COHENITE
Cohenite – Fe3C – is a native element that occurs abundantly in meteorites but is found on Earth only in specific geological settings. It has no economic use.
It is strongly magnetic, a property that does not aid in its identification because native iron – with which it is associated – is also magnetic.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: White; compared to Fe, it is yellowish.
It tarnishes to light bronze and then to golden yellow.
Habit: Idiomorphic, myrmekitic, lamellar, dendritic, crusts.
Cleavage: {100} good, {010} good, {001} good.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 5.5 – 6
Parting: No.
Streak: No information available.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 7.2 – 7.65
2. Geology and Deposits
Cohenite is abundant in meteorites, an occurrence that will not be discussed here.
In terrestrial geological settings, cohenite is stable only in igneous rocks that formed under strongly reducing conditions and contain native iron. Such conditions arose only when basaltic magmas intruded coal deposits, as at Disko (Greenland) and Bühl (Germany).
3. Mineral Associations
In occurrences of native iron in basalts (Bühl, near Kassel, Germany), cohenite is associated with the following minerals:
– rock-forming silicates (olivine, augite, plagioclase),
– Fe-Ti oxides (ilmenite, magnetite),
– alteration minerals (goethite, kaolinite, montmorillonite, calcite, aragonite, quartz),
– sulfides (pyrite, pyrrhotite, molybdenite, sphalerite), and
– native iron, wüstite, troilite, taenite (Fe,Ni), and schreibersite – (Fe,Ni)3P).
4. Transmitted Light Microscopy
This does not apply, as cohenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Polishing cohenite takes longer than polishing native iron, but it is of good quality, without polishing scratches.
PLANE POLARIZED LIGHT – PPL
Reflection color: Cream-white, slightly more cream-colored than native iron.
Compared to the color of cohenite, cohenite’s color is cream or slightly brownish.
Compared to the color of pyrrhotite, cohenite’s color is pale cream.
Compared to the color of pentlandite, cohenite’s color is very similar.
Pleochroism: Weak, but distinct.
Reflectivity: 57 – 59%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Mild, weak anisotropy.
Internal reflections: No.
May be confused with: Schreibersite is quite similar and occurs in the same paragenesis.
General Characteristics:
Grain shape: terrestrial cohenite (in contrast to meteoritic cohenite) forms fine lamellar intergrowths with native iron, known as “pearlite.” If the iron has been replaced by iron oxides or iron hydroxides, the intergrowth is termed oxide-pearlite or hydroxide-pearlite, respectively. Larger cohenite grains may occur within the pearlite. Aggregates of cohenite and native iron may be encased in thin crusts of native copper.
Cleavage may be visible.
Lamellar twinning can be seen only with high-quality equipment and trained eyes.
Alteration of cohenite produces graphite and native iron.
