STILPNOMELANE

Stilpnomelane – (K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36·nH2O – is a rarer phyllosilicate. It does not constitute ore.

It is classified in the Stilpnomelane Group and is isostructural with franklinphilite, parsettensite and chalcodite. It has three varieties. Under the microscope, the optical properties vary depending on varying degrees of the Fe oxidation state; distinguishes between ferro-stilpnomelan and ferri-stilpnomelan.

Macroscopically it is very easy to confuse stilpnomelane with biotite, with other brown micas and with dark chlorites. Also under the microscope, stilpnomelane is often confused with biotite.

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: Red, brown, black, dark green, dark reddish brown, golden brown.    

Habit: Plaques, scales or fibers with a comb structure. It forms feathery or radial groups. Crystals up to 1 cm.       

Cleavage: Brittle.       

Tenacity: Brittle.  

Twinning: No.       

Fracture:  Micaceous.      

Mohs Hardness: 3 – 4

Parting: Sometimes, on (010)         

Streak: White, gray.         

Lustre: Sub-vitreous, sub-metallic, pearly in the cleavages.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.59 – 2.96

 

2. Geology and Deposits

Stlpnomelane occurs associated with banded iron formations (BIFs) and associated shales and slates. Also in layers with limonite.

It occurs in metamorphic rocks of the blueschist and greenschist facies.

It is also found in some metamorphosed deposits of massive sulfides, when it is a product of surface alteration.

It was found in acid gneiss xenoliths in gabbros and granophyrs (Skaergaard, Greenland).

 

3. Mineral Associations

In banded iron formations, stilpnomelane is frequent and is associated with quartz, Fe oxides (hematite, magnetite, goethite), siderite, apatite, grunerite, riebeckite and phyllosilicates (chlorite (chamosite), greenalite, berthierina, minnesotaite).

            In metamorphic rocks of the blueschist facies, it occurs with glaucophane, lawsonite, garnet (andradite), clinochlore, tremolite, talc, phengite, zoisite, quartz, titanite, deerite, howieite and zussmanite.

In greenschist facies metamorphic rocks, in the pumpellyite-chlorite zone, stilpnomelane is common, occurring with chlorite, muscovite, epidote, albite, titanite, garnet, biotite, sericite, pumpellyite, actinolite, calcite and quartz.

In the paragenesis of metapelites formed under high pressure, it occurs with talc, fengite and chloritoid.

When associated with sulfides, it occurs with chalcopyrite, pyrite, pyrrhotite, quartz, chlorite and magnetite.

It never occurs alongside chloritoid.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.543 – 1.634     nβ: 1.576 – 1.745   nγ: 1.576 – 1.745

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Strongly pleochroic in yellowish, greenish and brownish:

X: pale yellow, golden yellow, pale brown, colorless.

Y = Z: deep brown, almost black, tan, deep yellowish brown, weak greenish yellow.

It closely resembles biotite and can occur intergrown with biotite.

Basal sections show practically no pleochroism and are dark, as is the case with biotite as well.

Relief: Medium to high, may show “relief pleochroism”, when the relief changes from high to medium, for example, rotating the stage, as in carbonates.  

Cleavage: {001} perfect, {010} imperfect, the two intersect at 90º.

Usually only one cleavage is visible under the microscope. This cleavage is of much lower quality than the biotite cleavage, with which stilpnomelan is easily confused, but biotite has mottled extinction, stilpnomelane does not.           

Habits: Plaques, lamellae, micaceous, fibrous, acicular, radiated or feathery aggregates. Very reminiscent of micas, especially biotite.

It can be shown to be intergrown with chlorite, paragonite and other micas.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Moderate to high birefringence, from 0.033 to 0.111, corresponding to colors between 2nd higher order (intense colors, with several mixed colors, very colorful) to 5th and 6th order, of white, cream and brown colors such as carbonates.

The strong colors inherent in the mineral mask the interference colors, as is usually the case with colored minerals.

Basal sections are isotropic!           

Extinction:  Parallel, but not mottled like that of the micas (important!)

Elongation sign: ES(+) (like all micas!)            

Twins: No.         

Zoning: Sometimes it is zoned.             

CONVERGENT LIGHT

Character: B(-), but the figure is pseudo-uniaxial!

Strong colors make it difficult to obtain the interference figure. 

2V angle: 0o, it can vary up to 20º – 30º. When at 0o, it simulates an uniaxial figure.         

Alterations: to clay minerals, chlorites and Fe oxides.          

May be confused with: biotite, which has similar habit and colors. However, biotite has stronger pleochroism, a much better quality cleavage and mottled extinction.

Chloritoid, chlorite and clintonite have much lower birefringences.

Tourmaline features higher relief and usually shows color zoning. 

Stilpnomelane 01: In PPL, yellow-brown stilpnomelane crystals with its typical lamellar shape, very similar to biotite. In this position, the big crystal shows its first pleochroism color.
Stilpnomelane 01: The same section of the previous image, now rotated 90 degrees. The big crystal shows the second pleochroism color, very similar to biotite.
Stilpnomelane 01: The same section as of the two previous images, now in XPL. Interference colors are very similar to the ones of biotite, but extinction is not mottled. As is always the case with strongly colored minerals, interference colors are masked by the strong own color of the mineral.
Stilpnomelane with riebeckite (blue), grunerite (light green) and quartz (colorless) associated with a banded iron formation, in PPL. Stilpnomelane can show a habit of lamellar crystals in radial aggregates, something that biotite hardly does.
In PPL, stilpnomelante in banded iron formation (BIF), forming a somewhat radial aggregate of lamellar crystals. Riebeckite in blue, quartz colorless.
Stilpnomelane 02: In PPL, lamellar crystals of stilpnomelane forming a radial aggregate. It is possible to perceive evidence of cleavage parallel to the length of the lamellae, much less developed than the cleavage of biotites, which is diagnostic.
Stilpnomelane 02: The same section as of the previous image, now in XPL. The vertical lamellae are extinct (parallel extinction!), but the extinction is not mottled as in biotite. Therefore, bad cleavage and absence of mottled extinction are diagnostic for stilpnomelane over biotite.
In PPL, unaltered stilpnomelane (on the right in the image) and stilpnomelane altered to Fe hydroxides (on the left in the image). The altered stilpnomelane no longer shows pleochroism and is very dark. Care must be taken not to confuse basal sections with altered crystals, as is also the case with biotite.
In PPL, stilpnomelane intergrown with fibrous riebeckite (crocidolite - blue) in banded iron formation. The irregular intercalation of fine blue crystals with fine orange crystals can be seen.
Stilpnomelane 03: In PPL, longitudinal sections of stilpnomelane on the left and basal sections of stilpnomelane on the right. The longitudinal sections show pleochroism, the basal sections practically do not show pleochroism.
Stilpnomelane 03: The same section as of the previous image, now in XPL. The basal sections are isotropic while the longitudinal sections show parallel extinction (but not mottled) and strong interference colors.
Stilpnomelane 04: In PPL, stilpnomelane with a habit where the lamellae are grouped in sets that tend to radials. Quartz colorless, Magnetite and/or hematite opaque.
Stilpnomelane 04: The same section as of the previous image, now in XPL. Stilpnomelane show strong interference colors.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of stilpnomelane. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with stilpnomelanea, like hematite and magnetite.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption