TROILITE

Troilite is a rare sulfide, found here on Earth in only a few very specific geological occurrences. It has no economic use.

Troilite is classified in the Pyrrhotite Group. It is the S analogue of achávalite (FeSe) and chemically the Fe analogue of covellite (CuS). It can transform into hexagonal pyrrhotite at T<122ºC, in the pyrrhotite-2H polytype.

Generally, the literature deals with pyrrhotite and troilite together, despite the great differences between these iron sulfides in terms of texture, structure, intergrowths, twins, occurrences, etc.

1. Characteristics

Crystal system: Hexagonal, bipyramidal dihexagonal.

Color: Gray brown, yellow brown, pale grayish brown, bronze.

Bronze brown when tarnished. 

Habit: Massive, granular.

Cleavage: No.

Tenacity: Malleable

Twinning: No.

Fracture: No information available.

Mohs Hardness: 3.5 – 4.5

Parting: No.

Streak: Brownish black

Lustre: Metallic.

Diaphaneity: Opaque.

Density (g/cm³): 4.67 – 4.79

 

2. Geology and Deposits

In meteorites, troilite is very common. It is common on the Moon, it also appears to be abundant on the surface of Mars and on Jupiter’s satellites. Troilite also occurs in slag and pulverized coal particles. These occurrences will not be addressed here.

In terrestrial geological occurrences troilite occurs in stratified ultramafic intrusive rocks and in the serpentinites that formed therefrom.

Also in occurrences of native iron in basalts.

 

3. Mineral Associations

It is associated with rock-forming minerals typical of ultramafic stratiform complexes: calcosodic feldspars (plagioclases), olivine, clinopyroxenes, rutile and chromite. In these complexes, it occurs together with the characteristic sulfides: pyrrhotite, pentlandite, mackinawite, cubanite, valeriite, chalcopyrite, pyrite and brezinaite (Cr3S4).

In occurrences of native iron in basalts (Bühl, near Kassel, Germany) it is associated with rock-forming silicates (olivine, augite, plagioclases), Fe-Ti oxides (ilmenite, magnetite), alteration minerals (goethite, kaolinite). , montmorillonite, calcite, aragonite, quartz), sulphides (pyrite, pyrrhotite, molybdenite, sphalerite), native iron, wüstite, cohenite, taenite (Fe,Ni) and schreibersite (Fe,Ni)3P).

 

4. Transmitted Light Microscopy

Not applicable, as troilite is completely opaque.

 

No image

5. Reflected Light Microscopy

Sample preparation: Due to its low hardness, troilite requires careful polishing and some polishing grooves are quite persistent, but good polishing is possible, except for the larger grains, as is also the case with pyrrhotite.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light brown, very similar to the color of pyrrhotite.

Pleochroism: Discreet in light brown.

Reflectivity: ~40%

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Distinct anisotropy between gray-brown and bluish.

Internal reflections: No.

May be confused with: pyrrhotite, which has the same reflection color, but its anisotropy colors and intensity are completely different.

 

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: distinc anisotropy between gray and bluish.

 

Wüstite:

Reflection color: medium gray.

Reflectivity: 18.65%

Iso/Anisotropy: isotropic..

 

 

 

 

 

 

 

 

 

In PPL, native iron and troilite in basalt from the Bühl occurrence (Kassel, Germany). Native iron (white), basalt silicates (dark gray and black), troilite (cream). Troilite forms partial or complete crusts around native iron, accumulations between drops of native iron and small accumulations in the rock itself. In CPL, the image is completely black; troilite anisotropy is weak.
In PPL, from the same polished section as the previous figure, trolite (cream) and native iron (white).
Same situation as former images. The troilite backings around the native iron can be very thin and there can be other minerals forming the crusts.
Detail view of one of the troilite crusts (cream) between native iron (white) and basaltic rock silicates (almost black). PPL. Bühl (Kassel, Germany).
In PPL, troilite (cream) also occurs without connection to native iron droplets in basaltic (dark) rock. Bühl (Kassel, Germany).
In PPL, troilite (cream, left), native iron (white, right) and silicates and alteration minerals (dark) in a highly altered sample of the native Fe occurrence from Disko (Greenland).
In PPL, troilite (cream), native iron (white), silicates (dark gray), and iron alteration minerals (bluish gray). Disko, Greenland.
<