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Large-Scale Chemical Mapping of Geological Samples

The study and visualization of the distribution of elements between different minerals in a rock, and sometimes within individual minerals (zoning), is of great interest to geoscientists as it can provide information about the mineralogy, paragenesis, origin and evolution of rock samples. Geoscientists often need overview maps of their samples at a relatively large scale of a few millimeters to centimeters.

Here we show two examples for element distribution maps recorded by WDS on SEM combined with the Rapid Stage, which is an additionally mounted Piezo-driven stage that allows fast mapping by continuous movement and seamless data acquisition.

The first example is a sulfide bearing ore from the metamorphic Rajapalot complex in central Finland (Sayab et al. 2024). The sample contains the principal ore phases linnaeite and cobaltite, with other mineralization including pyrrhotite, pyrite and chalcopyrite in a siliceous host rock with quartz, albite, orthoclase, chlorite, tremolite, biotite, titanite and other trace accessories (Fig. 1a, Fig. 1b).

Fig. 1a: BSE micrograph of a sulfide bearing ore from central Finland. Bright phases are sulfides, darker phases are silicates. FOV is 2.9 x 2.2 mm, pixel resolution is 2.41 µm.
Fig. 1b: Element distribution maps for a sulfide bearing ore recorded by WDS on SEM. The FOV spans 2.9 x 2.2 mm (1200 x 900 pixel) and mapping for each element was done in 1 h at 20 kV and 20 nA using the TAP crystal for Na-Ka, Mg-Ka and Al-Ka, the PET crystal for Si-Ka and S-Ka and the BRML60 multilayer for Fe-La and Co-La. Note the clear separation of the Co and Fe distribution that is based on the high spectral resolution of WDS.

The second example presented is a garnet peridotite from more than 80 km in the earth’s mantle below the African craton, brought up to the surface by the Newlands diamond-bearing kimberlite (South Africa). The thin section contains the minerals garnet, chromite, clinopyroxene and olivine (Fig. 2a, Fig. 2b).

Fig. 2a: BSE micrograph of a peridotite rock from South Africa. Bright phases are oxides, darker phases are silicates. FOV is 8.7 x 6.5 mm, pixel resolution is 7.24 µm.
Fig. 2b: Element distribution maps for a garnet spinel peridotite recorded by WDS on SEM. The FOV spans 8.7 x 6.5 mm (1200 x 900 pixel) and mapping for each element was done in 4 h at 20 kV using the TAP crystal for Mg-Ka, Al-Ka and Si-Ka and the BRML60 multilayer for Cr-La.

WDS can be used to map the distribution of all elements present in a sample. However, the energy-selective nature of this spectroscopic technique must be considered, which leads to a sequential mapping process. The real advantage of WDS mapping lies in the resolution of peaks that overlap in EDS (e.g. Fe - Co; Fig. 1b), as well as in the improved detection of trace elements and light elements.

At low magnification, scanning the beam over the entire field of view is not suitable for WDS, as the detected X-ray signal would drop sharply with increasing angle. Therefore, the SEM stage is usually moved for mapping larger areas with WDS, but this can be very time-consuming. The most efficient way for large area mapping with WDS is clearly to use the Rapid Stage, which allows fast sample movement and on-the-fly data acquisition.

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