EOS 205 Mid notes
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How many minerals are there
What is the difference between a mineral and a crystal?
A mineral is a crystal but not all crystals are minerals
Crystal just means a solid with flat faces due to the orered, geometricaly symetrical atomic strucutre
to be a mineral it must meet the mineral criteria
what is a mineral? (4)
-homogeneous solid (unit cell is the same as entire sample)
-definite (but not fixed) chemical composition
-highly ordered atomic arrangement
Is ice a mineral?
calcium carbonate (CaCO3)?
CaCO3 - dependent on its form
what is the unit cell?
the smallest divisible part of a mineral
i.e. quartz is made of millions of SiO4 attached together
one single SiO4 molecule is the unit cell
Why do minerals have the properties they have?
It basically comes down to each have its very own unique chemical composition and arrangement of atoms, cat ions and an ions.
+/- charge is created resulting in magnetism
crystal sturcture can determine color, luster, hardness etc.
ten most common rock forming minerals
- anorthite (plagi-feld)
- orthoclase (alki-feld)
what is the most common mineral group in the crust?
what are the layers of goldschmidts periodic table? (4)
- lithophile-silica liquid
- Chalcophile- sulfide liquids
- Siderophile- metalic liquids
which minerals are in the crust (4)
which minerals in the mantel? (3)
what is a silicate
most common in earths crust
made of a metal-Si-O
4 types of bonding and examples
ionic - electron substitution (NaCl)
covealent- share elctrons (diamond)
metalic- electrons free to roam (gold)
vanderwalls- weak +/- attractions (graphite)
paulings rules (4) #2 is ignored by prof
1) every cation has a coordination polyhedron of anions based on the radius sums and ratios
3)shared edges (and more so with faces) reduces stability
4)high valency/small CN dont shatre polyhedra
5)# of differing constituents in a crystal tend to be small
why is quartz commonly so pure?
its SiO4 such that the Si has a tetrahedral (CN 6) polyhedra which doesent allow room for substitutions and impuritues
what changes the CN (2)
for same structure- ion size
for same ion - pressure (depth)
Si and mantel discontinuieties
- the si in olivine (Y2ZO4) Z=tetrahedral CN-4
- changes to a CN4/6 then to a CN6 at the 410 and 660 discontinuities
what is a polymorph? Give 2 examples
same chemical composition different xstal structure
what is a chromophore?
give an example
an atom (or group) responsible for the color in a mineral.
Ex- in olivine and garnet Fe2+
is the chromophore
it sits in diffrent sites and thus makes differing color
- olivine (fayalite) Fe2Si... Fe=Y=CN6
- garnet (almandine) Fe3Al2Si3... Fe=X=CN8
sum up crystal field transitions?
interactions between light and unfilled d orbitials of transition series elements (Sc-->Zn)
the d orbitals are split into higher and lower energy levels
octa (3 low 2 high) creates Δo
tetra (2 low 3 high) creates Δt
presence and value of delta influences wavelength absorbtion
this explains the chromophore difference in olivine/garnet with Fe
crystal field splitting (Δ) depends on (3)
- 1) size of ion
- 2)oxidation state
Color is caused by (4)
- 1) crystal field transmissions/splitting
- (crhomophers, dif sites, dif CN)
2) color centers (defects in structure allow the build up of electrons)
3)charge transfer transitions (transfer of electrons from one energy level to another) [Fe2+
Two principle minerals that record seafloor spreading
magnetism requires (2)
1. transition series elements
2. magnetic moments (created by valence shells and spin)
3 types of magnetism
diamagnetic para magnetic ferrimagnetic
explain diamagnetism and an example
mineral response (suscebtability) opposite that of applied field
not net mag...goes away if applied gone
ex quartz and feldspars
Paramagnetism explan and examples
mineral suscebtability has the same direction as the applied field
retains a weak net field when applied removed
examples (biotite hornblende)
Ferri magnetism and examples
very strong magnetism
strong net retention
to remove must heat above currie pt
ex magnetite, illmanite
describe x ray diffraction (XRD)
identifies mineral by its unique xstal structure
xrays difract due to the lattice
structure of the mineral and the d-spacing
gives info on atom locations and planes
- mineral must be powderized
- mount on slide and rotate under xray
- optimum angle Θ gives max difraction (when waves in-phase with xray)
- creates chart
only tells you the xstal structure not the composition
Electron probe microanalysis (EPM)
used to determine composition but says nothing about strucutre
electron beam shot at a thin sample
phonton exictes electrons makes them jump to higher level
sample emmits xrays as e moves back to stable level
xrays and associated backscatter create a "finger print of the mineral"
isomorphs vs end members
isomorphs are the general structure i.e. olivine Y2
w/ Y=Mg, Fe & Z=Si
endmembers are the extremes of the iso morphs i.e.
- fosterite Mg2SiO4
- fayalite Fe2SiO4
3 factors that influence ionic substitution
1. size of ions (15% extensive, 30% limited)
2. charge (nuetrality must be preserved)
3. Temp, greater temps creates more tolerance
types of solid solutions/substitutions (3)
1. ionic (based on ion size and charge) Na+Al3+=Ca2+Mg2+
2. intersital (filling of voids in lattice structure)
3. omission (high charge cat ion may replace two lo charge) K+K=Pb2
what is exsolution? how does it work?
the unmixing and reordering of a solid solution into two separate minerals
creates lamellea (the white and pink lines in kspar)
At high T a molecules are more excited and easily manipulated so that ions with varrying radius may be tolerated bonding with each other.
As the mineral cools this diference in ion radius is not tolerated and it cleans up into ordered arrangement
what does phase mean
Mechanically separated w/ distinct boundaries as in exsolution lamallea
what does a chemical analysis tell you about a mineral, how do you use it?
it gives the % weight which can be converted into mols then a mole ration of cations to anions to determine a chemical composition and formula
when given __anions to ___cations match it up with memorized X#Y#Z#O# to dtermine the solid solution.
trick you may have to divide as it could be a large ration representing a multiple of the chemical formula
iso morph vs polymorph
isomorph is diffrent chemical formula but same shape CaCO3 MgCO3 FeCO3 all rombohedrals
polymorph is the same formula with diffrent shapes shuch as kyanite sillimantite and andalusite
What are the things that control symmetry (4)
reflection, rotation, inversion and rotoinversion
what is reflection what are the two components?
symetry is created by the reflection across a mirror plane
- reflection is the opperation
- the mirror plane is the opperator
the ___ fold refers to how many rotations it takes to achieve "same look"
- 1 fold = 360
- 2 fold = 180
- 3 fold = 120
- 4 fold = 90
- 5 is forbidden
- 6 fold = 60
360/degree of rotation to achieve symetry= fold number
what is inversion?
not quite the same as mirroring or rotation
in a way its rotated 180 (troughits axis not the cartiesin axis as in rotation and mirroring) and then rotated over a line 45deg between cartiesian axis
if in NE quadrant rotate the item 180 degrees through its center
then rotate it over the line running NW-SE so that the object ends up in the SW quadrant
rotate itme 180 degrees then invert it
what is the difference between habit and form
crystal form is the arragement/symetry etc of the faces/planes of the crystal
habit is how the common charectoristic-shape of the crystal
point groups and crystal systems?
who has most/Least?
point groups are unique combinations of symmetry operators...there are 32 of them
these 32 point groups make up the 6 crystal systems
cubic has the most (32) while triclinic has none
What are the three factors that govern ionic substitutions?
- *Ionic Radius: 15% extensive %30 limited
- Ex: Mg w/ Fe2+=extensive same size
- Mg w/ Ca2+=limited Ca2+ is larger
- *Charge: neutrality preservation
*Temp: higher T alllows for greater variations in CN
Give examples of simple ionic substitution in olivine and feldspar solutions
- Olivine Y2ZO4Y= Fe, Mg results in forsterite Mg2SiO4 & fayalite Fe2SiO4
- Feldspar MZ4O8M= Ca Na Al (K) results in orthoclase KAlSi3O8 & Albite NaAlSi3O8
What is the strongest type of magnetism? Name two minerals and describe their electronic state of their cations to achieve this magnetism.
Ferri-magnetism is strongest, magnetism is in direction of applied field, weak net magnetism remains, needs to be at currie pt to undo net magnetism
examples magnetite and illminite
cations must have half filled d-orbitals in the same direction as the applied field
what factor governs coordination geometry
give an example of 3 specific coordination numbers/geometrys in a comon rock forming mineral
ionic radius ratio
- andradite Ca3Fe2Si3O12
- Ca is in the cubic site CN 8
- Fe is in the octahedral site Cn 6
- Si is in the tetrahedral csite Cn 4
Quartz contains little to no Uranium wheres Zircon may have plenty. Explain using paulings rules.
1. cation surrounded by coordniation of anions based on radius ratio
- 3.shared edges and faces decrease stabaility
- 4.high valence low CN reduces sharing
- 5. # of constituents in an xstal tend to be small
specificly 1&4 show that qurtz is extremly pure as it is SiO2 (small high valence) very limited places for substitutions whereas zircon has lower valence, larger, and more constiuients which are easily subed for the Ur
Describe crystal field transition/splitting. what factors or variable sgenerate different colours by this mechanism?
- depends on
- identity of transition metal
- arrangement of empty 3d vallence shells
- the ions oxidation state
- CN of site
the above effects the size of Δ which controlls wavelenght absorption
- octahedral has 3 high and 2 low splitting centers
- tetrahedral 2 high and 3 low
give an example of crystal field transition where two diffrent chromophores produced two diffrent colors in the same mineral
Fe in garnet on cubic site
- oxidation states
- Fe3+ = brown
- Fe2+ = red
Give an example of a crystal field transition where one chromophore produced different colors in diferent minerals
- Fe2+ in garnet, CN 8 = red
- Fe2+ in olivine CN6 = green
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