Wiktionary
metacinnabar
n. (context mineralogy English) A isometric-hextetrahedral form of the mineral, cinnabar, containing mercury and sulfur and ranging from black to grayish white.
Wikipedia
Metacinnabar
Metacinnabar is the cubic form of mercury sulfide (HgS). It is the low temperature form and trimorphous with cinnabar ( trigonal structure) and the high temperature hypercinnabar ( hexagonal structure). It occurs with cinnabar in mercury deposits and is associated with native mercury, wurtzite, stibnite, marcasite, realgar, calcite, barite, chalcedony and hydrocarbons.
It was first described in 1870 for an occurrence in the Redington mine, Knoxville, Napa County, California.