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INFLUENCE OF SAMPLE SURFACE PREPARATION ON TIR SPECTROSCOPY RESULTS
CHRIS HECKER, EVELIEN ROST, FREEK VAN DER MEERMARTIN SCHODLOK, BGR
LABORATORY TIR SPECTROSCOPY
Directional – hemispherical reflectance measurementsBruker Vertex 70 and integrating sphere
THE ISSUE
TIR spectra influenced by e.g.: grain size, packing density, solid solution, crystallographic
orientation, surface roughness, exsolution …
Curse and a blessing Changes results but also contains additional information
Some issues intuitively known but not well characterized e.g. roughness, orientation (work in progress)
WHY NOW?
Mineral spectroscopy from point measurement to image
Issues always there but now more visible
PHD TOPIC EVELIEN
Determine (and correct) effects of: surface preparation Split Cut Grind Polish
Orientation of crystal axes Possibly pyroxenes, plag feldspars?..
PHD TOPIC EVELIEN
Determine (and correct) effects of: surface preparation Split Cut Grind Polish
Orientation of crystal axes Possibly pyroxenes, plag feldspars?..
On “simple” rock samples
ROCK SAMPLES - CONDITIONS
Simple modal mineralogy Spatially homogeneous (@3 cm sample spot) Uniform mineral size (non-porphyritic) LWIR active mineralogy 20x20x20 cm minimal => enough for several tests
ROCK SAMPLESGildehaus SStFontainebleau SSt Shanxi gabbro
Fine grained• Quartz• 6% porosity
Medium grained• Quartz• Kaolinite• ~20% porosity
Fine grained• Feldspar, pyroxene• intergranular
• Quartz, cummingtonite, hornblende
• No porosity except microcracks.
METHODS – ROUGHNESS
Surface roughness differences:
Split, Saw, Polish (grit of 4000)
washed, compressed air, oven dried @50°C for ~8 h.
Cooling in desiccator for ~12h
Used same surface of block, <20cm distance => avoid even unlikely anisotropy / inhomogen.
METHODS – SPECTRAL MEASUREMENTS
Bruker Vertex70 FTIR with DHR integrating sphere Measured range: 5000 – 500 cm-1
Spectral resolution: 4 cm-1
3 scans/sample surface
PRELIMINARY RESULTS - GABBRO
General shape is same Amplitude of signal
changes: Splitting: darkest Sawing: brighter Polished: brightest
PRELIMINARY RESULTS - GABBRO
Normalized from 0.0 to 1.0 Shape not exactly the
same Some of the shoulders
(e.g. [email protected]) are lower/higher, resp.
Relative peak heights slightly change
PRELIMINARY RESULTS – GILDEHAUS SST
Amplitude of signal changes: Splitting: darkest Sawing: brighter Polished: brightest
Shape of 8.2micron quartz lobe changes
PRELIMINARY RESULTS – GILDEHAUS SST
Normalized from 0.0 to 1.0 Shape of 8.2 feature
clearly different But not on second lobe
(asymmetric)
PRELIMINARY RESULTS – FONTAINEBLEAU SST
Inverse to Gildehaus: Splitting now brighter
than sawing Lobe shape of splitting
now sloping down to left (Polished not measured
yet)
INTERPRETATIONS
Explanations through traditional dispersion theory? (refractive index and ext coeff for ord and extord ray)
Differences in two sandstones: Porosity Cementation (Kaol vs ?amorphous quartz phase)
Instrument effects? Preferred reflection (polarization) on polished surfaces? Different results for polarized/specular light even with integr. sphere?
NEXT STEPS
Characterize the sample composition better (XRD, ?micro-XRF)
Characterize the sample surface better Morphology scanner Electron microscopy
See which parts can be explained by dispersion theory
INFLUENCE OF SAMPLE SURFACE PREPARATION ON TIR SPECTROSCOPY RESULTS
CHRIS HECKER, EVELIEN ROST, FREEK VAN DER MEERMARTIN SCHODLOK, BGR