Metacinnabar and tiemannite are e.nd members of HgS-HgSe series, In meta- cinnabar, selenium is often seen in place of sulfur, and zinc in place of mercury;in tiemannite, sulfur is frequently in place of selenium. Like minerals in many isomorphous series, these two minerals are quite similar to each other and show regular variation in physical-optical properties, X-ray diffraction patterns and infrared absorption spectra. Metacinnabar, which is associated with such sulfides as cinnabar and sphale- rite, was formed in a sulfides-rich environment at fairly low temperature (190- 220'C). Tiemannite is associated only with quartz without such sulfides as sphalerite and cinnabar; it w as therefore for med in a sulfides-poor environ went at a bit higher to mperature (230-265'C).