Pyrrhotites are a class of geologically important nonstoichiometric iron sulfides with the general composition of Fe_1-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.