Mineralogy Flashcards
What are the three natures of solid matter?
Crystalline
Amorphous
Glass
What is the structure of crystalline solid matter?
Atoms regularly ordered in three dimensions
What is the structure of glassy solid matter?
Atoms roughly the same distance apart and some areas with regular order
What is the structure of amorphous solid matter?
Truly randomly ordered
Why are crystalline solids so great/preferred?
Bond distances optimised
Bond angles optimised
Every atom in an optimum site
Everything in its place
=all energies minimised
Why do crystalline minerals form?
Most energy efficient way to keep atoms together
Why do non crystalline solids form?
If you freeze melts or solutions fast they may not have time to crystallise and form non crystalline solids
Short range order
Entropy
Radiation
What is devitrication?
The process of non crystalline solids reverting to crystalline forms over time
What is snowflake obsidian?
Where glass has been partially devitrified
What is a mineral?
A naturally occurring solid material usually crystalline with a chemical composition which is essentially constant
What is a rock?
A naturally occurring aggregate of mineral grains which may include several different minerals
Is glass a rock or mineral?
Considered as rocks since their compositions are very variable
What are crystals (structural wise)?
Ordered structures compromising of repeated stacking of single units in 3D
What is the individual building block of structure called?
Unit cell
What are the 7 crystal systems?
Cubic
Orthorhombic
Triclinic
Trigonal
Tetragonal
Monoclinic
Hexagonal
What are two ways of determining the crystal system of an unknown mineral
?
- Consider shape and size of unit cell through X-ray diffraction
2.; examine shape and symmetry of macroscopic crystals which reflects the symmetry of the unit cells
How is the shape of a crystal defined?
Symmetry elements:
- 6/4/3/2 fold axis
- mirror plane
- centre of inversion
What are the properties of a cubic crystal system?
X=y=z
A=B=Y=90
Properties the same in all directions=high symmetry
What are the properties of a tetragonal crystal system?
X=y=z
A=B=Y=90
Properties the same in all directions=high symmetry
What are the properties of an orthorhombic crystal system?
X!=y!=z
A=B=Y=90
Rectangle
What are the properties of a monoclinic crystal system?
X!=y!=z
A!=g!=90, b=90
What are the properties of a triclinic crystal system?
X!=y!=z
A!=b!=g
What are the groups of unit cells based on?
- Based on three principal axes (hexagonal and trigonal)
- Based on four principal axes
What is a gem?
Highly prized earth material, usually a mineral
Ie. Diamond, rubies, emerald
What are the compositions of some precious minerals?
Diamond C
Rubies and sapphires Al2O3
Emerald and aquamarine Be3Al2(SiO3)6
What are some precious ,metals?
Gold
Silver
Platinum
Electrum (alloy)
What is the composition of some semi-precious minerals?
Amethyst, citrine, tigers eye = SiO2= quartz
Tanzanite = Ca2Al3(SiO4)(Si2O7)O(OH)=zoisite
Peridot = (Mg,Fe)2SiO4 = olivine
What are some common non-mineral gems?
Amber = fossilised tree sap
Jet = coal
Pearls = biomineral of proteins and aragonite)
Lapis lazuli = rock of minerals lazurite, sodalite and calcite
What causes colour in gems?
- Small amount of transition metals
- Elements with hanging bonds
- Radiation damage
Give an example of transition metals causing colour in gems.
Cr in Al2O3 makes red ruby
Fe and Ti in Al2O3 makes blue sapphire
What is corundum?
Al2O3
What is beryl?
Be3Al2(SiO3)6
Give an example of hanging bonds causing colour in gems.
N5+ replacing C4+ in diamond makes it yellow
B3+ replacing C4+ in a diamond makes it blue
Give an example of radiation causing colour in gems.
Smoky colour in smoky quartz
Radiation damage + Fe makes amethyst purple
Are gems rare?
No
Particularly perfect forms of common minerals
What is the building block of earths crust?
Silicates
SiO4 tetrahedron
What are the properties of SiO2?
Silica SiO2
Chemically stable, doesn’t weather
Found in sedimentary rocks (ie sandstone), acidic igneous rocks (ie granite)
Macrocrystalline silica (quartz) vs crypto crystalline (chalcedony)
What can chalcedony be described as?
Jasper (red Fe3+)
Onyx (flat banding)
Agate (curved bands)
Flint (black, found in chalk)
Chert (blue layers in limestone)
What is disordered SiO2?
Opal
Disordered structure like a glass with irregular structure
Milky colour = opalescence
What is the definition of a solid solution?
Mineral has a range of compositions (members of a solution) between fixed limits (end members)
What are the end members for olivine?
Forsterite Mg2SiO4
Fayalite Fe2SiO4
What are olivines important for?
Basalts/gabbros
Peridotite and dunite
What is the most common mineral in earths crust?
Feldspars
What is the composition of feldspars?
(Na, Ca, K)Al- 2 Si2- 3 O8
What are the end members of the feldspars?
K orthoclase
Na albite
Ca anorthite
What are the solid solutions of feldspars?
Anorthite-albite = plagioclase feldspar
Albite-orthoclase = alkali feldspar
Mainly albite = ternary feldspar
What is a complete and incomplete solid solution?
Complete= all combinations between end members is possible. Albite and orthoclase
Incomplete = gap in combinations. Orthoclase and anorthite
What are the physical characteristics of feldspars?
Complex
Twinning
Exsolutin
Alteration
What rock types are plagioclase feldspars found in?
Ultra basic rocks
Basic rocks
Intermediate rock
What rock types are alkali feldspar found in?
Intermediate rocks
Acidic rocks
Alkaline rocks
What are the end members of mica?
Biotite Mg Fe rich, dark colour
Muscovite Al rich, colourless
What are the pyroxene end members?
Orthopyroxenes Mg Fe rich (no Ca)
Clinopyroxenes Ca
What are pyroxenes characterised by?
Two prominent cleavages on basal sections that intersect at 90degrees
What is the general formula for a pyroxene
?
X2+ +Y2+ [Si2O6]
Where X and Y are Ca. Mg, Fe, Na, Al
What are the big ten rock forming minerals?
Calcite
Olivine
Pyroxene
Amphibole
Muscovite
Biotite
Anorthite
Albite
Orthoclase
Quartz
What does bonding control?
Physical properties
What are physical properties?
Hardness
Optical nature
Cleavage
Conductivity
Magnetivity
What are the four types of bonding?
Ionic
Metallic
Covalent
Van der Waals
What are positive nd negative ions called?
+ cations formed by elements with low ionisation potentials
- anions formed by elements with high electronegativity
What are the properties of ionic bonding?
Sometimes soluble in water
Moderate hardness 3-5
Multiple perfect cleavages
High symmetry q
What is metallic bonding?
Sea of delocalised electrons
What are the properties of metallic bonds?
Moderately strong
Conductive
High symmetry
Malleability
Where does covalent bonding occur?
Between elements of the same electronegativity or ionisation potential
What are the properties of covalent bonds?
Strong
High hardness
Low symmetry
High refractive index
What is van der waals bonding?
Caused by transient fluctuations in electron density
What are the three Bravia’s lattices?
Primitive (corners) density =52%
Body centred 68%
Face centred 74%
Hexagonal close pack
What do the dense structures of metals mean?
Bond distance is minimised
Solids with high density
What it the tetrahedral arrangement of methane?
Bond angles are all 109.5
All bond distances are the same
Tetrahedral point symmetry
What is the tetrahedral arrangement of SiO4?
All bond angles 109.5
Bond distances vary but are the same in one mineral
Cubic point symmetry
What is the tetrahedral arrangement of CH3Cl?
Different electronegativity means different bond
HH-H bonds are 110
H-Cl angles 108
Distances are different
Trigonal point symmetry
What happens if you change the identity of one element in SiO4?
Optimum bond angles move away from regular
Lower symmetry
When does the SiO4 tetrahedral become distorted?
Ig you change 1 O for OH, F or Cl
Change central Si for Al
Place other ions nearby, changing local electron distribution density
Lower symmetry
Give an example of a mineral that is triclinic?
Al it’s
What are Goldschmidts rules?
Size matters
Charge matters
What is an onuma diagram?
Show partition coefficient versus ionic radius
Shows the best size to fit in a structure
Why does size matter?
Structures have sites that can accommodate ions of particular sizes
Optimum size is determined with an onuma diagram
Why does charge matter?
Some ions are the wrong charge for the site
Al3+ can’t substitute for Mg2+
Why do Mg and Fe substitute so much?
Roughly same size and charge
What is an ideal solid solution?
Perfect complete solid solution with two or more ons exchanging for each other without distortion of the structure
Ie olivine
How can the properties of members of an ideal solid solution be calculated?
From a linear combination of the properties of the end members (Vegards law)
What can be calculated from the properties of end members?
Density
Refractive index
Unit cell parameters
What is a limited solid solution?
When a solid solution forms to a certain degree but not completion
What is homovalent substitution?
Swapping an ion of a particular charge with another of the same charge
What defines a pyroxene?
X and Y site
Optimum radius 0.84 and 1 respectively
X can be Mg or Fe2+
Y can be Ca, Mg, Fe2+
Chain silicate
What are the end members of pyroxenes?
X and Y site
Optimum radius 0.84 and 1 respectively
X can be Mg or Fe2+
Y can be Ca, Mg, Fe2+
Chain silicate
What are Ca rich pyroxenes called?
Augite
End members diopside(mg) and hedenbergite (fe)
How many sites can Ca occupy in a pyroxene?
1 Y site
2Ca = wollastonite=pyroxenoid
What is the characteristics of pigeonite?
Breaks down over geological time to orthopyroxene and augite
Originally an Fe rich Magma with restricted stability
What is aliovalent substitution?
The exchange of ions with different charges by counteracting the charge elsewhere
Give examples of pyroxenes with aliovalent exchanges.
Jadeite: Na+ + Al3+ = Ca2+ + Mg2+
Aegirine: Na+ + Fe3+ = Ca2+ + Mg2+
Titanaugite: Ti4+ = Ca2+ + Mg2+
Where are sodic pyroxenes found?
Mantle rocks (omphacite)
Collision zones (jadeite)
Alkaline igneous rocks (aegirine)
What is the cleavage of pyroxenes?
90 degrees
What is the structure of an amphibole?
Double chain
A X Y sites
A Y2 X5 (Si,Al)8 O22 (OH,F)2
What are the optimum radii of amphiboles?
A = 1.3
Y = 1
X = 0.84
What are the end members of amphiboles?
Orthoamphibole
Clinoamphibole
What are the similarities between pyroxenes and amphiboles?
Mg and Fe solid solution is orthorhombic
Ca is monoclinic
Miscibility gap between Clino- and orthorhombic- forms
Sodic counterparts
What do amphiboles have that pyroxenes dont?
Extra Na site which can accept Na or K
More sites=can take wider range of elements
Get octahedral and tetrahedral Al
What is hornblende?
Amphibole equivalent of augite (Ca rich)
What are sorosilicates?
Bow shaped silicates Si2O7
Epidote groups = low grade metamorphic Ca2Al3SiO4(OH)Si2O7
Zoisite = metamorphic marls
Allanite = accessory in igneous rocks
What are cyclosilicates?
Ring silicates
Three fold axis = beitoite, catapleiite
Four fold axis = axinite
Six fold axis = beryl, cordierite
Nine-tetrahedral ring = eudialyte
What are phyllosilicates?
Sheet silicates separated by sheets of ions
Silicate tetrahedral shares three corners
Ie clays
What are clays?
Phyllosilicates
Mg = brucite layers = trioctahedral clays
Al = gibbsite layers = dioctahedral clays
How are Clay minerals formed?
By stacking various permutations of aluminosilicate and octahedral layers
What are the three groups of clays?
Kandites
Smectites
Illites
What is a kandite clay?
1 octahedral : 1 aluminosilicate structure
Eg china clay - kaolinite
What is smectite clay?
1 octahedral : 2 aluminosilicate structure
Eg montmorillonite
Inter layer gap where water and small ions can reside - clay swells in volume
What are illite clays?
1 octahedral : 2 aluminosilicate structure +cation layer in between blocks
What are mixed layer clays?
When many clay types are interfingered sequences of all three end members
What causes desiccation cracks?
Caused by swelling and shrinkage of smectites
Why does subsidence occur?
Building on clay-rich sediments will subside in drought or if too much water is abstracted from subsurface aquifers
What are clays regulators of?
Subsurface water composition as smectites allow the exchange of cations with water
What do clays do with ions transported by water?
Adsorption occurs, acculmalating pollutions
Can be reversed by pH change
How do you identify clays?
By Secondary Electron Microscope - different morphologies
By X Ray Diffraction
How can you identify a smectite with XRD
Have different patterns when wet vs dry
out of the ‘big ten’ minerals, what one is not covalently bonded?
calcite = ionic
how do rocks respond to stress?
mineral rotation and deformation (lower crust and mantle)
what is meant by competent?
the ability of a bond to keep a structure together/resistance to flow
ionic bonds=poor competcy
covalent = highly competent
what happens if you deform a sedimentary sequence with limestones/evaporates?
sandstones will remain competent but evaporates will behave incompetently ie flatted out and act as a lubricant
give an example of a deformation of a sedimentary sequence with limestones/evaporites>
alpine orogeny, 400-500m of triassic evaporites flattened into units a few m thick
what does a phase diagram show?
pressure vs temperature/composition to tell you which face is stable at any range.
what is polymorphism?
the same chemical formula being associated with more than one mineral
same chemistries, different structure
give examples of polymorphs
graphite and diamond C
sphalerite and wurtzite ZnS
Aragonite and Calcite CaCO3
Quartz, Tridymite, Cristobalite, Coesite, Stishovite SiO2
Andalusite, Sillimanite, Kyanite Al2SiO5
what controls what polymorph exists at a certain temperature and pressure?
le Chateliers principle - pressure squeezing atoms closer together
what are reasons for the actual mineral made not to reflect the phase diagram?
minerals grow too slowly
mineral cannot change from one polymorph to another
grow one polymorph when we expect the other (ie corals)
what can we constrain with a polymorph?
the peak metamorphic pressure and temperature a rock has experienced
ie. Al2SiO5 polymorphs
where do we frequently find coesite and stishovite?
impact crater - hugh pressure, low temper
what is the difference between quartz and ice?
both hexagonal
quartz = covalently bonded
ice = hydrogen bonded
how do hydrogen bonds work?
electrostatically attraction between molecules
polar bonds, H makes one side slightly pos, one side slightly neg
are hydrogen bonds strong or weak?
weak, incompetent
i.e. ice is a soft mineral
are glaciers a rock?
yasssss
what is the hardness of ice?
1.5
What are the consequences of hydrogen bonding>
Insufficient to restrict mineral deformation - ice flows under its own weight
Melting at ambient temperatures and pressures
What is impressive about ice?
Solid state of ice is less dense than liquid state - unique - all other mineral solids sink
Where do we find most covalent bonds?
The lithosphere
Why does ice have a reversed phase boundary? (Top left to bottom right)
Density of solid less than that of liquid state
Increased pressure goes from solid-liquid (instead of liquid to solid) (ie meltwater at bottom of glacier)
What are examples of ionuically bonded minerals?
Halite
Gypsum
What is kraken mare?
Sea of liquid methane on titan
How is methane similar to ice?
Increased pressure causes melting
At what temperature and pressure do all three phases of methane exist?
100K, 0.5atm
Why can we not see methane-bergs on titan?
Methane solid is denser than liquid methane so sinks
0.51gcm-3 vs 0.44gcm-3
What forms our perception of geology?
Being on a planet with a temperature range of -10 ->40 degrees C
What are the characteristics of Venus?
At surface 100kbar, 450 degrees c
What are the dark patches on the top of mountains on Venus?
Galena or Te-bearing mineral frost - seasonal
Galena is ionically bonded
What are the characteristics of Pluto?
Mountains of ice
Lava flows of water - cyrovolcanoes
Ammonium lavas
Snow caps of methane
Nitrogen glaciers
Give an example of a solid nitrogen glacier
Sputnik planitia, Pluto
What is nitrogen convection driven by?
Thermal convection - warmer at bottom due to lithosphere radiation
Ice rises up through solid nitrogen
Dunes of solid methane - breeze
Why does nitrogen flow?
Poorly bonded
- flows like plastic
What is adsorption?
Highly charged surfaces that attract ions and metals to them
Why can you not see clays in thin section?
Clays only form nanocrystals smaller than the wavelength of light
Appear as turbidity
Where do most rare earth elements come from?
Ion adsorption clay deposits
How are ion adsorption clay deposits created?
Waters rich in rare earth ions flowing over clay-rich sediments and leaving the rare earths stuck to the clay surfaces
How do you remove rare earth ions from clays?
Acidify clays with ammonium sulfate to liberate the ions
Why dont clays grow crystals big enough to see?
Too much disorder
Micas
What is an illite?
Cation layer where ions have been formerly filled
How do micas vary from an illite?
Region between 2:1 layers are filled with a formal cation layer without any water
What are trioctahedral and dioctahedral micas?
3= biotite
2= Muscovite
What is the charactistic of serpentine?
Buckled and warped layers
What causes buckling and warping in layers?
When layers in a sheet silicate dont qui6e match up
What is asbestos?
A form of chrysotile serpentine
Used as a fire retarding material
What causes asbestosis ?
Fibres created an illness caused by tiny asbestos needles penetrate and inflame lungs
What is perthite?
The intergrowth of albite and orthoclase as alkali feldspars break down
What is sandadine?
K feldspar existing in high temperatures
What is orthoclase?
Cooling feldspar very slowly, optically homogeneous
What is microcline?
Feldspar that upon cooling forms a low temp form, tartan twinning
How do you preserve sanadine?
Quench lava in water
Otherwise inverts to orthoclase then microcline over time
what is the name of the process by which a glass becomes a crystalline solid?
devitrification
what are the names of feldspar endmembers?
orthoclase
albite
anorthite
what is the name for the process whereby one soild breaks down into a mixture of two solids?
exsolution
