Stilpnomelane is a phyllosilicate mineral. It has the chemical formula K(Fe²⁺,Mg,Fe³⁺)₈(Si,Al)₁₂(O,OH)₂₇·n(H₂O).^[4]

Stilpnomelane
General
Category	Phyllosilicates Smectite group
Formula (repeating unit)	K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O)
IMA symbol	Stp[1]
Strunz classification	9.EG.40
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 21.72 Å, b = 21.72 Å c = 17.4 Å; α = 124.14° β = 95.86°, γ = 120°; Z = 6
Identification
Color	Black, greenish black, yellowish bronze, greenish bronze
Crystal habit	Platey, scaly and fibrous with comb structures; radiating groups
Cleavage	Perfect on {001}, imperfect on {010}
Tenacity	Brittle
Mohs scale hardness	3–4
Luster	Vitreous to dull
Streak	Gray white
Diaphaneity	Subtranslucent to opaque
Specific gravity	2.77 – 2.96
Optical properties	Biaxial (−)
Refractive index	nα = 1.543 – 1.634 nβ = 1.576 – 1.745 nγ = 1.576 – 1.745
Birefringence	δ = 0.033 – 0.111
Pleochroism	X: bright golden yellow to pale yellow Y and Z: deep reddish brown, to deep green to nearly black
2V angle	0–40 measured
Dispersion	None
References	[2][3][4]

Stilpnomelane occurs associated with banded iron formations. It is a metamorphic mineral associated with the blueschist and greenschist facies.^[2]

It was first described in 1827 for an occurrence in Moravia in the Czech Republic. The name is derived from the Greek stilpnos for shining, and melanos for black.^[3]