Troilite

Troilite occurs at only very few places on earth but is abundant on our moon and also on Mars. The most frequent occurrences on earth are within meteorites.
Troilite was named after Domenico Troili who collected and examined samples of a meteorite that fell on earth in Albareto (near Parma) in 1766.
Formula: FeS
Troilite is the iron-rich endmember of the pyrrhotite group, the iron-deficient variant of FeS with the formula Fe_(1-x)S (x = 0 to 0.2).
Space group: P-62c (No. 190)
Crystal system: hexagonal
Crystal class: -6m2
Lattice parameters: a = b = 5.962(2) Å, c = 11.750(3) Å, α = β = 90°, γ = 120°

A section of the meteorite Sikhote Alin that has Troilite inclusions.

Crystal structure (click on the picture to download the VESTA file):

(K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data,”J. Appl. Crystallogr., 44, 1272-1276 (2011).)

Although it is a stoichiometric 1:1 compound the Fe (brown) and S (yellow) atoms do have different coordination environments:
- FeS₆ (distorted) octahedra
- SFe₆ (distorted) trigonal prisms
This is analogous to NiAs; however, in Troilite the unit cell represents a superstructure of the simple NiAs-type structure (P6₃/mmc) based on small displacements of the Fe and S atoms from their ideal positions leading to an overall reduced symmetry (P-62c).
The hexagonal close-packed sulfur framework remains essentially the same.