Keystoneite
A nickel tellurite discovered in 1988 at Colorado's Keystone mine — the nickel analogue of zemannite.
Intro adapted from Wikipedia (CC BY-SA).
Keystoneite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Mg₀.₅NiFe³⁺(Te⁴⁺O₃)₃·4H₂O.
It crystallizes in the hexagonal crystal system (space group P6_3/m). It is a member of the Zemannite structural group.
Key properties: specific gravity 4.4; Pale yellow-green streak; Adamantine luster; optically uniaxial.
IMA symbol Kys, IMA status: approved, named in 1987.
A rare secondary mineral formed in the oxidized zone of a complex polymetallic hydrothermal mineral deposit.
It is found associated with Melonite, tellurium, paratellurite, magnolite, coloradoite, calaverite, gold, stibnite, pyrite, “limonite”, manganese oxides, quartz.
The name refers to its occurrence at the Keystone mine, Colorado, USA.