A black single crystal of the extremely rare mineral janggunite from the type locality: Janggun Mine, Bonghwa-gun, Gyeongsangbuk-do, Korea (Republic). Janggunite is a manganese mineral only known from three localities worldwide. — Photo: David Hospital · CC BY-SA 4.0
Janggunite
A rare mixed-valence manganese oxide described in 1975 from South Korea's Janggun mine.
Intro adapted from Wikipedia (CC BY-SA).
Janggunite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula (Mn⁴⁺,Mn²⁺,Fe³⁺)₆O₈(OH)₆.
It crystallizes in the orthorhombic crystal system.
Key properties: Mohs hardness 2–3; specific gravity 3.59; Brownish black to dark brown streak; Dull luster; optically biaxial.
IMA symbol Jgn, IMA status: approved, named in 1975.
Formed at a late stage of oxidation in a cementation zone of a manganese deposit.
It is found associated with Nsutite, todorokite, calcite, rhodochrosite.
The name refers to the Janggun mine, South Korea, its first locality.