sesquisulfides

Sesquisulfides are a class of minerals that contain the sulfide ion (S2-) in a 3:2 ratio with a metal. The term "sesquisulfide" is derived from the Latin "sesqui," meaning one and a half, and "sulfide," referring to the sulfide ion. These minerals are characterized by their unique chemical composition and crystal structures. Sesquisulfides are relatively rare and are often found in association with other sulfide minerals in hydrothermal veins and metamorphic rocks. They are typically dark in color and have a metallic luster. The most well-known sesquisulfide is nickel sulfide (NiS), which is a component of the mineral millerite. Other examples include cobalt sesquisulfide (CoS) and iron sesquisulfide (FeS). Sesquisulfides are of interest to geologists and mineralogists due to their distinctive properties and the economic value of some of the metals they contain.