The Low‐Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

Abstract

The low‐temperature magnetic properties of the many pyrrhotite varieties have not been studied extensively. Monoclinic pyrrhotite (Fe7S8) goes through the Besnus transition at ~30–34 K, which is used widely to diagnose its presence in bulk samples. Other pyrrhotite polytypes are assumed to be antiferromagnetic, although it has been suggested occasionally that some may also have remanence‐carrying capabilities. Here we compare the magnetic properties of monoclinic (4M) and hexagonal (3T) pyrrhotite at low temperatures. The 4M pyrrhotite records a Besnus transition consistently. Despite not recording a Besnus transition, 3T pyrrhotite has a magnetic remanence at room temperature and has distinctive room‐ and low‐temperature magnetic properties that cannot be explained by known or unidentified impurities (with abundances <0.1%). Some 3T‐pyrrhotite samples have exceptionally high (>700 mT) and stable coercivities below 50 K. The importance of this mineral in fossil or active gas hydrate and methane venting environments makes it important to develop a more detailed understanding of its occurrences and magnetic properties.

The Besnus Transition in Single‐Domain 4C Pyrrhotite

“…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 coercivities of these nodules, however, reveal a different temperature behavior with a pronounced increase down to T ≈ 50 K followed by nearly stable values, which contrast with the low‐temperature behavior of monoclinic 4C pyrrhotite.…”

“…The coercivities of these nodules, however, reveal a different temperature behavior with a pronounced increase down to T ≈ 50 K followed by nearly stable values, which contrast with the low-temperature behavior of monoclinic 4C pyrrhotite. Horng and Roberts (2018) assigned this magnetic behavior to hexagonal 3C pyrrhotite. It is worth noting that in contrast to 4C pyrrhotite, the low-temperature, magnetic, and structural behavior of the 3C polymorph is not resolved so far, and therefore, the effect of changes in the stacking modulation on the Besnus transition remains unknown.…”

Magnetic Mineralogical Approach for the Exploration of Gas Hydrates in the Bay of Bengal

“…The remanence does not recover totally during warming of RT‐SIRM. In the same zone (Z‐III) of NGHP‐01‐05C, the Besnus transition at ~35 K is typical of detrital pyrrhotite as evident in the RT‐SIRM cooling and the first derivative of magnetization curves (Dekkers, ; Horng & Roberts, ; Rochette et al, ; Figures i and j), but lack of such a transition is not diagnostic of authigenic pyrrhotite, which lacks a low‐temperature transition (Horng & Roberts, ). The remanence recovers totally during warming of the RT‐SIRM, which suggests the occurrence of a single domain pyrrhotite.…”

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…No unambiguous rock‐magnetic evidence for pyrrhotite is provided in any samples from this study, but Kars et al () in their study of Site U1437 recognized a candidate for the Besnus transition of pyrrhotite (Dekkers et al, ; Rochette et al, ) at ~32 K in ZFC LTSIRM warming curves for samples in intervals with lower methane contents within the overall methane‐rich zone between fluid anomalies 6 and 8. Diagenetic pyrrhotite which forms in methanic sediments has been reported to lack a Besnus signature (Horng & Roberts, ), so the presence of diagenetic pyrrhotite in the methane‐rich zones cannot be ruled out.…”