STILPNOMELANE

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Triclinic
    Chemistry : K(Fe,Mg)8(Si,Al)12(O,OH)27
    Rarity : Fairly common

Stilpnomelane is a mica that is found mainly in sedimentary iron deposits, associated with magnetite, goethite, hematite and pyrite. It is also known as a common constituent of weakly metamorphic rocks (chlorite schists, epidote schists), more rarely of glaucophane schists and of certain metamorphic metal deposits (Franklin in New Jersey and Nordmark in Sweden for example). Its name comes from the Greek stilpnos (brilliant) and melanos (black), thus evoking its luster and its color. Stilpnomelane occurs in lamellae or scales, in foliated masses, in fibrous, downy or radiate aggregates, not exceeding 1 cm, or in velvety coatings. Its color is reddish brown to black, golden brown or green-black with easy foliated cleavage. When the lamellae are thin, the resemblance to biotite is strong.

Main photo : Stilpnomelane from Les Moulins Mine, Valais, Switzerland © Mischa Crumbach

Stilpnomelane in the World

Stilpnomelane is common and its deposits are numerous. Among the most typical are the Animikian iron formation (Minnesota) and the Bald Knob iron mine (North Carolina) where it is well represented. Still in the United States, crystals forming aesthetic velvety aggregates are known at the Sterling mine near Antwerp (New York). Among its many other deposits, let us mention the Ilimaussaq alkaline complex near Narssaq (Greenland) and the polymetallic skarn of Tetukhe (Russia).

Right photo : Stilpnomelane de Clot della Mutta, Piemond, Italy © Alessandro Cantamessa


Stilpnomelane in France

In France, stilpnomelane is known on the island of Groix (Morbihan), in the Aure valley (Hautes-Pyrénées), at the Costabonne mines (Pyrénées-Orientales), at the Bois de la Roche quarry (Loire-Atlantique), at the Clausis mine near St-Véran (Hautes-Alpes), etc...

Twinning

No twinning known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 3 to 4
Density : 2.59 to 2.96
Fracture : Micaceous
Streak : White, gray

TP : Translucent
RI : 1.543 to 1.745
Birefringence : 0.033 to 0.111
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Sulfuric acid

Magnetism : NoneRadioactivity : None

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