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Nelenite (brown) and franklinite (metallic, black), from Franklin, Franklin Mining District, Sussex County, New Jersey, US
Category Phyllosilicate
(repeating unit)
(Mn,Fe)16As3Si12O36(OH)17 or
Strunz classification 9.EE.15
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/m
Unit cell a = 23.24, b = 13.41
c = 7.38 [Å]; β = 105.21°; Z = 2
Color Light to medium brown
Crystal habit Coarsely granular to massive
Cleavage Perfect on {0001}
Mohs scale hardness 5
Luster Vitreous, resinous
Streak Light brown
Diaphaneity Transparent
Specific gravity 3.46
Optical properties Uniaxial (-)
Refractive index nω = 1.718 nε = 1.700
Birefringence δ = 0.018
Pleochroism Visible ε = colorless and ω = light brown
References [1][2][3]

Nelenite is a rare manganese iron phyllosilicate arsenate mineral found in Franklin Furnace, New Jersey.

Its formula is (Mn,Fe)16As3Si12O36(OH)17[3] or (Mn,Fe)16(Si12O30)(OH)14[As3+3O6(OH)3][1]

Discovery and occurrence[edit]

It was first describe in 1984 for an occurrence in the Trotter Mine, Franklin Mining District, Sussex County, New Jersey. It was named for Joseph A. Nelen of the Smithsonian Institution.[1][3] It has also been reported from Montgomery County, Virginia and the Suceava district of Romania.[1] At the type locality in New Jersey it occurs associated with actinolite, calcite, willemite, tirodite, rhodonite, apatite, lennilenapeite, stilpnomelane, microcline and talc.[3]