Troilite and its assemblages from enstatite-rich meteorites at the nanoscale
Noemi Mészárosová, Roman Skála, Mariana Klementová, Lukáš Palatinus, Jozef Veselý, Petr Harcuba
MAPS, Version of Record online: 09 November 2026
“A transmission electron microscopy (TEM) study of four troilite grains from enstatite chondrites (Y-691, Y-791790, Y-86004, A-881314) and two grains from aubrites (Y-793592, Mayo Belwa) addressed their crystal structures and nanoscale mineralogical relationships. The crystal structure of troilite in enstatite-rich meteorites is predominantly consistent with the space group 𝑃̅62𝑐, except for the FeS phase from Mayo Belwa, which exhibits a complex modulated structure consistent with a trilling of the modulated NC pyrrhotite superstructure. TEM energy-dispersive X-ray spectroscopy (TEM-EDS) analyses confirmed the incorporation of chromium and titanium into the troilite crystal structure, even in samples containing nanometer-sized daubréelite exsolution lamellae (A-881314, Y-793592). Nanometer-sized inclusions of (Mn,Mg,Fe)S monosulfide solid solution occur in troilite in Y-791790. Crystallographic orientation relationships between troilite and associated sulfide phases (daubréelite and keilite) indicate semi-coherent intergrowths at the nanoscale. The absence of nanometer-sized daubréelite in Y-86004 and Y-791790 suggests rapid cooling of the Cr-Fe sulfide solution and quenching of the chemical composition of the phase stable above 700°C, consistent with the impact melt origin of these meteorites. Additional weak, diffuse diffractions observed in selected area electron diffraction paterns for troilites from Y-86004, Y-791790, and Y-793592 are attributed to the development of vacancy ordering with variable periodicity along the c-axis, corresponding to NC-type superstructures. The detailed nature of vacancy ordering and the contributions of Cr and Ti substitution to the observed superstructure features remain unresolved and require comprehensive structure analysis.
