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X-ray crystallography. What tools do we have to ID minerals so far?. How do we know atomic structure of minerals?. Atoms are 1-5Å apart so how do we see this?. X-rays. Between 0.1 and 10Å (1Å = 0.1 nm). X-rays. Amplitude Wavelength. X-ray interference. Constructive is in phase.

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how do we know atomic structure of minerals
How do we know atomic structure of minerals?
  • Atoms are 1-5Å apart so how do we see this?
x rays
X-rays
  • Between 0.1 and 10Å (1Å = 0.1 nm)
x rays1
X-rays
  • Amplitude
  • Wavelength
x ray interference
X-ray interference

Constructive is in phase

Destructive if out of phase

wave behavior
Wave behavior

Interference and diffraction

If a wave encounters a barrier with several holes, the

propagating wave will have sharper peaks of intensity at

certain angles.

x rays interact with atoms
X-rays interact with atoms
  • Atoms cause scattering
  • Diffraction: coherent scattering of waves off a periodic arrangement of matter
generation of x rays
Generation of X-rays
  • Need
    • Vacuum environment
    • Source of electrons
    • Large accelerating voltage
    • Target metal
generation of x rays1
Generation of X-rays
  • Tungsten filament as source of e-
  • E- sent to target metal (usually Cu)
  • X-rays generated off Cu
  • Higher voltage, faster e-, more energy
x ray crystallography1
X-ray Crystallography

X-ray generation

W

C

a

t

h

o

d

e

C

u

A

n

o

d

e

(

-

)

(

+

)

electrons

X

-

r

a

y

s

two types of x rays generated
Two types of X-rays generated
  • Continuous spectra (white radiation)
  • Electrons hit target surface, loose energy, stop
  • NO change of target electron configuration
  • Removed by filtering
two types of x rays generated1
Two types of X-rays generated
  • Characteristic radiation
  • Electrons interact with target electron configuration
  • Fingerprint of target metal
two types of x rays generated2
Two types of X-rays generated
  • Bombarding electrons dislodge electron from target
  • K-shell or L-shell
  • If vacancy in K-shell filled from L-shell, Kradiation
  • If vacancy if K-shell filled from M-shell, K radiation
two types of x rays generated3
Two types of X-rays generated
  • K radiation has more energy than K radiation
two types of x rays generated4
Two types of X-rays generated
  • K radiation has more energy than K radiation
  • Quantized energy and characteristic of metal target
slide17

Table 2-1: Wavelengths in Nanometer Units of Characteristic Emission Lines for Selected Elements.

Element K[alpha] K[alpha]1 K[alpha]2 K[beta]1 L[alpha]1

Cr 0.229092 0.228962 0.229351 0.228480 ****

Cu 0.154178 0.154051 0.1544330 0.139217 1.3357

Mo 0.071069 0.070926 0.071354 0.063225 0.54062

W 0.021060 0.020899 0.021381 0.018436 0.14763

slide18
Characteristic radiation more intense than continuous
  • Want Ka radiation to go to sample so need to filter everything else
x ray detection
X-ray detection
  • Photographically
  • Electronic detectors
  • Result: see planes of atoms and what orientation planes are in
  • Don’t get image of individual atoms
x ray diffraction
X-ray diffraction
  • Works thanks to Bragg Equation
  • X-rays diffract at specific angles based on spacing of atomic planes
x ray crystallography2
X-ray Crystallography

Destructive and constructive interference of waves

Bragg Equation:

in phase

in phase

Y

x

q

q

q

d

x ray crystallography3
X-ray Crystallography

nl=2dsinq n is the “order”

As soon as the crystal is rotated, the beam ceases (This is diffraction, not reflection)

Only get diffraction at certain angles!

Relation between l and d and q

Y

x

q

q

d