Kermesite

Kermesite or antimony oxysulfide is also known as red antimony (Sb₂S₂O) . The mineral’s color ranges from cherry red to a dark red to a black. Kermesite is the result of partial oxidation between stibnite (Sb₂S₃) and other antimony oxides such as valentinite (Sb₂O₃) or stibiconite (Sb₃O₆(OH)). Under certain conditions with oxygenated fluids the transformation of all sulfur to oxygen would occur but kermesite occurs when that transformation is halted.