Troilite and its assemblages from enstatite‐rich meteorites at the nanoscale

Abstract 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 , except for the FeS phase from Mayo Belwa, which exhibits a complex modulated structure consistent with a trilling of the modulated N C 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 patterns 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 N C ‐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.