Nelenite
Nelenite (brown) and franklinite (metallic, black), from Franklin, Franklin Mining District, Sussex County, New Jersey, US
General
Category	Phyllosilicate
Formula (repeating unit)	(Mn,Fe)16As3Si12O36(OH)17 or (Mn,Fe)16(Si12O30)(OH)14[As3+3O6(OH)3]
IMA symbol	Nln[1]
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
Identification
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	[2][3][4]

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

Its chemical formula is (Mn,Fe)₁₆As₃Si₁₂O₃₆(OH)₁₇ or (Mn,Fe)₁₆(Si₁₂O₃₀)(OH)₁₄[As³⁺₃O₆(OH)₃]^[2]

Discovery and occurrence

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.^[2][4] It has also been reported from Montgomery County, Virginia and the Suceava district of Romania.^[2] At the type locality in New Jersey it occurs associated with actinolite, calcite, willemite, tirodite, rhodonite, apatite, lennilenapeite, stilpnomelane, microcline and talc.^[4]