Basics and potential of cathodoluminescencein metamorphic petrology

Jens Götze

Institute of MineralogyTU Bergakademie Freiberg, Germany

1. Basics of luminescence- physical basics- instrumentation- general applications

2. Application of CL in metamorphic petrology- identification of minerals, mineral distribution- primary growth structures- secondary features

(deformation - recrystallization - fluid flow - alteration - mineral neoformation)

- examples of technical processes

3. Conclusions

ContentContent

Basics of luminescenceBasics of luminescence

LiteratureLiterature

Literature

Unwin-HymanBoston

1988

Cathodoluminescenceof geological materials

D.J. Marshall

Unwin-Hyman Ltd.

Literature

Springer-VerlagBerlin, Heidelberg, New York2000

ISBN 3-540-65987-0

Literature

TU Bergakademie FreibergAkademische BuchhandlungMerbachstraße, PF 203D-09599 Freiberg

ISBN 3-86012-116-2

Literature

All-Russia Institute of Mineral Resources(VIMS)Moscow2002

ISBN 5-901837-05-3

History of cathodoluminescence

History of cathodoluminescence

History of Cathodoluminescence

1879 CROOKSLuminescence studies on crystals after bombardmentwith a cathode ray

1965 SIPPEL, LONG & AGRELLFirst application for thin section petrography

1965 SMITH & STENSTROMCathodoluminescence studies with the microprobe

1971 KRINSLEY & HYDECathodoluminescence studies with the SEM

1978 ZINKERNAGELFirst CL microscope in Germany

Basics of luminescenceBasics of luminescence???

LuminescenceLuminescence

= transformation of diverse kinds of energyinto visible light

Basics of luminescence

Luminescence of inorganic and organic substances

results from an emission transition of anions, molecules

or a crystal from an excited electronic state to a ground

state with lesser energy.

(Marfunin1979)

Basics of luminescence

Fluorescence = luminescence emission with alifetime < 10-8 s

Phosphorescence = luminescence emission with alifetime > 10-8 s

Basics of luminescence

Main processes of luminescence

(1) absorption of excitation energy and stimulationof the system into an excited state

(2) transformation and transfer of the excitation energy

(3) emission of light and relaxation of the systeminto an unexcited condition

Schematic model ofluminescence processesSchematic model ofluminescence processes

Excitationby energy

Emissionof light

e-

electrons cathodoluminescence

thermal excitation

biological processes

UV photoluminescence

thermoluminescence

bioluminescence

Basics of luminescence

Sample

Back scattered electrons

Secondary electrons

Primary electron beam

Cathodoluminescence

Auger electrons

Scattered electrons

X-rays

Unscattered electrons

Specimen current

Basics of luminescence

incidentelectron beam

continuum radiation(Bremsstrahlung)

cathodoluminescence

Sample surface

back scatteredelectrons

primary X-rayexcitation

secondary electrons

2-8 µm

Electron beam interaction with a solidElectron beam interaction with a solid

Basics of luminescence

The band modelThe band model

valence band

conduction band

insulatorconductor semiconductor

E

Energy levels in a band scheme for different crystal types

valence bandvalence band

conduction band

conduction bandband gap

band gap

Basics of luminescence

valence band

conduction band

insulatorconductor semiconductor

E

Electron transitions in a band scheme for different crystal types

E (photonenergy)

E

Basics of luminescence

Conduction band

Valence band

E

activator

(b) insulator(broad interband spacing)

(a) semiconductor(small interband spacing)

Positions of ion activator energy levels in a band scheme for different crystal types

acceptor

donor

Basics of luminescence

Valence band

E

activator

luminescence

(e)(a) (b) (c)

trap

(d)

Conduction band21

Basics of luminescence

Electron transitions and luminescence processes in a solid

Conduction band

Valence band

E

activator

luminescence

exci

tatio

n

radiationlesstransition

Basics of luminescence

Electron transitions and luminescence in a solid with ion activation

The configurational coordinatemodel

The configurational coordinatemodel

Basics of luminescence

excitedstate

groundstate

Configurational coordinate diagram for transitions according to the Franck-Condonprinciple with related absorption and emission bands, respectively.(modified after Yacobi & Holt 1990)

absorptionband

emissionband

Basics of luminescence

Excitation (1) and emission (2) spectra of Mn2+ in calcite (after Medlin 1964)

1

2

Stokes shift

Basics of luminescence

The sensitivity of the electronic states of the Mn2+ ion in octahedral coordination to changesin the intensity of the crystal field splitting Dq and representation in a configurational diagram(modified after Marfunin 1979 and Medlin 1968)

How can we use

the luminescence signal ??

How can we use

the luminescence signal ??

Basics of luminescence

Visualization of the real structure of solids by CL

CLPol

Luminescence centres

intrinsic

lattice defects(broken bonds, vacancies)

extrinsic

trace elements(Mn2+, REE2+/3+, etc.)

Basics of luminescence

Types of luminescence centres

transition metal ions (e.g., Mn2+, Cr3+, Fe3+)

rare earth elements (REE2+/3+)

actinides (especially uranyl UO22+)

heavy metals (e.g., Pb2+, Tl+)

electron-hole centres (e.g., S2-, O2

-, F-centres)

crystallophosphores of the ZnS type (semiconductor)

more extended defects (dislocations, clusters, etc.)

Basics of luminescence

Detection of the cathodoluminescence emission

(2) CL spectroscopy(1) CL microscopy

contrasting of different phases

visualization of defects, zoningand internal structures of solids

oapatite

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

300 400 500 600 700 800 900 1000 1100

wavelength [nm]

rel.

inte

nsity

[cou

nts]

Sm3+

Eu2+

Sm3+

Nd3+

Dy3+

determination of the real structure

detection of trace elements, theirvalence and structural position

CL emission spectraCL emission spectra

The crystal field theoryThe crystal field theorylocal environment of the activator ion

- The activator-ligand distances in the different excitedstates and the slope of the energy levels depend on theintensity of the crystal field(expressed as crystal field splitting = 10Dq)

- The stronger the interaction of the activator ion with thelattice, the greater are the Stokes shift and the width of the emission line.

(Burns, 1993)

Basics of luminescence

The crystal field theoryThe crystal field theorylocal environment of the activator ion

Factors influencing values of or 10Dq are:

- type of the activator ion (size, charge)

- type of the ligands

- the interaction distance

- local symmetry of the ligand environment

etc.

Basics of luminescence

300 400 500 600 700 800

wavelength [nm]

0

16000

12000

8000

4000

rel.

inte

nsity

[cou

nts]

zircon

Dy3+

Dy3+

Dy3+

Dy3+

zircon

scheelite

anhydrite

calcite

fluorite

apatite400 500 600 700 [nm]

Dy3+

Tb3+

Dy3+ Sm3+Dy3+

Sm3+

Sm3+

Sm3+

Tm3+

Influence of the crystal field on CL emission spectraInfluence of the crystal field on CL emission spectra

(1) influence of the crystal field = weak

CL emission spectra are specificof the activator ion

CL spectra of narrow emissionlines (e.g. REE3+)

Basics of luminescence

Influence of the crystal field on CL emission spectraInfluence of the crystal field on CL emission spectra

(2) influence of the crystal field = strong

CL emission spectra are specificof the host crystal

CL spectra of broad emissionbands (e.g. Mn2+, Fe3+)

Mn2+calcite

300 400 500 600 700 800wavelength [nm]

100

400

300

200

rel .

inte

n si ty

[co u

n ts]

Mn2+ activated CL of CaCO3:

aragonite green (~560 nm)

calcite yellow-orange (~610 nm)

magnesite red (~655 nm)

Basics of luminescence

perl (aragonite CaCO3)

sea lily stalk (calcite CaCO3)500 µm

Visual and spectral detection of Mn2+ activated CL in natural carbonates

Basics of luminescence

Influence of the crystal fieldon the broad CL emission bands in solid solutions

Influence of the crystal fieldon the broad CL emission bands in solid solutions

plagioclase

Fe3+Mn2+

0

800

600

400

200rel.

inte

nsity

[cou

nts]

300 400 500 600 700 800 900wavelength [nm]

750

740

730

720

710

700

690

680

wav

elen

gth

[nm

]0 20 40 60 80 100

An content [mol-%]

Position of the Fe3+ activated CL emission band in plagioclases in relation to theanorthite content

IRred

Basics of luminescence

InstrumentationInstrumentation

Scanning Electron Microscope JEOL 6400with OXFORD Mono-CL detector

samplesilica-glassfibre guide

primary electron beam

mirror

Cathodoluminescence detector on a scanning electron microscope

OXFORD Mono CL

Hot-cathode luminescence microscope HC1-LM(designed by Rolf Neuser, Bochum)

cold-cathode microscopehot-cathode microscope

InstrumentationInstrumentation

leaded glassviewing point

microscope

thin section

coronapoints

condenser

light sourcedischarge tube

curved electrodehigh

voltage

cable

optic axis

specimen

microscopeobjective

focuscoil

cathode

Cathodoluminescence techniquesCathodoluminescence techniques

CL microscopySEM-CL

polished thin (thick) section

defocused electron beam, stationary mode

heated filament („hot cathode“)14 kV, 0.1-0.5 mA

ionized gas („cold cathode“)

glass optics: 380-1200 nm (Vis - IR)

analytical spot ca. 30 µm

true luminescence colours

resolution 1-2 µm

polarizing microscopy, (EDX)

polished sample surface

focused electron beam,scanning mode

heated filament20 kV, 0.5-15 nA

mirror optics: 200-800 nm (UV - IR)

analytical spot ca. 1 µm

panchromatic CL images (grey levels)

resolution << 1 µm

SE, BSE, EDX/WDX, cooling stage

Cathodoluminescence imagingCathodoluminescence imaging

zircon

CL

SE

BSE

10 µm

CL

Polmi

500 µm

quartz

Cathodoluminescence microscopySEM cathodoluminescence

Cathodoluminescence microscope HC1-LM

Computer aidedimage analysis

vacuumpumps

electronicsteerage

CL microscope withattached CCD basedviedeo camera

Wehnelt cylinder

Electron gun

Hot-cathode luminescence microscope HC1-LM

sample is fixedupside down inthe sample holder

transparency !

screen

sampleholder

brassclips

Sample chamber

microscope

sample

Light sourcefilamentelectron

beam

Polarisingmicroscopy

Cathodoluminescencemicroscopy

Cathodoluminescence microscopyCathodoluminescence microscopy

Sample preparationSample preparation

Sample preparationSample preparation

1. Polished thin section sample holder(glass)

epoxy resin

sample(~25 µm)

48 m

m

28 mm

application for all CL equipments

Sample preparationSample preparation

2. Polished section Sample holder(plastics)

application for SEM-CL („cold-cathode“ microscopes)

epoxy resin

samplepiece

Sample preparationSample preparation

4. Sample holder for fluid inclusionpreparates

sample holder(metallic)

glassplate

48 m

m

28 mm

application for all CL equipments

samplepiece

Sample preparationSample preparation

3. Polished sample piece

samplepiece

application for SEM-CL („cold-cathode“ microscopes)

Sample preparationSample preparation

5. Pressed tablet (powder samples)

application for SEM-CL („cold-cathode“ microscopes)

pressed tabletof powder sample

Sample preparationSample preparation

for SEM-CL and „hot-cathode“ CL microscopes

Coating with conducting material

- to prevent the built up of electrical charge during electron irradiation

- coating material: C, Au, Al, Ag, Cu

DocumentationDocumentation

DocumentationDocumentation

Digital video cameraKAPPA 961-1138 CF 20 DXC

Digital micrographsConvetional photos/slides

Nikon photo cameraKodak Ektachrome 400 HC

Advantages of CCD:

high spatial resolution

high sentsitivity

- analysis of minerals withvery low CL intensities

- low accumulation time

direct combination withimage analysis

Spectral CL measurementsSpectral CL measurements

High-resolution CL spectroscopyHigh-resolution CL spectroscopy

14 kV, 0.2 mA< 5s accumulation time

adaptation(30µm screen)

Silica-glassfibre guide

150, 600, 1200 lines/mm 0.4 nm resolution

data processing

adaptation

High-resolution CL spectroscopyHigh-resolution CL spectroscopy

Triple-gratingspectrograph

Nitrogen-cooledCCD-detector

Silica-glass fibre guide

CL microscope

Factors influencingCL properties/intensityFactors influencingCL properties/intensity

Factors influencing the CL intensityFactors influencing the CL intensity

time(especially transient CL)

sample preparation(sample surface, thickness, etc.)

sample coating(quality, thickness, material, etc.)

temperature

analytical conditions(acceleration voltage, beam current, vacuum, etc.)

Analytical parameters influencing cathodoluminescenceAnalytical parameters influencing cathodoluminescence

-120 -100 -80 -60 -40 -20 0 +20

Specimen current at 20 kV [nA]

rela

tive

inte

nsity2

0

1000

3

Variation of the intensity of quartz CLwith beam current(modified after Hanusiak 1975)

rela

tive

inte

nsity

1

100

10

100

temperature [°C]

1

0.1 1 10

Variation of CL intensity with sampletemperature for quartz(modified after Hanusiak & White 1975)

Sensitizing and quenching

Interaction between two or more ions with transfer of ecitation energyfrom one ion to another resulting in changes of their luminescence.

Quenching of luminescence:

(1) concentration quenching(self quenching)

(2) quenching by ions with intensecharge transfer bands (e.g. Fe2+, Co2+)

(3) quenching due to lattice defects

(4) thermal quenching

Sensitizing of luminescence:

(1) emission-reabsorption(„cascade“ luminescence)

(2) resonance radiationless

(3) nonresonance radiationless

typical sensitizer ions:

(1) ions with intensive absorptionbands in the UV (e.g. Tl+, Cu+, Pb2+)for sensitization of Mn2+

(2) ions of transition metals (e.g. Mn2+) forsensitization of REE3+

(3) REE2+/3+ for sensitization of REE3+

excitationemission

activatoractivator

excitationradiationlesstransition

luminescence emission concentration quenching

Intensity of the Mn2+ activated CL (ca. 560 nm) in dependenceon the Mn content in feldspar

Intensity of the Mn2+ activated CL (ca. 560 nm) in dependenceon the Mn content in feldspar

10

8

6

4

2

0

Inte

nsity

/ a.

u.

0 20 40 60 80 100 120 140 160Mn content [ppm]

plagioclases

alkali feldspar

rel.

CL

inte

nsity

mol-% Mn0 0.5 1.0 1.5

apatiteVysoki Kamen

300 µm

o

o

o

o

o

o

0.85

4.86 wt-% Mn

3.413.81

2.97

1.26

Concentration quenching („self quenching“) of Mn2+ activated CL in apatite

from Kempe & Götze (2002)

Mineral groups and minerals showing CLMineral groups and minerals showing CL

in general all insulators and semiconductors

elements diamondsulfides sphaleriteoxides corundum, cassiterite, periclasehalides fluorite, halitesulfates anhydrite, alunitephosphates apatitecarbonates calcite, aragonite, dolomite, magnesitesilicates feldspar, quartz, zircon, kaolinite

technical products (synthetic minerals, ceramics, glasses !)

no luminescence of conductors, iron minerals andFe-rich phases

General applications of CL in geosciencesGeneral applications of CL in geosciences

identification of minerals, mineral distribution andquantification

typomorphic properties(CL colour, CL behaviour, spectral characteristics)

crystal chemistry(trace elements, internal structures, zoning)

reconstruction of geological processes

characterisation of technical products(also non-crystalline phases !)