Stibnite, sometimes called antimonite,'s a sulfide mineral with the formula Sb2S3. This soft grey material crystallizes in an orthorhombic space group. It's the most important source for the metalloid antimony. The abbreviation for antimony, Sb,'s taken from stibnite. Formation, structure, reactivity Sb2S3 forms when antimony(III) compounds are treated with hydrogen sulfide. This reaction gives a black precipitate: » 2 Sb3+ + 3 H2S → Sb2S3 + 6 H+ This reaction's reversed by hydrochloric a… (More on Stibnite)