Stilpnomelane
General
CategoryPhyllosilicates
Smectite group
Formula
(repeating unit)
K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27·n(H2O)
IMA symbolStp[1]
Strunz classification9.EG.40
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 21.72 Å, b = 21.72 Å
c = 17.4 Å; α = 124.14°
β = 95.86°, γ = 120°; Z = 6
Identification
ColorBlack, greenish black, yellowish bronze, greenish bronze
Crystal habitPlatey, scaly and fibrous with comb structures; radiating groups
CleavagePerfect on {001}, imperfect on {010}
TenacityBrittle
Mohs scale hardness3–4
LusterVitreous to dull
StreakGray white
DiaphaneitySubtranslucent to opaque
Specific gravity2.77 – 2.96
Optical propertiesBiaxial (−)
Refractive indexnα = 1.543 – 1.634 nβ = 1.576 – 1.745 nγ = 1.576 – 1.745
Birefringenceδ = 0.033 – 0.111
PleochroismX: bright golden yellow to pale yellow Y and Z: deep reddish brown, to deep green to nearly black
2V angle0–40 measured
DispersionNone
References[2][3][4]

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 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]

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Handbook of Mineralogy
  3. ^ a b Stilpnomelane on Mindat
  4. ^ a b Stilpnomelane on Webmineral