Magnetic anisotropy behaviour of pyrrhotite as determined by low- and high-field experiments

Abstract

S U M M A R YHere we report on the sources of magnetic anisotropy in pyrrhotite, an iron sulphide present in many rocks as an important carrier of the Natural Remanent Magnetization. While the magnetic hysteresis parameters of pyrrhotite are well known, the existing database concerning its anisotropy behaviour is patchy and ambiguous. Therefore, a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized. Before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring Curie temperatures, hysteresis loops, IRM acquisition curves, and FORC diagrams (the latter three all at room temperature). First the variation of the low-field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 A m −1 . Existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence. This enables estimating the grain size for unimodal pyrrhotite distributions in rocks. Measured trends are better fitted with an exponential function than with a Rayleigh Law style function. Based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples (of the original 11) were selected for the high-field anisotropy determinations within the basal plane. Those data were acquired with a torque cantilever-type magnetometer. As expected, most single crystals showed a pure 6-θ curve within their basal plane because of the easy axis configuration. In some crystals, however, lower harmonic terms overlapped the 6-θ term. This may be the dominant source of the observed variation in magnetic anisotropy properties. Torque data of three of the six samples were of sufficient quality to allow evaluation of K 1 . Re-evaluation of existing torque data and including the present newly derived determinations, yields for the anisotropy constant of pyrrhotite within the basal plane K 1 : (2.7 ± 0.2) 10 4 Jm −3 . This is over an order of magnitude more precise than the sparse existing K 1 data; only the value reported by Mikami and co-authors in 1959 agrees with the new determination. With this firmly established K 1 value meaningful anisotropy models are now possible for pyrrhotite-bearing rocks.Keywords: Magnetic and electrical properties; Magnetic fabrics and anisotropy; Magnetic mineralogy and petrology; Rock and mineral magnetism.

Transtension y transpresion del Jurasico Medio-Superior al Cretacico Inferior durante la construccion del arco magmatico en Chile central: evidencia a partir de enjambres de diques maficos.

“…7b). In these experiments, demagnetization close to 320°C in some samples of CrMDS indicate the presence of monoclinic or mixed monoclinic-hexagonal pyrrhotite (e.g., Martín-Hernández et al, 2008) which agrees with petrographic observations. Samples of the four sites of CrMDS show weak ferromagnetic behavior, with K m between 824 and 394x10 -6 [SI] (Fig.…”

Effects of Magnetic Anisotropy on Total Magnetic Field Anomalies

“…AMS was determined in a field of 200 A/m, which is higher than the geomagnetic field (approximately 40 A/m). Because some minerals, e.g., pyrrhotite (Martin‐Hernandez et al, ; Worm, ; Worm et al, ) or hematite (Guerrero‐Suarez & Martin‐Hernandez, ), have been reported to display field dependence of susceptibility and AMS, test measurements were made on a subset of samples in 5 A/m, 40 A/m, and 200 A/m. These showed no field dependence on either mean susceptibility, principal susceptibility directions, AMS degree, or AMS shape.…”

Fabrica Magnetica y Microestructuras a traves de los Andes de Tierra del Fuego, Argentina.

“…Therefore we infer the presence of pyrrhotite (mixture of monoclinic and hexagonal; Dunlop and Özdemir, 1997;Martín-Hernández et al, 2008). The rise of k around 500ºC probably corresponds to the breakdown of pyrrhotite into magnetite (Dunlop and Özdemir, 1997;Martín-Hernández et al, 2008). Consequently, it is inferred that the magnetic behavior of these samples (F4.3, F7.8 and Y7.3) is governed by pyrrhotite.…”

“…Petrovký and Kapička (2006) suggest that the temperature of a sharp Hopkinson peak can be used as the Curie point. Therefore we infer the presence of pyrrhotite (mixture of monoclinic and hexagonal; Dunlop and Özdemir, 1997;Martín-Hernández et al, 2008). The rise of k around 500ºC probably corresponds to the breakdown of pyrrhotite into magnetite (Dunlop and Özdemir, 1997;Martín-Hernández et al, 2008).…”