Which mineral is slowly killing the citizens of Argo City in Supergirl?

The mineral slowly killing the citizens of Argo City in Supergirl is Kryptonite.

In the film’s Krypton flashbacks, Argo City survives the planet’s destruction but its soil becomes poisoned by Kryptonite. The residents are gradually exposed to its radiation, creating a slow-moving catastrophe rather than an instantaneous disaster. Zor-El ultimately seeks a way to send Kara away before the city’s people perish.

Kryptonite is one of Superman mythology’s defining elements: a radioactive substance from Krypton that weakens or can kill Kryptonians. Green Kryptonite is the best-known variety, while red, gold, and other forms have different effects. The film uses the material in an unusual way, portraying it as an environmental threat to an entire Kryptonian community.

A common mix-up is confusing Kryptonite with the green radiation of Barenton’s sun. The sun’s radiation makes Kara powerless during that sequence, but Kryptonite is the mineral poisoning Argo City.