The singular conductive properties of natural covellite were investigated through magnetic AC susceptibility measurements. Two natural hexagonal covellite lamellae coming from Calabona (Italy) and Butte (USA), respectively, were investigated from 30 to 1.5 K. At 1.63(5) K, an abrupt increase of the diamagnetism (Meissner effect) was observed, thus evidencing a transition from a poor conducting to a superconducting state. The transition is independent on small applied DC fields (< 50 Oe), whereas larger values prevent the superconducting state to be attained. These features confirm covellite as a Class I superconductor.
This peculiar property of covellite, never observed before on natural materials, can be mainly related to its structural features. The layered framework of covellite, where CuS3 planes are isolated by covalently bonded S2 planes, provides an electron excess, which can be easily involved in the bulk conduction.