Esterite

Esterite is a copper zinc tin sulfide mineral with the chemical formula Cu2ZnSnS4. It is a member of the stannite group of quaternary sulfide minerals and is related to other Cu–Sn–S and Cu–Zn–Ge sulfides such as stannite (Cu2FeSnS4) and kesterite (Cu2ZnGeS4).

Crystallography and appearance: Esterite crystallizes in the tetragonal crystal system and typically occurs as fine-grained masses, though rare, well-formed crystals have been reported. It has a metallic luster and a gray to black color, with a dull to submetallic appearance in massive forms. It is opaque and has a relatively high density for a mineral in its class.

Occurrence and associations: Esterite forms in hydrothermal ore deposits and in skarn and other metamorphic environments. It is usually found in association with other sulfide minerals such as sphalerite, chalcopyrite, galena, and pyrite. Its presence can indicate a Cu–Zn–Sn sulfide assemblage in a given deposit.

Significance and uses: Esterite itself is not a major ore mineral for copper, zinc, or tin. However, its Cu2ZnSnS4 composition places it in a broader group of earth-abundant quaternary sulfide minerals that have attracted attention for research into thin-film photovoltaic absorber materials, similar to synthetic CZTS (copper zinc tin sulfide) devices. In mineral collections, esterite is valued for its role in illustrating the diversity of the stannite family and its relation to related sulfide phases.