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 < 30 mu (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) 1MIIb 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:

  Phase	STARKE (1969)	RIETVELD reflection	RIETVELD transmission
  	wt%	wt%	wt%
  quartz	30	31	30
  muscovite	39	44	42
  chlorite	19	16	19
  albite	10	6	8
  microcline	-	3	1
  rutile	-	< 1	< 1
  accessories	2	-	-

Table 1: Quantitative analysis results for metashale Böhlscheiben

Fig. 1: Measurement and difference curve for metashale Böhlscheiben, reflection geometry
Co-K-alpha radiation, Fe filter, URD 6, 10-100 degree 2 theta, step width 0.02 degree, 8s per step

Fig. 2: Measurement and difference curve for metashale Böhlscheiben,
transmission geometry, Cu-K-alpha radiation, graphite monochromator, XRD 3000 TT, 6-78 degree 2 theta, step width 0.03 degree, 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 I_00l 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

  • Bergmann, J., Kleeberg, R., Taut, T., 1994:
    A new structure refinement and quantitative phase analysis method basing on predetermined true peak profiles.
    Z. f. Kristallographie, Supplement issue No. 8, Europ. Cryst. Meeting 15, Book of Abstracts p. 580.

  • Bergmann, J., Kleeberg, R.,Taut, T., Haase, A., 1997:
    Quantitative Phase Analysis Using a New Rietveld Algorithm - Assisted by Improved Stability and Convergence Behavior.
    Adv. X-Ray Analysis 40 (1997).

  • Hill, R.J.; Tsambourakis, G. and Madsen, I.C., 1993:
    Improved petrological modal analysis from X-ray powder diffraction data by use of the Rietveld method
    I. Selected igneous, volcanic and metamorphic rocks.
    Journal of Petrology, 34, Part 5, p. 867-900.

  • Starke, R., 1969:
    Bestimmung der mineralischen Zusammensetzung des Tonschiefers TB durch quantitative Phasenanalyse.
    Berichte deutsch. Ges. geol. Wiss. B. Mineral. Lagerstättenforsch. 14 p. 73-77.