Mineral X is assumed to be a new member of the thalcusite homologous series with the structural formula TlCu2NS2N+1 with N=1.5. It was found in loose ussingite-analcime boulders on the Taseq slope towards the Narsaq Elv in the northern part of the Ilimaussaq complex in South Greenland in association with chalcothallite, cuprostibite, galena, sphalerite, bornite, antimonian silver and seina-jokite. Supergene alteration has resulted in a number of secondary Cu-Sb-minerals. The primary ore minerals appear to have crystallized contemporaneously under low S-fugacities.