Wiktionary
janggunite
n. (context mineralogy English) An orthorhombic black mineral containing hydrogen, iron, manganese, and oxygen.
Wikipedia
Janggunite
Janggunite is a rare manganese oxide mineral with formula Mn(Mn,Fe)O(OH).
It was first described in 1976 for an occurrence in the Janggun mine, Bonghwa-gun, Gyeongsangbuk-do, South Korea and named for the locality.