On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

Abstract

In monoclinic 4C pyrrhotite (ideal formula Fe7S8), ordered vacancy distribution forms a superstructure with strong ferrimagnetism that makes this mineral a major remanent magnetization carrier in the Earth's crust. The pronounced decrease in isothermal remanence magnetization at about 32 K, known as Besnus transition, is a characteristic trait that marks the low‐temperature transition in 4C pyrrhotite, and it is used as a key to identify this mineral phase in rock samples. Here we take a nearly pure single pyrrhotite crystal (Fe6.97S8) from the Swiss Alps to study its Besnus transition in a broad mineral‐magnetic approach that combines detailed structural analysis with static and dynamic magnetization experiments. All the magnetic properties inferred from the experimental data are discussed in the context of a recent model that explains interacting anisotropy fields in the 4C pyrrhotite caused by the vacancy arrangement as the origin of the Besnus transition.

The Besnus Transition in Single‐Domain 4C Pyrrhotite

“…At 10 K, B c = 276 mT is by a factor of about 3.6 higher than the value at 300 K (Figures and a), while M r / M 3T increases from 0.55 at 300 K (Figure ) to 0.67 at 10 K (Figure b). The relative increases of the two parameters associated with the Besnus transition are less pronounced than what has been reported previously for MD and mechanically treated 4C pyrrhotite (Dekkers et al, ; Kind et al, ; Koulialias, Schäublin, et al, ; Rochette et al, ; Volk et al, ). Finally, the B c and M r / M S values of our SD 4C pyrrhotite at 300 K are similar to those of the multiphase nodules from recent and fossil methanic sedimentary deposits reported by Horng and Roberts ().…”

“…The shift of T crit suggests that an external 3 T field affects the Fe 2+ spin rotation and thus the Besnus transition. The IRM 10K after FC is about 3% higher than the ZFC_IRM 10K , but both have nearly the same decay upon heating with T crit = 31 K. Furthermore, overall losses of both IRM 10K curves are markedly less pronounced to those obtained from MD pyrrhotite (Kind et al, ; Koulialias, Schäublin, et al, ). With this in mind we conclude that the rotational behavior of Fe 2+ spins, that is, change in multiaxial magnetocrystalline anisotropy, associated with the Besnus transition (Koulialias, Charilaou, et al 2018), has a less distinct effect on the remanence properties of 4C pyrrhotite in the absence of domain walls.…”

“…Before ball milling, the sample has amplitude‐dependent in‐phase ( χ ′) and out‐of‐phase ( χ ′′) susceptibilities typical of MD 4C pyrrhotite (Figures a and b). The χ ′ drops at about T = 35 K to form a local minimum at T ≈ 32 K followed by a local maximum at T = 31 K. The χ ″ peaks at 33 K, which indicates the critical temperature ( T crit ) of the Besnus transition (Koulialias, Schäublin, et al, ). Below this temperature the amplitude dependence ceases, that is, domain walls in the 4C pyrrhotite become strongly pinned (Kind et al, ).…”

“…Above the Besnus transition, hysteresis curves of the SD pyrrhotite exhibit relatively high B c and M r / M 3T compared to MD samples (Dekkers et al, ; Koulialias, Schäublin, et al, ). At 10 K, B c = 276 mT is by a factor of about 3.6 higher than the value at 300 K (Figures and a), while M r / M 3T increases from 0.55 at 300 K (Figure ) to 0.67 at 10 K (Figure b).…”

“…The low‐temperature anomaly in the magnetization at T ≈ 35 K, also known as the Besnus transition, is a diagnostic feature of natural and synthetic 4C pyrrhotite (Besnus & Meyer, ; Rochette et al, ). Magnetic information about this transition has so far been obtained from samples with relatively large crystallites in the multidomain (MD) magnetic state (Dekkers et al, ; Koulialias, Schäublin, et al, ; Rochette et al, ), from shock‐induced samples containing MD and single‐domain (SD) particles (Louzada et al, ), from crushed samples with significant contribution of SD particles (Dekkers et al, ; Wehland et al, ), or from nonideal SD pyrrhotite samples (Bezaeva et al, ). Complementary information from phase pure, SD monoclinic pyrrhotite has so far not been established although this mineral phase could be widespread in sedimentary environments such as gas hydrate‐bearing sedimentary deposits (e.g., Kars & Kodama, , ).…”