FUNDAMENTALS OF EARTH SCIENCES 1

ES1-LAB SESSIONFALL SEMESTER 2018

Global-regional

Regional-local

local

Microscopic

ANALYTICAL TOOLS:

Planet-scaleRegion

Outcrop

TIME

Rock sample

Polarizing microscope
SEM
SEM-EDS
EPMA
Mass spectrometer
XRD
XRF
…

POLARIZING MICROSCOPY

http://www.olympusmicro.com/

ANISOTROPIC MINERAL
Incident white light(unpolarized)
2 distinct polarized rays
vibrating at right angle
and out of phase

DOUBLE REFRACTION
BIREFRINGENCE
Retardation

FAST RAYSLOW RAY

The slow ray lags behind thefast ray (retardation).
“Extraordinary” ray “Ordinary” rayBased on Imperial College Rock Library1st POLARIZER

In the sample light is split intoa fast and slow ray withvibration directions
determined by the orientationof the mineral sample
2nd POLARIZER(ANALYZER)

The fast and slow ray
recombine at the analyzer andinterfere→ Interference colors
Interference colors depend on(1) Mineral optical properties (retardation,wavelengths of fast and slow ray)(2) Crystal orientation(3) Crystal thickness

Based on Imperial College Rock Library
http://www.earth.ox.ac.uk/~oesis/atlas/metmins/

1st POLARIZER

2nd POLARIZER(ANALYZER)

When the mineralrotates, it becomes
extinct (black)
every 90º.
Extinction occurs
when the vibration
direction of theslow and fast ray isparallel to thepolarizers.
EXTINCTION

https://www.ig.cas.cz/en/structure/departments/tectonics-and-geodynamics/microstructural-analysis

Quartz (SiO2)
Plane polarized light (PPL)

Calcite (CaCO3)

Fosterite (Mg2SiO4)
Mg-rich Olivine

Crossed polarized light (CPL)

http://www.earth.ox.ac.uk/~oesis/nws/nws-a98-16.html
http://www.earth.ox.ac.uk/~oesis/atlas/metmins/

ISOTROPIC MINERAL (e.g. garnet)

Isotropic mineral
Rotating an isotropic mineral does not affectthe outgoing light because the physicalproperties of the mineral is the same in all
direction.

GarnetPlane polarized light (PPL)

Crossed polarized light (CPL)SCANNING ELECTRON MICROSCOPE (SEM)

Secondary electrons→ sample image

High-energy
electron beam

Electron-sample interactions
produce a number of signals:- Secondary e- Backscattered e- (BSE)- Diffracted backsc. e- (DBSE)- X-rays- Visible light (Cathodolumin.)- Heat

BSE and DBSE
→ Structure andorientation of minerals

X-rays (EDS)→ Chemical composition
Visible light (cathodo.)→ Chemical composition

http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html

Energy-Dispersive X-Ray Spectroscopy (EDS)
X-rays of different energies characteristic of different elements areseparated. Results can be displayed as graphs (A) or elemental maps (B)
(A)

(B)

BSE image of a schist

http://serc.carleton.edu/research_education/geochemsheets/eds.html

Cathodoluminescence microscopy“Cathodoluminescence (CL) is the emission of photons of characteristic wavelengthsfrom a material that is bombarded by high-energy electrons. The electron beam istypically produced in an electron microprobe (EPMA) or scanning electron microscope(SEM-CL) or in a cathodoluminesce microscopy attachment to a petrographic microscope(Optical-CL).”
Darrell Henry, Louisiana State University (http://serc.carleton.edu/research_education/geochemsheets/CLTheory.html)

Trace elements promoting CL: Mn2+, Cr3+, Fe3+, Ti4+, rare earth elements (REEs).

http://www.bgs.ac.uk/scienceFacilities/laboratories/mpb/cathodo.html

X-ray powder diffraction (XRD)
X-rays are produced and directed at a rotating sample. The angle at which X-rays are
diffracted depend on the spacing between atoms in the crystal lattice of a mineral.
Diffracted rays

Bragg’s law
nλ=2d sin Θ

n: integer
λ: wavelength of incident X-rays
d : spacing of atomic planes2ΘΘ : angle of incidenceThis technique is used for mineral identification and reconstruction of mineral structures.

http://serc.carleton.edu/research_education/geochemsheets/techniques/XRD.html

X-ray fluorescence (XRF)
An intense X-ray beam is directed at a sample. Some of the energy is absorbed by thesample and used to dislodge electrons of lower energy levels (usually K and L) =ionization. These electrons are replaced by electrons from outer orbitals, a processaccompanied by the emission of radiations in a range of wavelengths. The emitted
wavelengths are characteristic of the atoms present in the sample.
The wavelength is given by
Planck’s law:

E = hc/ λE = energy
h = Planck constant
λ = wavelengthc = speed of light in a vacuum
This technique is used to determine
the abundance of major and traceelements in geological samples.
Wikipedia
http://serc.carleton.edu/research_education/geochemsheets/techniques/XRF.html