# A combined experimental and theoretical study of the prototypical polymorphic transformation from marcasite to pyrite FeS2

## TL;DR

This study explores how marcasite transforms into pyrite FeS2, revealing a significant energy barrier that explains why marcasite remains stable at lower temperatures.

## Contribution

The paper combines experiments and theory to map the marcasite-to-pyrite transformation and quantify its kinetic barrier.

## Key findings

- The transformation from marcasite to pyrite is thermodynamically favorable but hindered by a kinetic barrier of ~3 eV.
- Marcasite remains stable below 450 °C due to the high energy barrier.
- Annealing experiments and theoretical calculations align, confirming the transformation pathways.

## Abstract

We present an investigation of the prototypical polymorphic structural transformation from marcasite to pyrite FeS2 studied by combining annealing experiments and theoretical calculations. These experiments have become possible due to the availability of laboratory-synthesized high-purity marcasite samples. We constructed an annealing temperature, time, and phase composition map of marcasite based on a series of isothermal annealing experiments at different temperatures and heating times. To understand the microscopic mechanisms and pathways of the transformation, we performed theoretical calculations that yield an agreement with the experimental results. Based on the combined results, we show that the transformation of marcasite to pyrite, while thermodynamically favorable, is hindered by a kinetic barrier of the order of 3 eV. As a result, marcasite can remain stable for extended times at temperatures below 450 °C.

TOC illustrates the polymorphic transformation of FeS2 from marcasite to pyrite, highlighting the energy barrier (ΔG) associated with the phase transition.