Pennantite is a manganese aluminum silicate hydroxide mineral of the chlorite group. The composition of Franklin material, determined by [Dunn] and published by Bayliss (1983) is: SiO2 23.9, Al2O3 19.1, Fe2O3 2.6, MgO 3.4, ZnO 15.9, MnO 27.4 wt. %.
Pennantite was found at Franklin by [Dunn], but has not been found at Sterling Hill. The unit-cell given above is from Bayliss (1983).
Franklin pennantite occurs as thin platy layers of 1 mm crystals in several different assemblages. Some pennantite-1a appears to be epitactically grown on willemite. It is dark red, sometimes reddish-brown, with vitreous luster and a perfect cleavage. The hardness and texture is that of a chlorite. Subspherical aggregates are common. There is no discernible fluorescence in ultraviolet.
Pennantite was first described from Franklin as "a Mn-chlorite" by Dunn et al., (1983a). It coats secondary willemite crystals, and is associated with prehnite, datolite, roeblingite, charlesite, and numerous other species in a highly hydrated, recrystallized assemblage. (Dunn, 1995)

 Location Found: Franklin
 Mineral Note: Pennantite replaces grovesite in name, "The Picking Table, Fall 1990"
 Year Discovered: 1946
 Formula: Mn52+Al(AlSi3O10)(OH)8
 Essential Elements: Aluminum, Hydrogen, Manganese, Oxygen, Silicon
 All Elements in Formula: Aluminum, Hydrogen, Manganese, Oxygen, Silicon
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
 To find out more about this mineral at minDat's website, follow this link   Pennantite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.471. 'Pennantite-1a'

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 31, No. 2 - Fall 1990, pg. 3Notes from the Laboratory & Changes to the List of Species From Franklin and Sterling Hill, Pete J. Dunn, John L. Baum, Pennantite
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