In the back yard of the federal police headquarters in Roraima, the northernmost state of Brazil, giant sacks lie strewn and overflowing with a jet-black, gravel-like mineral: cassiterite.
Although less high-profile than other items seized during a crackdown on illegal mining in this Amazon state - including a Sikorsky S-76 helicopter painted in the colours of the Brazilian flag - cassiterite has become so sought-after that it is nicknamed "black gold".
Cassiterite is the chief ore of tin, a less heralded but critical mineral for the energy transition. It is used in coatings for solar panels, lithium-ion batteries and solder for electronics.
According to the International Energy Agency, the demand for critical energy minerals is due to almost triple by 2030.
In a world market heated by demand from multinational companies and with prices for tin on the rise-up 29% in the first six months of this year - Brazil has become one of the world's largest exporters of the metal.
However, as well as increasing profits and commodity exports, the rush for cassiterite has become a new environmental and policing problem.
Considered a conflict mineral in the European Union and the US, cassiterite has increasingly attracted illegal mining gangs in the Brazilian Amazon.
Criminals have also profited from the illegal extraction of manganese and copper, which are also vital to the energy transition. Prices for these minerals have also rocketed this year.
The search for critical metals gained momentum after Brazil's government launched initiatives to encourage mining investment, given the growing interest of international mining firms in the country's mineral wealth.
The development bank BNDES and the Brazilian mining multinational Vale are planning to launch an investment fund to support domestic projects to produce critical minerals with the government publishing an investors' manual.
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