Pennantite Locality: Virginia District, Lordsburg, Lordsburg Mining District, Pyramid Mountains, Hidalgo County, New Mexico, USA Description: This is the EDS graph for the green minerals. — Photo: Jerry Cone · Public domain
Pennantite
The manganese-rich member of the chlorite group — a greenish phyllosilicate of manganese deposits, named for Wales's Pennant Hills.
Intro adapted from Wikipedia (CC BY-SA).
Pennantite is an IMA-recognized mineral species in the silicate class, with the chemical formula Mn²⁺₅Al(Si₃Al)O₁₀(OH)₈.
It crystallizes in the orthorhombic crystal system. It is a member of the Chlorite, Clay structural group.
Key properties: Mohs hardness 2–2.5; specific gravity 2.89–3.07; Pearly on cleavage faces luster; optically biaxial.
IMA symbol Pnn, grandfathered status — known before the IMA’s founding, named in 1946.
developed during hydrothermal alteration of manganese deposits, as crusts on manganese minerals, and in veins cutting ore.
It is found associated with Banalsite, analcime, paragonite, pyrophanite, garnet, barite, ganophyllite, kutnohorite, kellyite, willemite, prehnite, datolite, roeblingite, clinohedrite.
The name refers to the Welsh mineralogist Thomas Pennant (1726–1798).