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BGMN Application: Copper Shale (Kupferschiefer) - an example for automatic multiphase analysis of sample series -

Project:

Investigation of the influence on water quality by mining dumps

Supported by:

Federal Ministry for Research and Technology

Authors:

Dr. R. Kleeberg, Dipl.-Min. J. Mibus

Aim of this work:

was to characterize weathering processes in old mining dumps of copper mines near Mansfeld/Germany.

The deposited material is mainly carbonatic black shale (containing quartz, illite-muscovite, calcite, dolomite, plagioclase and ore minerals like sphalerite, galena and pyrite) and some sulfatic-carbonatic wall rock (Zechstein).

Drilling profiles in old dumps where investigated to look for changes in mineralogy and geochemistry by dissolution/precipitation processes. One question was the buffering capacity of the carbonatic material. We had to look for changes in carbonate content and the neoformation of gypsum and other secondary minerals dependent on the depth.

Analytical problems:

• about ten detectable phases, some of them in concentrations near the detection limit

• high number of samples, no extensive sample preparation and time consuming measurement possible

• gypsum tends to multiple cleavage/preferred orientation

• sphalerite, often near the detection limit, tends to {110} cleavage

• muscovite-illite has mainly 2M1 polytype, but disordering features result in anisotropic line broadening

• problems in grain statistics because the sample changer was not able to rotate

Analysis:

Sample preparation:

hand ground in an agate mortar, sieved < 30 um, filled without pressure in an Al sample holder (thick sample); minimization of preferred orientation by sandpaper.

Measurement:

URD-65 (SEIFERT-FPM GmbH), Co long fine focus tube; secondary monochromator; angular range 7°...79°, 0.03° step size, 5 sec per step

Starting model for calculation, identical for all samples:

10 phases (quartz, albite, calcite, dolomite, muscovite 2M1, gypsum, sphalerite, pyrite, galena, cerussite)

Refined parameters:

total number 102-148, dependent of automatic reducing of the order of preferred orientation correction in the case of low phase content

lattice parameters, crystallite size line broadening for all minerals

different orders of spherical harmonics preferred orientation correction, up to 6th order for sphalerite

anisotropic line broadening for muscovite

K site occupation for muscovite (tendency for substitution by hydronium ion)

strain broadening description for the carbonates (partially iron-zinc-magnesium-substitution?)

zero point (limited) and sample displacement

automatic selected background polynom At first, a batch of about 40 measurements was prepared by simple renaming of one input file. After the first run, only the sphalerite reflections showed poor fitting in some samples. A new setting of limits for reducing the order of preferred orientation model resulted in better fitting in a second run. Remaining rest peaks in difference plot can be caused mostly by grain statistics.

The calculation time needed was about 15-25 minutes per sample. The full batch was done over night, the Rietveld operator slept at home. In some samples, up to 3 phases (mostly cerussite, galena and pyrite) were not detectable. In this cases, the program calculated small concentrations (below 1 wt%) and errors in the same magnitude. If all parameters of such a 'dummy' phase were shifted to the defined limits, the error was insignificant and a concentration of 0 was calculated.

Results:

The quantitative results were useful and agree with chemical parameters. Dissolution of calcite and gypsum in the upper and precipitation of gypsum in lower part of the profile were verified directly. The calculated K-site occupation in muscovite-illite was about 0.65 (averaged). This is a suitable value for such a sediment. Lattice parameters of calcite indicate a small substitution of Ca by Fe and/or Zn.

Examples:

Results:

Remark:

Fig. 1: Rietveld refinement plot of sample mib32
2401 measuring values, 667 peaks, 144 parameters
Start: Tue Nov 25 03:19:23 1997; End: Tue Nov 25 03:39:57 1997
R_p=9.20% R_pb=16.63% R=9.40% R_wp=11.65% R_exp=7.71%

Fig. 2: Rietveld refinement plot of sample mib15
2401 measuring values, 667 peaks, 148 parameters
Start: Mon Nov 24 21:24:58 1997; End: Mon Nov 24 21:44:24 1997
R_p=8.60% R_pb=16.17% R=8.70% R_wp=11.24% R_exp=7.59%