Rocks minerals and crystals
Download
1 / 19

Rocks Minerals and Crystals - PowerPoint PPT Presentation


  • 120 Views
  • Uploaded on

Rocks Minerals and Crystals. By Guest Scientist Dr. David Walker LDEO-Columbia University. Rocks are made of minerals.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Rocks Minerals and Crystals' - ostinmannual


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Rocks minerals and crystals

Rocks Minerals and Crystals

By Guest Scientist

Dr. David Walker

LDEO-Columbia University


Rocks are made of minerals
Rocks are made of minerals

This pallasite meteorite rock came from the edge of the core of an unknown asteroid in our solar system. This thin slab is lit from both the front and back. Magnesium silicate olivine forms amber-colored crystal windows through iron crystals of kamacite and taenite ( the polished metal).


Minerals are crystalline
Minerals Are Crystalline

Geometrical crystal shapes suggest ordered structures.


Periodic 3d atomic order crystals
Periodic 3D atomic order = crystals

External morphology in regular geometric shapes suggests internal periodic structure, such as for:

Layered silicate

chlorite

Ring silicate

beryl (gem=emerald)


How to learn the atomic order
How to Learn the Atomic Order?

  • Put X-ray beams through crystals.

  • X-rays are short electromagnetic waves of wavelength (l) between 0.1 and 10 Angstroms.

  • If waves hit periodic array with spacing d l then COOPERATIVE SCATTERING occurs ( = DIFFRACTION ).

  • This is NOT the same as taking an X-ray picture in a medical lab and magnifying it.


Cooperative scattering
Cooperative Scattering

Waves on Pond with Array of Duck Decoys

Ripple train approaches line of ducks

d

d

d

l


Map view of pond surface
Map View of Pond Surface

As the ripple train passes, each duck bobs up and down sending out new waves.

Those waves interfere with one another.

Both + & -

l

d


Condition for Scattering: l=d sina1

wave

)

d

no wave

a1

sin a1 = l/d

To keep parallel beams

at angle a1 in phase

must be l.

l

wave


Condition for Scattering: nl=d sina

1l =d sina1

wave

a1

)

no wave

d

wave

n = 1

a1

no wave

a2

2l =d sina2

n = 2

wave

For small a [ l >> d] get many beams. Large n resembles continuous scatter.


l

Wavelength must be shorter than d

n l = d sin a means sin a =n l /d

Maximum a is 90o – diffraction directly sideward - for which sin a  1

Giving n l /d 1 or n l d

Smallest n l when n = 1

The  easiest to satisfy for n = 1

So l  d to keep sin a  1

 Otherwise no diffraction!

d

a = 90o


nl = d sina is satisfied both forward and backward from the array, as well as on either side.

n l = d sin a

n=2

n=2

n=1

n=1

l

d

a

a

a

a

n=1

n=1

n=2

  • NOTICE for fixed l , smaller d gives bigger a

  • Spots or wave beams spread as ducks become closer.

  • Spots or wave beams spread as you move away from ducks.

n=2


XRD is not like medical X-ray imagery!

Spots spread as duck converge.

Spread grows with

distance from ducks.

Spots spread as fingers spread

Medical X-ray

XRD


Laser grid diffraction demonstration

LASER

Laser/grid diffraction demonstration

  • Spots absent in nonperiodic fabric

  • Spot symmetry same as that of grid

  • Spots rotate with grid rotation but not XY

  • Spots spread with grid tilt or smaller d

  • Spot spacing s grows with S

s

)

a

S

d


Mineral crystals diffract x rays
Mineral Crystals Diffract X-rays

Therefore: X-rays are waves !

Crystals are periodic arrays !

l  d !

This 1912 demonstration won Max von Laue the Nobel Prize in physics for 1914.

X-ray beam


For mineralogists
For Mineralogists

  • Symmetry of spots  symmetry of array

  • Spacing of spots  array spacing of scattering atoms

  • Intensity of spots  atomic weight occupancy distribution.

    This makes possible

    crystal structure

    analysis.

    Library of patterns is reference resource of ‘fingerprints’ for mineral identification!

Chain

silicate

diopside

(along chains)


1915 nobel prize to the braggs
1915 Nobel Prize to the Braggs

Father and son team showed that XRD could be more easily used if diffraction spots treated as cooperative scattering “reflections” off planes in the crystal lattice.

Planes separated in perpendicular direction by dhkl

Angle of beam and reflection from lattice plane is 

Braggs’ Law:n l = 2 dhkl sin 

XRD Mineral identification done from tables of the characteristic Bragg dhkl which are

calculated from l and  observations.


Powder xrd for mineral id
Powder XRD for mineral ID

d

2 = 90

d

d

d

dhkl

Powdered sample

2hkl

X-ray beam in

2 = 0

Make list of dhklfrom measured2hkl

using n l = 2 dhkl sin 

Compare with standard tables <JCPDS>


Exercise

s

LASER

Exercise

  • Measure screen to image distance (S).

  • Measure distance from middle of pattern to first spot (s).

  • Measure spacing of grid (d).

)

a

S

l = d sS

d

Compute wavelength l of laser light from n l = d sin a

Use l derived to measure the d of a larger or small grid spacing


Website references
Website References

  • http://www.icdd.comCommercial library of the JCPDS powder patterns of over 60,000 crystal structures.

  • http://www.ccp14.ac.ukXRD applications freeware and tutorials.

  • http://webmineral.comFun resource for mineralogy, especially crystal shapes.

  • http://ammin.minsocam.orgMineralogical Society of America’s site including “Ask A Mineralogist”.


ad