The importance of temperature-dependent diffraction data in
understanding magnetic changes across the pyrrhotite λ-transition.
Abstract
Pyrrhotites are a class of geologically important nonstoichiometric iron
sulfides with the general composition of Fe1-xS (0 ≤ x ≤
0.125) and are found in a variety of intergrown polytypes,
conventionally separated into the antiferromagnetic hexagonal and the
ferrimagnetic monoclinic (4C) varieties. Both structures undergo
magnetic phase transitions, where antiferromagnetic polytypes display
the λ-transition at ~490K and the 4C polytype shows the
Besnus transition at ~30K. However, recent studies have
shown the relationship between pyrrhotite polytypes and their magnetic
behavior to be more complex and new non-monoclinic polytypes (e.g., 3C)
have been described that are also capable of retaining a spontaneous
magnetization at room temperature. These advances raise the level of
detail needed for the characterization of pyrrhotite in rock magnetic
and paleomagnetic studies. This study demonstrates the utility of
combining X-ray diffraction data collected as a function of temperature
with low- and high-temperature magnetic measurements to characterize
natural samples. We analyze two natural samples that contain mixtures of
4C, 5C, and 6C polytypes and describe how their polytypes and magnetic
properties vary as a function of temperature across the λ-transition and
how and when pyrite and greigite form. We also report the effect on
natural samples of an annealing protocol commonly used to elevate the
concentration of the 4C polytype in synthetic samples and found that
annealing instead transformed some antiferromagnetic pyrrhotite into a
form whose diffraction pattern most closely resembles the 4C polytype
and displays room temperature spontaneous magnetization, but lacks the
characteristic Besnus transition.