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Chapter 12
Additional Analytical Methods
Analytical MethodsTechnique
TypeTechnique application
SubdivisionsSpecific
applicationDescription Destruction
Light microscopyGeneral surveys
Features larger than 1μm
TransmittedTransparent
mineralsPetrographic microscope –
light from below sampleNon
Reflected Opaque minerals (ore minerals)
Petrographic microscope – light from above sample
Non
DiffractionFurther identification,
lattice parameters and crystal structure
X-RayPowders of single
minerals or mixtures (heavy atom
position)
X-Ray beam scattered at differing intensities at different
anglesSemi
NeutronPowders of single
minerals or mixtures (light atom position)
Neutron beam scattered at differing intensities at different
anglesSemi
Particle Microscopy
High resolution imagingFeatures smaller than
1μm
Transmission Electron Microscopy
Images of structural defects: dislocations,
twin and phase boundaries
Accelerated (high voltage)electron beam deeply penetrates small area
Non
Scanning Electron Microscopy
Image sample morphology and
determine compositional
variations
Accelerated (low voltage)electron beam shallowly
penetrates large areaNon
Atomic Force Microscopy
Image arrangement of individual atoms in
surface of crystals
Measure electrostatic repulsion intensity of atoms in sample in close contact with atoms of a
crystal tip
Non
Analytical Methods
Technique Type
Technique application
SubdivisionsSpecific
applicationDescription Destruction
Chemical AnalysisAccurate chemical
compositions of minerals
Microprobe
Quantitative point analyses in polished sections; mostly only
Na and higher
Accelerated electron beam with two detectors: energy
dispersion and x-ray detector compared with standard
Non
X-Ray Fluorescence
Quantitative analyses of rock in
powder; gives chemical elements –
major and trace
High-energy polychromatic X-ray beam produces secondary fluorescent X-rays which are analysed for wavelength and
energy
Semi
Optical emission & absorption
Mostly for liquid sample quantitative chemical analyses
Light beam excite or absorb valence electrons from sample;
secondary beams dispersed into separate wavelengths of
measurable intensities
Complete
Mass spectrometry
Measure amounts of different isotopes - mainly radiometric
dating and determination of stable isotopes
Ionization of atoms, ions accelerated and into magnetic
field which deflects ions – degree of deflection dependant
on ion mass and charge
Complete
Analytical MethodsTechnique
TypeTechnique application
Subdivisions
Specific application
DescriptionDestructi
on
Spectroscopy Investigate structural
environments
Infrared & Raman
Information on symmetry, bond
lengths and angles,
coordination polyhedra
IR radiation or laser beam passed through sample
and intensity of light measured. Absorption of
light corresponds to energy differences of
vibrational levels in the crystal
Non
X-Ray absorption
Compositional edges in mineral
grains
Measure the difference in absorption of X-rays
relative to the intensity of the rays
Non
Nuclear magnetic resonance
Determine the occupancy of an
element in different
structural sites
Nuclei of atoms in mineral spin to cause magnetic
field which is placed inside a large magnetic field.
Magnetic resonance when applied field = energy
difference in spin levels. Specific for different
chemical and crystallographic environments
Non
Chapter 13
Mechanical properties and deformation
Stress-strain Mechanical properties – expression of history of crystal Definitions:
• Stress (σ) Force per surface area
• Strain (ε) Deformation resulting from the stress
• Deformation Elastic
• When stress removed strain returns to original value Plastic or ductile
• Active dislocations cause permanent changes in structure and shape, but material stays in tact
Work-hardening• Stress needed for creating increasing strain increase rapidly as
multiplying dislocations interfere with each other Brittle (Failing)
• Material has reach its ultimate strength and fractures completely
Deformation
Stress applied to crystal Deforms crystal on crystallographic
slip planes (hkl) with displacements along crystallographic slip directions [uvw]
Slip is not instantaneous but propagate along this slip plane, breaking one bond at a time but resulting in a complete displacement of the two parts of the crystal
Deformation
Fig 13.2; 13.3; 13.4
Dislocation microstructures
Present in most crystals even at ideal growth conditions
Number of dislocations generally increase with deformation
Development and propagation of dislocations are influenced by each other or other obstacles such as inclusions
Loops, diffusion of vacancies (climb)
Dislocation microstructures
Loops Fig. 13.6, 13.7
Dislocation microstructures
►Diffusion of vacancies (climb)
►Fig. 13.8, 13.9
Mechanical twinning
A mechanical stress cause part of crystal to flip into new orientation about a plane
New orientation related to old orientation by mirror plane
Thus: Geometric twinning relationship
Fixed small sized deformation unlike slip which is a continuous arbitrary deformation and can be large
Mechanical twinning
Fig 13.10, 13.11