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. Qtz@8.62) 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

c.a.hecker@utwente.nl