Metashale Böhlscheiben

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Variable Divergence Trimer Multithreading combined refinement

BGMN Application:

Reference Sample Metashale Böhlscheiben

Author:
Dr. R. Kleeberg

Aim of this work:
This standard rock sample examined well was selected in order to test the convergence behavior of the BGMN program on a real sample with several low symmetrical phases and to compare different measuring geometry.

Measurements:
The measurements were carried out at a sample of particle fraction <30µm (step by step ground and sieved) both in Bragg-Brentano geometry (reflection, URD 6) and in Debye-Scherrer geometry (transmission through flat sample, XRD 3000 TT). For the case of transmission measurement, the measurement parameters (goniometer radius, slits, sample thickness, step width, counting time) were chosen so that a resolution sufficient for qualitative phase analysis was reached. Pulse statistics and peak profiles useful for normal peak search programs were still achieved in spite of smaller intensities compared to reflection geometry (see fig. 2). Consequently, the measuring time was in contrast to HILL et al. (1993) in the order of magnitude of the "normal" measurements in reflection geometry.

Calculation:
Refinement was carried out with muscovite as a 2M1-polytype as above, chlorite (ripidolite) disorder model and albite with isotropic width model and complex texture correction as well as with quartz. The difference curve of first refinement showed weak remaining peaks at 0.324 nm. That corresponds to a small potassium feldspar content presumed already in former times already. Therefore, microcline (without texture modeling and with limited peak broadening) was included in the model during the second refinement.

Results: (recalculated using BGMNwin on December 2005)
Table 1: Quantitative analysis results for metashale Böhlscheiben
Phase STARKE (1969) RIETVELD reflection RIETVELD transmission
wt% wt% wt%
quartz 30 31 32
muscovite 39 41 40
chlorite 19 18 17
albite 10 7 8
microcline - 1 1,5
rutile - <1 <1
accessories 2 - -

metash1.png
Fig. 1: Measurement and difference curve for metashale Böhlscheiben, reflection geometry
Co Kα radiation, Fe filter, URD 6, 5°-80° 2Θ, step width 0.03°, 5s per step

metash2.png
Fig. 2: Measurement and difference curve for metashale Böhlscheiben, transmission geometry, CuKα radiation, graphite monochromator, XRD 3000 TT, 6°-80° 2Θ, step width 0.04°, 10s per step. The results in table 2 show good agreement for both types of measuring geometry. In this case, the analyses seem to predict systematically more quartz and muscovite as well as less chlorite compared to the recommended values of STARKE (1969). For the interpretation of these deviations, reliable estimates of the model errors are still missing.

In the difference diagrams, the better modeling of the I00l of muscovite in transmission geometry is noticeable. Polarization effects on the mica crystallites textured strongly would be able to be cause for poor modeling the 00l reflection intensities.

References