EOS 205 Mid notes

3800

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

-naturally occurring

-homogeneous solid (unit cell is the same as entire sample)

-definite (but not fixed) chemical composition

-highly ordered atomic arrangement

Ice yes

sugar no

CaCO₃ - dependent on its form

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

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.

• quartz
• biotite
• muscovite
• anorthite (plagi-feld)
• orthoclase (alki-feld)
• albite
• olivine
• pyroxene
• amphibole
• calcite

Silicates

ALCS

• atmophile-gas
• lithophile-silica liquid
• Chalcophile- sulfide liquids
• Siderophile- metalic liquids

• quartz
• feldspars
• amphiboles
• micas

• olivine
• pyroxene
• garnet

most common in earths crust

made of a metal-Si-O

ionic - electron substitution (NaCl)

covealent- share elctrons (diamond)

metalic- electrons free to roam (gold)

vanderwalls- weak +/- attractions (graphite)

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

its SiO₄ such that the Si has a tetrahedral (CN 6) polyhedra which doesent allow room for substitutions and impuritues

for same structure- ion size 

for same ion - pressure (depth)

• the si in olivine (Y₂ZO₄) Z=tetrahedral CN-4
•  changes to a CN4/6 then to a CN6 at the 410 and 660 discontinuities

same chemical composition different xstal structure

1) quartz-->coesite-->shitsovite

2)olivine-->wadsleyite-->ringwoodite

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

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

Δt<Δo<Δc

this explains the chromophore difference in olivine/garnet with Fe

• 1) size of ion
• 2)oxidation state
• 3)CN

• 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⁺+Ti⁴⁺]+photon----->[Fe³⁺+Ti⁴⁺]

4) impurities

Magnetite (Fe₃O₄)

Illmanite (FeTiO₃)

1. transition series elements

2. magnetic moments (created by valence shells and spin)

diamagnetic   para magnetic     ferrimagnetic

weakest form

mineral response (suscebtability) opposite that of applied field

not net mag...goes away if applied gone


ex quartz and feldspars

mineral suscebtability has the same direction as the applied field

retains a weak net field when applied removed

examples (biotite hornblende)

very strong magnetism

strong net retention

to remove must heat above currie pt

ex magnetite, illmanite

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

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 are the general structure i.e. olivine Y₂ZO₄ w/ Y=Mg, Fe & Z=Si


endmembers are the extremes of the iso morphs i.e. 

• fosterite Mg₂SiO₄
• fayalite Fe₂SiO₄

1. size of ions (15% extensive, 30% limited)

2. charge (nuetrality must be preserved)

3. Temp, greater temps creates more tolerance

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

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

Mechanically separated w/ distinct boundaries as in exsolution lamallea

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

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

reflection, rotation, inversion and rotoinversion

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

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

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 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

• *Ionic Radius: 15% extensive %30 limited
• Ex: Mg w/ Fe²⁺=extensive same size
•       Mg w/ Ca²⁺=limited Ca²⁺ is larger

• *Charge: neutrality preservation
• Ex:K⁺+K⁺=>Pb²⁺

*Temp: higher T alllows for greater variations in CN

• Olivine Y₂ZO₄
• Y= Fe, Mg results in forsterite Mg₂SiO₄ & fayalite Fe₂SiO₄

• Feldspar MZ₄O₈
• M= Ca Na Al (K) results in orthoclase KAlSi₃O₈  & Albite NaAlSi₃O₈

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

ionic radius ratio

• example 
• 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

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

• 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

• example 
• octahedral has 3 high and 2 low splitting centers
• tetrahedral 2 high and 3 low

Fe in garnet on cubic site

• oxidation states
• Fe³⁺ = brown
• Fe²⁺ = red

• Fe2+ in garnet, CN 8 = red
• Fe2+ in olivine CN6 = green