Most sulphide minerals on or near the Earth’s surface are not very stable. In an oxygen-rich environment, they often transform into various sulphates, oxides, hydroxides, and, less commonly, sulphoxides. Stibnite (Sb2S3), the sulphide of antimony, is also one of these minerals; its steel-grey, columnar, acicular crystals are often oxidized to yellow or white antimony ochre (stibiconite, valentinite, sénarmontite, etc.). An intermediate product of this transformation is the reddish-coloured kermesite (antimony sulphoxide, formula Sb2S2O), the most beautiful specimens of which are found in the vicinity of Pernek and Pezinok.
In the picture: Kermesite, Pernek (Slovakia). FOV: 7 cm. Photo by Sándor Szakáll.