Towards a Better Understanding of the Structure of Nano-Minerals at Ambient and Extreme Conditions.

The high-pressure (HP) behavior of nano-crystalline mackinawite (n-FeS) with particle sizes of 6, 7, and 8 nm has been investigated by high-energy X-ray total scattering and pair distribution function analysis. An irreversible first-order structural phase transition from tetragonal mackinawite to orthorhombic FeS-II was observed at about 3 GPa. The transition is induced by the closure of the van-der-Waals gap in the layered mackinawite structure. A grain size effect on the transition pressure and the compressibility was observed. The n-FeS study is an example of a broad class of nano-crystalline minerals where the total scattering (TS) approach provides significant new information on local-, intermediate- and long-range structure. Under extreme conditions, of pressure in this case, straightforward modifications allow quantitative descriptions of the transformations mechanisms.