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Alum shale

Wall with alum shale in Andrarum, Skåne.

Photo: Mikael Erlström, SGU.

Alum shale

Alum shale contains several of the rare and innovation-critical metals used for the development of the green technology needed to respond to the climate transition. At the same time, alum shale contains organic matter, uranium and sulphide minerals that require special handling in order to avoid a potential environmental risk.

Alum shales are rocks where most of the constituent minerals originate from the sediments that were once deposited in the sea that covered large parts of Sweden, some 500 million years ago. These minerals consist primarily of clay minerals, fine-grained quartz and feldspar, as well as a small amount of heavy minerals. These minerals are mixed with organic matter, carbonate and phosphate from dead algae and organisms. The slow sedimentation and large amount of organic matter on the oxygen-free sea floor have also resulted in the incorporation of several innovation-critical metals in the alum shale.

Alum shale is underlying younger bedrock or forms the bedrock surface in parts of Skåne, Västergötland, Östergötland and Närke. The formation is also found on Öland, and as scattered occurrences under thick limestone layers on Gotland. In addition, the formation is found as a surface rock along the mountain edge, from Idre in Dalarna via the Storsjö area in Jämtland, the Tåsjö area in Ångermanland – Västerbotten to Torneträsk and Treriksröset. Alum shale formations also occur as thrusted and deformed layers within the Caledonian bedrock in Jämtland, Ångermanland and Västerbotten.

Metals contents of alum slate

Alum shale contains at various concentration a number of the metals and minerals currently included in the EU list of Critical Raw Materials. Most of the rare earth elements are found in alum shale. The concentration of these metals range from a few grams per tonne, to just under a hundred grams per tonne. Cobalt (Co), gallium (Ga), hafnium (Hf), niobium (Nb), bismuth (Bi), indium (In), antimony (Sb), lithium (Li) and tungsten (W) can also be found in alum shale, at concentrations from a few tens of grams per tonne down to less than one gram per tonne . Phosphates/phosphorus minerals, vanadium (V), barium (Ba), titanium (Ti ), strontium (Sr) and magnesium (Mg) are found in concentrations from a few hundred, to a few thousand grams per tonne . There is no reliable information on the content in alum shale of the platinum group's metals or other materials listed in the EU Critical Raw Materials list.

Mineable concentration for each metal or mineral in the alum shale depend on their abundance, available technology for extraction, demand, and the situation on the world market. Over time, additional metals and minerals may also be added to the EU list, which are not currently considered critical but where innovations and accessibility can change the status of their importance. In the future, this may change the relevance of alum shale as a natural resource.

Last reviewed 2022-06-17