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Cleuson, Val de Nendaz, Wallis, Switzerland

The Cleuson locality is a small radioactive anomaly with several ore veins in finely layered host rock. One of the veins is the type locality for Cleusonite, see the literature list at the bottom. The dump, created by building a small road, was screened with a scintillation counter. Samples showing an activity above base level were cut and polished. The main ore mineral found was arsenopyrite (Asp). The entire sample shows strong flow patterns, even visible in the polished slabs. The sample contains are larger amount of rutile, either as clusters of small crystals, as rounded and partly fractured or dissolved blebs or as entirely scattered fragments within the flow patterns. Euhedral zircon is also present. An other sample from the banded wall rock contained small grains of chalcopyrite with a gray rim showing brown internal reflections, which most likely is goethite. The matrix consists of quartz, muscovite and albite as determined by Raman spectroscopy.

Polished slab with a quarz-arsenopyrite vein. Width at the base: 7 cm


Enlarged part from the upper picture. The arsenopyrite crystals can easily be seen:


The polished section with Nic //, Obj. 10x, air. Arsenopyrite in quarz, with a twin crystal (100 µm bar):


Same with Nic +:


Clusters of rutile needles, Nic //, Obj. 50x, air.


Same with Nic +, Obj. 50x, air


Rutile in state of fracturing and dissolution. Nic //, Obj. 20x, air, 100 µm scale bar


Rutile in state of fracturing and dissolution. Flow structures are dominating in the sample. Nic //, Obj. 20x, air, 100 µm scale bar

Rutile_2_100um 1

Euhedral zircon in quartz matrix (with internal reflections). Nic //, Obj. 50x, air, 50 µm scale bar


Zircon (right) and rutile (left). Nic //, Obj. 50x, air, 50 µm scale bar


View of the second sample with finely distributed chalcopyrite (width: 3.5 cm):


Chalcopyrite from the second sample, with a rim most likely goethite, Nic //, Obj. 50x, air


Same with Nic +, Obj. 50x, air


Micro raman spectroscopy

Several cavities with the size of about 100-500 µm were found in the sample and analysed. The spots marked in red all contained native sulfur, probably as a weathering product of arsenopyrite. Nic //, Obj. 20x, air, 100 µm scale bar


Rruff reference for sulfur at 532 nm (blue), results from this study (black). The first peak at about 85 cm-1 is confirmed as a sulfur peak by the McCreery group standard raman spectra page (http://www.chem.ualberta.ca/~mccreery/ramanmaterials.html).

Cleuson Rruff

For further informations on the region this article might be interesting:

Wülser, P.-A., Meisser, N., Brugger, J., Schenk, K., Ansermet, S., Bonin, M., Bussy, F.: Cleusonite, (Pb,Sr)(U4+,U6+)(Fe2+,Zn)2(Ti,Fe2+,Fe3+)18(O,OH)38, a new mineral species of the crichtonite group from the western Swiss Alps. European Journal of Mineralogy.17.(2005).933-942.