IRON

Native iron – Fe – , also known as telluric iron, is a very rare native element due to iron’s high oxidation potential. It holds no economic importance other than as a collector’s mineral.

Two types of native iron are distinguished based on carbon content. Type 1 contains 1.7–4% carbon, with native iron grains measuring 1 mm and a structure composed of pearlite (ferrite + cohenite) and cohenite ((Fe,Ni,Co)₃C). Type 2 contains less than 0.7% C and is found as grains ranging from 1 to 18 mm in size within basaltic rocks.

Native iron always contains varying amounts (up to 4%) of Ni. Additionally, it contains C, Co, Cu, S, and P. There are four varieties of native iron: carltonite, kamacite, martensite, and nickel-kamacite.

Native iron is magnetic

1. Characteristics

Crystal system: Cubic hexaoctahedral.        

Color: Steel-gray to black, typically oxidized to red hues.     

Habit:  Compact, lamellar, massive, droplets – crystals are unknown.      

Cleavage: Perfect {001} or {100}; literature diverges.       

Tenacity: Malleable.        

Twinning: on {111} and {112}.       

Fracture: Irregular.       

Mohs Hardness: 4 – 4.5

Parting: On {112}.         

Streak: Steel-gray.         

Lustre: Metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 7.3 – 7.9

           

2. Geology and Deposits

The formation of native iron requires basaltic lavas to intrude into carbon-rich sediments—a rare geological occurrence.

Consequently, native iron is found in association with both igneous and sedimentary rocks. Native iron is a major constituent of the Earth’s core.

Significant deposits of native iron exist on Disko Island, Greenland (Denmark), featuring blocks weighing up to 25 tons and composed of Type 1 native iron. Other well-known occurrences are found in Siberia and Germany (Bühl, near the city of Kassel).

Native iron also occurs in volcanic fumaroles, within petrified logs, and mixed with goethite and organic matter.

Extraterrestrial native iron (found in meteorites) is abundant but will not be discussed here.

 

3. Mineral Associations

It is associated with the constituent minerals of the mafic rocks in which it occurs, such as calcic-sodic feldspars (plagioclase), olivine, clinopyroxenes, orthopyroxenes, oxides (ilmenite, magnetite, rutile, ulvöspinel), and sulfides (pyrite, pyrrhotite, pentlandite).

Furthermore, it is associated with minerals characteristic of the paragenesis, such as cohenite (Fe3C), wüstite (Fe,O), taenite (Fe,Ni), troilite (FeS), schreibersite ((Fe,Ni)3P), and graphite (C).

 

4. Transmitted Light Microscopy

Not applicable, as native iron is completely opaque.

 

5. Reflected Light Microscopy

Sample preparation: Polishing native iron requires care regarding the temperature reached during grinding and polishing; otherwise, the iron’s characteristics change. Its polishing hardness is high, though lower than that of magnetite. Polished iron tarnishes very rapidly, which is itself a diagnostic feature.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Pure white to light gray. Slightly bluish when compared to cohenite, which is part of the paragenesis.       

Pleochroism: No.      

Reflectivity: 58%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic, completely dark throughout the rotation of the stage.        

Internal reflections: No.     

May be confused with: various other minerals.

Platinum can look very similar, but it does not exhibit the rapid tarnishing that polished native iron undergoes, and it shows a slightly darker reflection color.

Cohenite, which is found in association, has a slightly more yellowish color and is anisotropic.       

General Characteristics: 

Grain shape: generally occurs as droplet-like segregations or spongy aggregates. Better-developed grains—which are quite rare—have a polygonal shape and appear strongly intergrown.

Tarnishing or atmospheric corrosion: atmospheric moisture rapidly attacks native iron associated with basalts. Oxidation processes can be prevented by coating the section with varnish.

Exsolution intergrowths of lamellar cohenite, oriented parallel to the {111} planes of the native iron, are known as pearlite. The formation of cohenite or pearlite as the groundmass depends on the carbon content (consult specific literature). Pearlite subsequently alters to oxipearlite or hydroxipearlite.

Twinning does not occur, although it is abundant in meteorites and metallurgical products.

Cleavage does not occur.

Zoning does not occur.

Wüstite inclusions may be present.

Rims surrounding the native iron may consist of ilmenite, pyrrhotite, cohenite, native copper, or magnetite.

Native iron rims occur around mackinawite.

Alteration clearly reveals tabular cohenite crystals in native iron samples with high cohenite content.

Alterations: native iron may alter to colloform hematite, goethite, or magnesioferrite. Wüstite alters to magnetite and native iron.

Oriented intergrowths of native iron and magnetite, well-developed, may occur.

 

Overview of the minerals of the native iron paragenesis:

None shows birreflectance nor internal reflections. Pleochroism is weak (in cohenite and troilite) or absent.

Native iron:

Reflection color: pure white with a bluish tint.

Reflectivity: 58%

Iso/Anisotropy: isotropic, completely black.

 

Cohenite:

Reflection color: white cream

Reflectivity: 57 – 59%

Iso/Anisotropy: weak anisotropy, compare individual grains.

 

Schreibersite:

Reflection color: light cream

Reflectivity: 52%

Iso/Anisotropy: anisotropic between gray and brown.

 

Taenite:

Reflection color: dark gray (like silicates!)

Reflectivity: 7% (extremely weak)

Iso/Anisotropy: Isotropic.

 

Troilite:

Reflection color: light brown, as pyrrhotite.

Reflectivity: 40%

Iso/Anisotropy: distinct anisotropy between gray and bluish.

 

Wüstite:

Reflection color: medium gray.

Reflectivity: 18.65%

Iso/Anisotropy: isotropic.

Native iron (white) in basalt from the Bühl occurrence (Kassel, Germany), viewed in plane-polarized light (PPL) with a 5x objective. A wide range of shapes and sizes of native iron droplets is visible, as well as variations in droplet density within the rock. Iron masses weighing up to 13.7 kg were found at this locality. The available sample contains regions with iron and regions completely devoid of iron; the contact between two such regions is shown in the next image.
In PPL, from the same sample as from the previous image, the contact between iron-rich and iron-poor regions of the sample.
The iron droplets typically exhibit jagged contacts with the surrounding silicates and may display crusts—continuous or otherwise and of varying thickness—composed of various minerals; troilite (cream-colored, like pyrrhotite) is very common. The sample is the same as the one in the two previous images.
In plane-polarized light (PPL), native iron (white) in basalt (dark colors) from the Bühl occurrence (Kassel, Germany), showing variations in shape and size in detail. Under crossed polars (XPL), the image is completely dark.
In PPL, native iron (white) and other phases (troilite, etc.) from the Disko (Greenland) occurrence, showing a high degree of alteration to secondary iron minerals (goethite) and others, which exhibit bluish-gray colors.
In PPL, native iron (white) and other phases (troilite, etc.) from the Disko (Greenland) occurrence, showing a high degree of alteration to secondary iron minerals (goethite) and others, which exhibit bluish-gray colors.