Basics and potential of cathodoluminescence in metamorphic petrology Jens Götze Institute of Mineralogy TU Bergakademie Freiberg, Germany ContentContent 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 BasicsBasics ofof luminescenceluminescence LiteratureLiterature Literature Cathodoluminescence of geological materials D.J. Marshall Unwin-Hyman Boston Unwin-Hyman Ltd. 1988 Literature Springer-Verlag Berlin, Heidelberg, New York 2000 ISBN 3-540-65987-0 Literature TU Bergakademie Freiberg Akademische Buchhandlung Merbachstraße, PF 203 D-09599 Freiberg ISBN 3-86012-116-2 Literature All-Russia Institute of Mineral Resources (VIMS) Moscow 2002 ISBN 5-901837-05-3 HistoryHistory of of cathodoluminescencecathodoluminescence History of Cathodoluminescence 1879 CROOKS Luminescence studies on crystals after bombardment with a cathode ray 1965 SIPPEL, LONG & AGRELL First application for thin section petrography 1965 SMITH & STENSTROM Cathodoluminescence studies with the microprobe 1971 KRINSLEY & HYDE Cathodoluminescence studies with the SEM 1978 ZINKERNAGEL First CL microscope in Germany BasicsBasics ofof luminescenceluminescence ??? Basics of luminescence LuminescenceLuminescence = transformation of diverse kinds of energy into 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) Fluorescence = luminescence emission with a lifetime < 10-8 s Phosphorescence = luminescence emission with a lifetime > 10-8 s Basics of luminescence Main processes of luminescence (1) absorption of excitation energy and stimulation of the system into an excited state (2) transformation and transfer of the excitation energy (3) emission of light and relaxation of the system into an unexcited condition Basics of luminescence UV photoluminescence Excitation thermal excitation thermoluminescence by energy biological processes bioluminescence electrons cathodoluminescence e- SchematicSchematic model model of of Emission luminescenceluminescence processes processes of light Basics of luminescence Primary electron beam Back scattered electrons Cathodoluminescence Secondary electrons X-rays Auger electrons Sample Specimen current Scattered electrons Unscattered electrons Basics of luminescence ElectronElectron beam beam interaction interaction with with a a solidsolid incident electron beam Sample surface secondary electrons back scattered electrons primary X-ray excitation 2-8 µm continuum radiation cathodoluminescence (Bremsstrahlung) TheThe band band modelmodel Basics of luminescence E conduction band conduction band band gap band gap conduction band valence band valence band valence band conductor semiconductor insulator Energy levels in a band scheme for different crystal types Basics of luminescence E conduction band E (photon energy) E valence band conductor semiconductor insulator Electron transitions in a band scheme for different crystal types Basics of luminescence E Conduction band donor acceptor activator Valence band (a) semiconductor (b) insulator (small interband spacing) (broad interband spacing) Positions of ion activator energy levels in a band scheme for different crystal types Basics of luminescence (a) (b) (c) (d) (e) E 1 2 Conduction band trap luminescence activator Valence band Electron transitions and luminescence processes in a solid Basics of luminescence Conduction band E radiationless transition luminescence excitation activator Valence band Electron transitions and luminescence in a solid with ion activation TheThe configurational configurational coordinate coordinate modelmodel Basics of luminescence excited state absorption ground band state emission band Configurational coordinate diagram for transitions according to the Franck-Condon principle with related absorption and emission bands, respectively. (modified after Yacobi & Holt 1990) Basics of luminescence Stokes shift 2 1 Excitation (1) and emission (2) spectra of Mn2+ in calcite (after Medlin 1964) Basics of luminescence The sensitivity of the electronic states of the Mn2+ ion in octahedral coordination to changes in the intensity of the crystal field splitting Dq and representation in a configurational diagram (modified after Marfunin 1979 and Medlin 1968) HowHow can can we we use use thethe luminescence luminescence signal signal ?? ?? Basics of luminescence Visualization of the real structure of solids by CL Pol CL Luminescence centres intrinsic extrinsic lattice defects trace elements (broken bonds, vacancies) (Mn2+, REE2+/3+, etc.) Basics of luminescence Types of luminescence centres transition metal ions (e.g., Mn2+, Cr3+, Fe3+) rare earth elements (REE2+/3+) 2+ actinides (especially uranyl UO2 ) 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 (1) CL microscopy (2) CL spectroscopy contrasting of different phases determination of the real structure visualization of defects, zoning detection of trace elements, their and internal structures of solids valence and structural position 2600 apatite Sm3+ 2400 o 2200 2000 3+ 1800 Sm Dy3+ 1600 3+ 1400 Nd Eu2+ 1200 rel. intensity [counts] 1000 800 300 400 500 600 700 800 900 1000 1100 wavelength [nm] CLCL emissionemission spectra spectra Basics of luminescence TheThe crystal crystal field field theory theory (Burns, 1993) local environment of the activator ion - The activator-ligand distances in the different excited states and the slope of the energy levels depend on the intensity of the crystal field (expressed as crystal field splitting = 10Dq) - The stronger the interaction of the activator ion with the lattice, the greater are the Stokes shift and the width of the emission line. Basics of luminescence TheThe crystal crystal field field theory theory local 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 InfluenceInfluence of of thethe crystal crystal field field on on CLCL emissionemission spectra spectra (1) influence of the crystal field = weak Dy3+ Dy3+Sm3+Sm3+Dy3+ 16000 3+ Dy zircon zircon ] s 12000 t n scheelite u o c [ 8000 Dy3+ y anhydrite t i 3+ s Sm n 3+ e Dy t 3+ calcite n 4000 Dy 3+ i Tm 3+ . Sm l e r fluorite 0 Tb3+ 300 400 500 600 700 800 apatite wavelength [nm] 400 500 600 700 [nm] CL spectra of narrow emission CL emission spectra are specific lines (e.g. REE3+) of the activator ion Basics of luminescence InfluenceInfluence of of thethe crystal crystal field field on on CLCL emissionemission spectra spectra (2) influence of the crystal field = strong 400 2+ Mn 2+ ] calcite Mn activated CL of CaCO3: s t n 300 u o c [ y t aragonite green (~560 nm) i s 200 n e t n calcite yellow-orange (~610 nm) i . l e r 100 magnesite red (~655 nm) 300 400 500 600 700 800 wavelength [nm] CL spectra of broad emission CL emission spectra are specific bands (e.g. Mn2+, Fe3+) of the host crystal Basics of luminescence 500 µm sea lily stalk (calcite CaCO3) Visual and spectral detection of Mn2+ activated CL in natural carbonates perl (aragonite CaCO3) Basics of luminescence InfluenceInfluence of of thethe crystal crystal field field onon thethe broad broad CL CL emissionemission bands bands in in solidsolid solutionssolutions 800 plagioclase red IR 750 740 600 730 720 400 710 2+ 3+ Mn Fe 700 200 wavelength [nm] 690 rel. intensity [counts] 680 0 20 40 60 80 100 0 300 400 500 600 700 800 900 An content [mol-%] wavelength [nm] Position of the Fe3+ activated CL emission band in plagioclases in relation to the anorthite content InstrumentationInstrumentation Scanning Electron Microscope JEOL 6400 with OXFORD Mono-CL detector primary electron beam OXFORD Mono CL mirror sample silica-glass fibre guide Cathodoluminescence detector on a scanning electron microscope Hot-cathode luminescence microscope HC1-LM (designed by Rolf Neuser, Bochum) InstrumentationInstrumentation hot-cathode microscope cold-cathode microscope microscope microscope objective leaded glass thin section viewing point specimen corona condenser points optic axis cathode discharge tube focus light source coil h curved electrode hig ge lta vo ble ca CathodoluminescenceCathodoluminescence techniques techniques SEM-CL CL microscopy polished sample surface polished thin (thick) section focused electron beam, defocused electron beam, scanning mode stationary mode heated filament heated filament („hot cathode“) 20 kV, 0.5-15 nA 14 kV, 0.1-0.5 mA ionized gas („cold cathode“) mirror optics: 200-800 nm (UV - IR) glass optics: 380-1200 nm (Vis - IR) analytical spot ca. 1 µm analytical spot ca. 30 µm panchromatic CL images (grey levels) true luminescence colours resolution << 1 µm resolution 1-2 µm SE, BSE, EDX/WDX, cooling stage polarizing microscopy, (EDX) CathodoluminescenceCathodoluminescence imaging imaging SE Polmi zircon BSE quartz CL CL 10 µm 500 µm SEM cathodoluminescence Cathodoluminescence