidophiles FeS (l)
Inner core - Fe, Ni - (s)
Why do the crust and the mantle have different copositions?
The mantle partially melts to form magma due to radioactive heating - this rises and solidifies to form the crust
seen as cycles of of melting and recrystalisation to from essentially ionic mixture
This leads to mineral formaton and the concentration of elements in certain rocks - Since the earth’s formation the distribution of the elements has changed dramatically
Mg rich solids precipitate out (and fall) before the iron containing solids which leads to the crust being rich in Fe and the mantle rich in Mg
Why was life not possible in the early years of Earth?
The temperature in the accretion period was too high for abundant C, N, H2O to be bound. However, ice and C, and N compounds are rich in the asteroid belt. The formation of plamets like Jupiter scattered volatile rich planetisimals into the solar system
During the first 600x106 years large impacts would have sterilised the earth with no life possible
After the possibility of life increases
What is a mineral?
A solid compound found in nature with a cryystalline structure and unique chemical composition
e.g. halite = NaCl
What is a rock?
A mixture of usually 3-4 mnerals with some glassy amorphous materials - no unique chemical composition
e.g. granite
What is the study of minerals and rocks?
Mineralogy - study of minerals
Petrology - study of rocks
How are rocks classified?
Igneous - solidified magma#
Metamorphic - rocks changed by high T and P
Sedimentary - rom action of air, water or biological process
Rocks may have been through more than ne process
What are the major mineral classes?
Silicates: 70% of crust - greatest ,mass of minerals e.g. Mg2SiO4, Fe2SiO4
Oxides: include many important ores e.g. Fe2O3, Fe3O4, TiO2
Sulfides: HgS, FeS (pyrite) MoS2
Carbonates: e.g. CaCO3
Sulfates: CaSO4.2H2O
Phosphates: Ca5(PO4)3OH
Halides: NaCl, CaF2
Elements: gold, silver, carbon (graphite / diamond)
What are minerals classified by?
Crystal structure
density
colour
refractive index
cleavage
property - fluoroescence, magnetism, radioactivity
What structures can minerals have?
Range from amorphous to crystalline
Quartz - crystalline
Glass - amorphous
Unique composition may have more than one crystal structure - These are called polymorphs
Only one polymorph will be thermodynamically stable at a particular temperature and pressure
Can solid state compounds mix?
Some are miscible and can mix on the atomic scale usually due to similar radii of matal ions
The ratio of elements is non stochiometric
What is important about the Si-O bond?
It is the strongest bond containing O and is used to polymerise a range of structures
Si-O skeleton is -vely charged so requires cations for charge and occupy spaces
What are the factors determining site occupation?
Cation size (ionic radii) - major influence of crystal structure
A little bit of covalency which will be determined by the relative electronegativity and charge density
How can SiO4 tetrahedra link?
Share cornersvia bridging Os
What are some examples of SiO4 tetrahedra polymerisation?
What is the structure of olivines?
Exhibit solid solution between two end members forserite (Mg2SiO4) and faylite (Fe2SiO4)
Solid solution is possible because of similar size of Mg2+ and Fe2+ ions that can replace each other without causing strain in the structure
From the phase diagram when a melt is cooled the crystalline solid will be rich in Mg and the liquid rich in Fe
When is zoning often present?
common in minerals where solid solutions are present and depends on the rate of cooling
Very slow cooling gives large crystals of the end members.
Zoning and crystalline size give an indication of geological process conditions during crystalisation
What are otho and clinopyroxenes?
Orthopyroxenes contain Mg2+ and Fe2+ which are a similar size leading to solid solution
Clinopyroxenes contain Ca2+ whihc is much larger
The structures of these two are different and often separate into different phases (both solids) called intergrowths
What does the phase diagram of ortho/clinalpyroxene look like?
HAs an area of 2 phases in the solid phase area that is cslled the immiscibility dome.
Two phases are observed because at these temperatures a single silicate structure is not stable cotaining Ca2+ and Mg2+ ions.
Outside the dome a single solid solution is observed
The size of the ions is a function of T and P due to vibration - at higher T the ions vibrate more and are effectively bigger which can stabalise a structure
Describe layered silicates
Formulas can be complex because lots of ions can fit between the layers and they contain variable water content
Structures can cintain single, double or mixtures of silicate sheets
Various metal ions can be accomidated between the silicate layers
Describe Framework silicates
All the corners of the SiO4 tetrahedra are shared e.g. SiO2 which has many polymorphs
Incluydes Feldspars - used as abrasives and moonstone - constructed by substituting Al3+ into crystolographic sites pf SiO4
When large pores are present which can accomidate other molecules These are referred to as zeolites which can be used as catalysts and in ion exchange processes
What are extrinsic and intrinsic defects?
foreign atoms inetuded in the sturcture either by addition or by sustitution
Atoms are displaced from their crystolographic position or their missing from the structure (point defects)
Other defects arise from the relative dislacement of places of atoms (dislocations)
What causes colour in diamond?
Caused by extrinsic and intrinsic defects
Extrinsic: carbon substituted by nitrogen (yellow), boron (blue)
Colour due to extra (N) or absence (B) of an electron
Intrinsic: Point defects and dislocation give colours like brown, opurple, green, pink
usually caused bybradiation or heat - heat can alter the structural defects and change the colour
What are ruby and saffires made of?
Al2O3
Pure it looks hard and transparent
Has hexagonal close packed structure with 2/3 interstitial sites filled with aluminium atoms
Extrinsic defects are most important: substituion of Al for Cr gives red colour (ruby)
All other oclours are called saffire - Ti = blue, V = purple, Fe = yellow-green Different oxidation states give different colour
What is the origin of colour?
All essentuially arise from the excitation of an electron
The original location and final destination is dependent on the atomic composition and structure
How do extrinsic defects absorb light?
d-d transitions, metal - ligand transitions, logand to metal transitions
In crystals containing two metals of different oxidatuon states charge transfer also occurs
How does excitation occur across a band gap?
In a crystal with billion of atoms molecular orbitals are shown as bands collectively
The band gap energy determines which freuencies are absorbed (colour)
In diamond energy required to excite across the band gap is outside the visible region - no colour
If there is no band gap all visible frequncies are absorbed and itappears black
How do extrinsic impurities affect the band gap?
Can introduce and extra band
In diamond the N impurity contains an extra electron not used in bonding genrate a filled band between the filled and empty conduction bands
B impurity introduces an empty set of orbitals
Excitation across this gap is in the visible region leading to colour
How do intrinsic defects affect colour?
Missing atoms or structural imperfections can lead to ‘trapped’ electrons that can be excited in the visible region
Usually coused by radiation or heat
NaCl irradiated with gamma rays is blue - CL2 is illiminated leaving a solvated/trapped electron which is required for charge balance
Heat can change the structure and therefore the colour - heating blue NaCl will cause structural rearrangement and colour loss
