The presence of uranium minerals in the Atrevida mine has been known for years. The first notice was published in Mineralogistes de Catalunya (Viñals et al., 1990; Bareche, 1990). A posteriori, the presence of uranium in the Silurian and Carboniferous deposits in the Prades mountains area was described by Carles Canet (Canet, 2001).
The article of Bareche states that uranium minerals were collected in an open pit with a huge size baryte vein. It is also reported that Geiger counter research throughout the demarcation showed no signs of other uranium mineralizations. However, in the Tacha mine there are certain materials (with baryte and nickel minerals) that show a certain radioactive emission.
Crystals identified as kahlerite-zeunerite. Col. E. Bareche (image available in the Joan Astor photos
Museu Mollfulleda de Mineralogia d’Arenys de Mar and online: Issuu, Astor, 2014).
Figure 2.- Zeunerite crystals. Col. E. Bareche. Photo: Joan Astor (image available in the Joan Astor photos
Museu Mollfulleda de Mineralogia d’Arenys de Mar and online: issuu, Astor, 2014).
In the image that illustrates the article (figure 1) it can be seen a group of apple-green tabular crystals, pseudohexagonal, with small brown globular aggregates, identified as kahlerite-zeunerite. To reach this conclusion an EDS spectrum is presented where can be seen some elements: arsenic, uranium, copper, and iron.
Figure 3.- EDS spectrum of the specimen in Figure 1 (Bareche, 1990).
Kahlerite is a hydrated uranyl and iron arsenate of the chemical formula Fe2+(UO2)2(AsO4)2·12H2O, tetragonal crystalline system, belonging to the Autunite Group. It was discovered in 1953 and is named in honour of Franz Kahler (1900-1995), a geologist at the Carinthian Landesmuseum in Klagenfurt, Austria.
Zeunerite has similar composition as kahlerite, but with copper in the place of iron: Cu(UO2)2(AsO4)2·12H2O. It is also tetragonal and belongs to the same Autunite Group. Named in honour of Gustav A. Zeuner (1828-1907), physicist and engineer, director of the Freiberg School of Mines in Saxony, Germany. This mineral, like others in the same group, is easily dehydrated by losing four molecules of water, to become metazeunerite: Cu(UO2)2(AsO4)2·8H2O.
But can be assigned kahlerite to the paragenesis of the Atrevida mine? Many mineralogists, in collections, have specimens identified as “kahlerite” from this mine. That’s why it has been decided to study one of these specimens. It shows green tabular crystals in a baryte cavity (Figures 4 and 5).
Figure 4.- Green tabular Crystal. FOV 3 mm. Col. & photo J. Rosell. Plan APO MOTIC with stacking.
Figure 5.- The same crystal. FOV 1.5 mm. Col. i foto J. Rosell. Col. & photo J. Rosell. Plan APO MOTIC
with stacking and long distance focus.
By SEM-EDS spectroscopy it has been obtained a composition in which iron is not detected (Figures 6, 7 and 8). The composition is consistent with zeunerite-metazeunerite. But it also has been noticed that Ba and S appear in the results.
Figure 6.- SEM image of the metazeunerite crystal (RM3412). FOV 0.4 mm. Photo: J. Rosell.
Figure 7.- EDS results.
Figura 8.- EDS spectrum of sample RM3412.
The presence of Ba and S is detected since the sample has not been coated with carbon, not polished, in which the zone of impact of the electron beam is more diffuse, and the baryte (BaSO4) of the surroundings is also detected. When the electron beam “impact” on the mineral surface, and X-rays are emitted from the excited atoms, some of this X-rays excite other elements of the adjacent minerals, which emit new X-rays, in this case Ba and S (figure 9).
Figure 9.- Emission of X-ray by adjacent minerals (BSE: backscattered electrons).
Looking at the sample analysed in the 1990 article, it can be observed that there are several globular aggregates on the kahlerite-zeunerite crystals, most likely scorodite, arseniosiderite, or even goethite, all of them iron containing. Could these aggregates have masked the results of the EDS analysis? It is not known. The appearance of the crystals, it must be said, is quite different from what it has been analysed (RM3412). It would only be necessary to re-analyse the old sample with the devices currently available, which are much more sensitive and accurate. Should also be encouraged anyone who has specimens labelled as “kahlerite” from Atrevida mine to re-analyse them. This will allow to confirm (or not) a very unusual species as kahlerite.
In Catalonia, kahlerite has been identified as a rarity in the Eureka mine (Castell-estaó, La Torre de Cabdella, Lleida), as submillimetric tabular crystals (<40 μm) in radial groups that pseudomorph gersdorffite with uraninite and digenite (Castillo-Oliver et al., 2020).
References
Viñals, J., Bareche, E., Coca, J. (1990): “Mines de Catalunya: Vimbodí (II)”. Mineralogistes de Catalunya, vol. 4, n. 8, 213-227.
Bareche, E. (1990): “L’urani a la mina Atrevida”. Mineralogistes de Catalunya, vol. 4, n. 8, 228-230.
Canet, C. (2001): Dipòsits sedimentàrio-exhalatius del Paleozoic del SW dels Catalànides: model de dipòsit. Tesi Doctoral. Director: Melgarejo i Draper, Joan-Carles. Universitat de Barcelona. Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals. Barcelona. 414 pp. http://hdl.handle.net/10803/667072 [view Dec., 2021].
Castillo-Oliver, M., Melgarejo, J.C., Torró, L., Villanova-de-Benavent, C., Campeny, M., Díaz-Acha, Y., Amores-Casals, S., Xu, J., Proenza, J., Tauler, E. (2020): “Sandstone-Hosted Uranium Deposits as a Possible Source for Critical Elements: The Eureka Mine Case, Castell-Estaó, Catalonia”. Minerals, vol. 10, p. 34.
Astor, J.: El món dels minerals. Classe VII, fosfats, arsenats i vanadats (3a part). Disponible en línia. https://issuu.com/joanastor/docs/fosfats_3a_part [publ. 2014, view Dec., 2021].
