X-ray%20diffraction%20and%20minerals

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X-ray diffraction and minerals
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Is this mineral crystalline?
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Braggs law
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Why work on single crystals?
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Why work on crystal powder?
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The powder X-ray diffraction pattern of an
amorphous solid
- No sharp peak - Broad hump
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Braggs law n? 2 d sin ? 1) What do we
know? ?, i.e. the wavelenth of the X-ray
radiation 2) What do we assume? n 1 (Peaks for
higher n are weaker.) 3) What do we want to
know? d, i.e. the interplanar spacings of the
lattice
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X-ray spectra used to be recorded on film strips
rolled up within a round chamber.
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The distance from the center of each line to
the center of the hole (where X-rays entered the
chamber) was proportional to the angle 2-theta.
The intensities of the lines were originally
estimated by a human eye, on a scale of 1 to 100,
before detectors became routine.
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Ion order-disorder can be detected by X-ray
diffraction. This is very different from the
lack of order found in an amorphous solid.
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Powder X-ray diffraction is a routine technique
to measure the amount of crystalline SiO2
(quartz) present in mineral dust or soil. A
chemical analysis will not distinguish the SiO2
of quartz from the silicate skeleton of clays
and many other minerals.
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Laue diffraction experiment
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Large spots aluminum. Small spots silicon.
Laue photographs are used to study the epitaxial
relationships between thin films and the material
on which they are grown.
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How to solve crystal structures?
The electron density ( ) at a point X, Y, Z in a
unit cell of volume V is (X,Y,Z) 1/V Fhkl cos
2 (h ? X k ? Y l ? Z) - Therefore if we
know Fhkl and (for each h, k, l) we can compute
for all values of X, Y, and Z and plot the values
obtained to give a three-dimensional electron
density map. Then, assuming atoms to be at the
centres of the electron density peaks, we would
have the entire structure.
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The unit cell is described as being the smallest
regular repeat unit in a crystalline
lattice. These cells are defined by three unit
lengths (a, b, c) along the crystallographic
axe,s and the three interaxial angles (?, ?, ?).
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E this tube is the X-ray source. Inside it,
there is a 40,000 volt difference between a
tungsten filament and a copper target.
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What radiation does the target metal emit? A
spectrum (i.e. many wavelengths) with two sharp
peaks.
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A cathode filament is heated so that it boils off
electrons. A large voltage (20-100kV) is
maintained between the filament and the target (a
metal such as Mo, Cu, Co, Fe or Cr). The
electrons are accelerated and hit the target
metal.
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H scintillation counter which measures the
intensity of the diffracted X-ray beams. It is
connected to a goniometer which measures the
2-theta angles at which diffracted beams are
detected.
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G the sample chamber holds the specimen. Samples
are ground to a fine powder before mounting them
in the chamber. X-rays enter from the left, are
diffracted by the powder, and leave the chamber
to the right.
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A graphite monochromator is used to let only one
specific X-ray wavelength escape the X-ray source.
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piezoelectricity production of electrical
polarization in a material by the application
of mechanical stress - phonographs -
microphones - quartz watches
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When a chemical analysis will not tell you what
mineral this is....
This Anglo-Saxon brooch contains an inlay of
CaCO3, but is it calcite or aragonite (2 common
polymorphs)?
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When detecting twinning matters ! Piezoelectric
crystals may not display that property if they
are twinned. Twinning can show up in - external
forms - re-entrant angles (non-convex morphology)
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Indices of diffracted X-ray peaks are usually
written without parentheses. 111, 222 and 333
correspond to the 1st, 2nd and 3rd order
reflections of the (111) planes. 222 is produced
when the X-rays of successive planes have a path
difference of 2wavelength (two lambdas).
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Braggs law predicts at which angles the peaks
will be diffracted, but not their
intensities. Diffraction intensities are
influenced by the atomic number (Z) of the atoms
in the structure, by the shape and size of the
specimen, and by other factors related to the
machine. We use the peak intensities to determine
where the atoms are in the unit cell.
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Structures with lighter elements can be studied
using neutron diffraction. Neutrons are
scattered by the nucleus, and their scattering
varies less from element to element. whereas
X-rays are scattered by the electron cloud, and
light elements barely re-emit them.
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Mineral structure can be destroyed by radiation
damage.