Mineralogy,%20Diffraction

1
Mineralogy, Diffraction

• Carleton College

2
X-rays History

• Nature of x-rays

3
(No Transcript)
4
(No Transcript)
5
History Wilhelm Conrad Roentgen

• Roentgen was born on March 27, 1845 in Lennep
  (Germany).
• He was educated in Utrect and Zurich and became
  professor of physics at Strassburg (1876),
  Giessen (1879), Würzburg (1888), and Munich
  (1899).

6
History Wilhelm Conrad Roentgen

• He received the Nobel Prize in 1901. Roentgen
  refused to patent his discoveries and rejected
  all commercial offers relating to them.
• In his later years, he was embittered by the
  suggestion that he had taken credit for his
  laboratory assistant's discovery, and withdrew
  from public life.
• Roentgen died on February 10, 1923 of carcinoma
  of the rectum, and was buried beside his wife in
  the family grave in Giessen.

7
History Wilhelm Conrad Roentgen

• Roentgen was working in his laboratory at the
  Physical Institute of the University of Würzburg,
  Germany, experimenting with a Crookes tube.

8
History Wilhelm Conrad Roentgen

• This tube is a glass bulb with positive and
  negative electrodes, evacuated of air, which
  displays a fluorescent glow when a high voltage
  current is
• passed though it. When he shielded the tube with
  heavy black cardboard, he found that a greenish
  fluorescent light could be seen from a platinum
  screen 9 feet away.

9
History Wilhelm Conrad Roentgen

• He concluded that a new type of ray emitted from
  the tube, passed through the covering, and casted
  shadows of solid objects. The rays passes through
  most substances, including the soft tissues of
  the body, but left the bones and most metals
  visible.

10
History Wilhelm Conrad Roentgen
11
History Wilhelm Conrad Roentgen

• One of his earliest photographic plate from his
  experiments was a film of his wife, Bertha's hand
  with a ring, was produced on Friday, November 8,
  1895.

12
X-ray production

• X-rays are produced when an electron boiled
  from filament are caused to strike a target of
  atoms by the force of a high voltage field.
  Which is seen in the next slide

13
X-ray production
14
X-ray production

• Deceleration of electrons as they approach atoms
  in the target creates a white background of
  x-rays called the Brehmstrallen radiation.

15
X-ray production

• X-rays are produced when there is a sudden
  deceleration of electrons. In practice, X-rays
  are produced when an extremely high voltage
  (15-60
• Kv) is applied to a filament (typically a
  tungsten cathode) in a vacuum. The electrons are
  then accelerated into a metal target (typically a
  copper
• anode). The result is two particular types of
  X-radiation.

16
X-ray production

• The first type is known as white radiation and
  consists of a broad, continuous spectrum
  containing many wavelengths of radiation. It is a
  result of the very rapid deceleration of
  electrons as they encounter the strong electric
  fields of target metal. As the electrons collide
  they lose energy (often designated delta-E) and
  that energy goes into making X-ray photons. That
  energy, delta-E is related to the frequency of
  the X-ray radiation by Planck's Constant,

17
X-ray production

• ?E hv
• Where h planks constant
• Vfrequency of the x-ray
• remember that v c/l
• Cspeed of light, lwavelength
• therefore, ?Ehc/ l

18
X-ray production
19
X-ray production

• White radiation

20
X-ray production

• Superimposed on this background are peaks of
  intense x-rays that have wavelengths that depend
  on the atoms involved.
• These peaks of characteristic wavelengths are
  produced when an atom losses an electron from an
  inner orbital.

21
X-ray production

• The peaks are labeled
• Ka, Kb, La, Lb, etc
• depending on the specific energy levels involved.

22
X-ray production

• Laboratory production of X-rays

23
X-ray production

• Target metal anode (pure element)
• Filament cathode

24
X-ray Diffraction Experiments

• Space group
• Unit Cell

25
What is Diffraction?

• Diffraction, generally defined as a departure of
  a ray from the path expected from reflection and
  refraction.

26
What is Diffraction?

• Sets of narrow slits and ruled gratings were
  observed to produce diffraction patterns when
  the spacing of the slits is similar to the
  wavelength of light used.

27
What is Diffraction?

• Because all of the slits in a diffracting grating
  are illuminated by the same source of light, the
  set of slits may be considered to be a set of
  light source all in phase with one another.
• Light rays traveling perpendicular to the grating
  will remain in phase.

28
What is Diffraction?

• Light rays traveling at an angle Ø to the will
  not be in phase, except for a special angles such
  that S sin Ø nl, where S is the spacing of the
  slits, l is the wavelength of light and n is an
  integer.
• We may use this expression to find l for a laser
  or S for a diffraction grating.

29
Diffraction

• Because of spacing of planes of atoms in crystals
  is similar to the wavelength of x-rays
• Diffraction of X-rays by Crystals is possible.

30
Diffraction

• Atoms in a crystal behave like little x-ray
  sources.

31
Diffraction

• X-ray reflections

32
Diffraction

• The figure to the left illustrates a modern X-ray
  diffraction pattern of the mineral vesuvianite
  (type locality is Mt. Vesuvius). The diffraction
  pattern is recorded on photographic film as a
  series of spots (this is actually a negative).
  The spots do not represent atoms. They do
  represent layers, or planes of atoms within the
  crystal structure. The spacing of the spots is
  proportional to the distance between the
  different diffracting layers in the crystal. Can
  you recognize any symmetry in this diffraction
  pattern? If you look carefully you should be
  able to convince yourself that the center of
  the pattern corresponds to a fourfold rotation
  axis perpendicular to the plane of the page. You
  should also be able to recognize that the axes
  a1 and a2 represent mirror planes. The symmetry
  of the crystal is reflected in the symmetry of
  the X-ray pattern. Although the symmetry
  elements are not sufficient to identify the
  mineral, you will soon be able to recognize
  that the symmetry observed implies that this
  mineral belongs to the tetragonal crystal
  system. The symmetry, together with a
  measurement of the separation of the spots would
  be sufficient to identify this diffraction
  pattern as belonging to the mineral vesuvianite.

33
Braggs equation

• nl 2dsin Ø

34
Diffraction

• Diffraction of x-rays by crystals is possible
  because the spacing of planes of atoms in
  crystals is similar to the wavelength of x-rays.

35
Powder Diffraction

• X-ray diffraction by mineral powders is one of
  the mineral identification and characterization
  techniques most used by geologists.

36
Powder Diffraction

• Powder diffraction experiment requires only as
  small quantity of a mineral.
• 10-500mg
• Sample preparation is very simple and fast
• Reliable accurate results are obtained in a
  relatively short time, 10 minutes to 2 hours.

37
Powder Diffraction

• The principle behind PD experiment is the random
  orientation of crystals in a mineral powder.

38
Powder Diffraction

• If the powdered crystals are randomly oriented,
  then for all sets of planes (hkl) some of the
  crystals in the powder will be in the correct
  orientation (usually horizontal) with respect to
  the x-ray source to satisfy Braggs law.

39
Powder Diffraction

• In other words, at least a few of the mineral
  grains will diffract for each of the planes (hkl)
  during a scan through 2 Ø angle.
• The more the finely ground the powder, the more
  likely that all orientations are presented in
  abundance.

40
Powder Diffraction

• The ideal powder size is 5-10 microns.

41
Powder Diffraction

• Here at Carleton, we have an automated powder
  diffractometer that yields digital computer
  output.

42
Powder Diffraction

• Unknown minerals my be identified from powder
  diffraction data using ICDD Powder Diffraction
  File.

43
Powder Diffraction

• Intensity and 2 Ø or dhkl values are used in the
  search.
• Computer searches of the file may lead to a
  unique match with a known powder diffraction
  patter.

44
Powder Diffraction

• Because of the chemical composition of most
  minerals is variable and some aspects of a
  mineral structure may depend on its history, the
  obtained diffraction pattern may not exactly
  match the standard data for a given mineral
• This makes identification more challenging

45
Powder Diffraction

• Once a mineral has been identified, the Powder
  Diffraction File data card my be used to index
  the observed diffraction peaks.
• Miller indices, 2 Ø, d values, may be used to
  determine the Unit Cell Parameters of the sample.

46
Diffraction Summary

• Diffraction pattern is like a finger print of the
  crystal structure
• d-values reflect the unit cell parameters
• intensities reflect the atoms/molecules

47
Sample preparation for PD

• Sample preparation procedures are critical inn
  being able to obtain accurate and reproducible
  XRD results.

48
Sample preparation for PD

• Care should be exercise in order to avoid
  introducing errors resulting from factors such
  as
• non-representative sampling
• contamination
• material loss

49
Sample preparation for PD

• alteration of composition due to
• Over grinding,
• hydration
• dehydration
• oxidation

50
Sample preparation for PD

• Sample height displacement
• non-uniformity of the sample surface

51
Sample preparation for PD

• The Backloading Technique

52
Sample Preparation of PD

• Two-piece round sample holder

53
Sample Preparation of PD

• PW1811/27 Sample holder and
• PW1770/10 Sample Preparation Kit

54
Sample Preparation of PD

• Use the following guidelines to prepare a
  back-mounted sample
• Invert the holder ring and clamp it onto
  preparation table

55
Sample Preparation of PD

• Spread Powder level using spatula. Do not pack
  or compress

56
Sample Preparation of PD

• Press powder with block

57
Sample Preparation of PD

• Remove excess powder with a knife blade

58
Sample Preparation of PD

• Clean mating surface with small brush or edge of
  your thumb

59
Sample Preparation of PD

• Affix back plate

60
Sample Preparation of PD

• Invert the prep table while holding sample ring
  stationary and depress the release clamp, and
  remove the sample