Crystals of the very rare tellurites cliffordite (yellow) and mackayite (green) from the type locality: San Miguel Mine, Moctezuma, Sonora, Mexico. — Photo: David Hospital · CC BY-SA 4.0
Mackayite
Mackayite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Fe³⁺Te⁴⁺₂O₅(OH).
It crystallizes in the tetragonal crystal system (space group I4_1/acd).
Key properties: Mohs hardness 4.5; specific gravity 4.86; Pale green streak; Vitreous luster; optically uniaxial.
IMA symbol Mky, grandfathered status — known before the IMA’s founding, named in 1944.
Of rare occurrence in the oxidized zone of Au–Te deposits, an alteration product of tellurium.
It is found associated with Tellurium, tellurite, emmonsite, rodalquilarite, poughite, cliffordite, sonoraite, pyrite, alunite, barite, quartz.
The name refers to John William Mackay (1831–1902), Irish–American Comstock Lode mining magnate, in honor of his endowment of the School of Mines, University of Nevada, Reno, Nevada, USA.