Most Swedish mines extract ore from hard crystalline rock, whereas alum shale is softer and contains numerous metals and minerals dispersed throughout. It also contains sulphides and weakly radioactive elements.
Sources: Platåbergens Geopark, “Alunskiffern”, https://www.platabergensgeopark.se/wp-content/uploads/2022/10/Faktablad-%E2%80%93-Alunskiffern-Plata%CC%8Abergens-Geopark2.pdf