Stilpnomelane

K(Fe²⁺,Mg,Fe³⁺)₈(Si,Al)₁₂(O,OH)₂₇·n(H₂O)IMA symbolStp^[1]Strunz classification9.EG.40Crystal systemTriclinicCrystal classPinacoidal (1)
(same H-M symbol)Space groupP1Unit cella = 21.72 Å, b = 21.72 Å
c = 17.4 Å; α = 124.14°
β = 95.86°, γ = 120°; Z = 6IdentificationColorBlack, greenish black, yellowish bronze, greenish bronzeCrystal habitPlatey, scaly and fibrous with comb structures; radiating groupsCleavagePerfect on {001}, imperfect on {010}TenacityBrittleMohs scale hardness3–4LusterVitreous to dullStreakGray whiteDiaphaneitySubtranslucent to opaqueSpecific gravity2.77 – 2.96Optical propertiesBiaxial (−)Refractive indexn_α = 1.543 – 1.634 n_β = 1.576 – 1.745 n_γ = 1.576 – 1.745Birefringenceδ = 0.033 – 0.111PleochroismX: bright golden yellow to pale yellow Y and Z: deep reddish brown, to deep green to nearly black2V angle0–40 measuredDispersionNoneReferences^[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