Troilite – most frequently from outer space

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-62(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.

Picture: André Knöfel, CC BY-SA 3.0 de
https://commons.wikimedia.org/w/index.php?curid=7803661


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.

For a 3D interactive version on sketchfab, see here:

https://skfb.ly/6PHBS

Reference:
Lunar Troilite: Crystallography
H. T. Evans Jr
Science 1970, 167, 621-623

DOI: 10.1126/science.167.3918.621

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