Titel: µ-EDXRF based classification of chromites. A quick approach for testing hand specimen and drill cores
Dieter Rammlmair, Wilhelm Nikonow
Veranstaltung: GeoKarlsruhe 2021
Chromites from ophiolites and layered intrusions show a wide variety in Cr#, Mg# and Cr/Fe ratios. These ratios provide information on the genesis of the chromite deposits e.g. the place of formation.
By means of µ-EDXRF this information can be obtained in a very quick way from cut hand specimen.
Unfortunately, chromites show a number of very intensive diffraction signals due to the polychromatic beam of the µ-EDXRF system, which even after calculating the minimum of two detectors has still some influence on the chemistry of a single chromite grain.
To reduce this problem, chromite aggregates were segmented and for each individual grain, the shapes were used to calculate the mean spectra for both of the detectors (D1/D2), the minimum (min) of both, the mean, the detector minus minimum of both (D1/2-min) to localize the position of the diffraction signals in the spectrum.
All individual grain D1 and D2 spectra are plotted against each other to detect those grains showing a minimum of diffraction, to reduce the diffraction impact on the chemical signal.
Assuming that the chemical pattern of chromite grains within a texturally homogeneous sample should not be to extreme regarding Cr# and Mg# those grains of minimum D1/D2 deviation were selected to represent the chromite chemistry. Validation is done by microprobe analysis.
In a second step these “pure” pattern of selected samples referring to individual tectonic levels of an ophiolite are used as endmembers for hyperspectral classification of chromite of various samples into individual groups/tectonic levels.
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