The lepidocrocite-maghemite-haematite reaction chain-I. Acquisition of chemical remanent magnetization by maghemite, its magnetic properties and thermal stability

Abstract

SUMMARY We report on the magnetic properties and the acquisition of a chemical remanent magnetization (CRM) in a field of 100 μT as a function of temperature and time during the lepidocrocite–maghemite–haematite reaction chain. The development of CRM was monitored at a series of 13 temperatures ranging from 175 to 550 °C; data acquisition was done at the specific formation temperatures for durations of up to 500 hr. Up to acquisition temperatures of 200 °C it takes a considerable time (up to 7 hr) before the CRM is measurable. This time decreases with increasing temperature, reflecting the activation energy of the reaction to form the first maghemite. During the lepidocrocite conversion, formation of two types of maghemite is suggested by two peaks in the CRM versus time curves. Magnetic properties were analysed after various stages in the reaction. They indicate a mixture of superparamagnetic and single‐domain maghemite. The first reaction product (obtained after annealing at 200 °C) is a fine‐grained yet crystalline maghemite (labelled type A). Before massive maghemite formation occurs, the coercive and remanent coercive forces go through a minimum at intermediate temperatures of 250–300 °C (annealing for 2.5 hr). This minimum lowers to 200–250 °C with increasing annealing time (500 hr). This is probably the result of two processes acting simultaneously—formation of superparamagnetic maghemite particles of a second less crystalline maghemite type (labelled type B) and removal of stacking faults in type A maghemite. The second process is suggested by analogy to the behaviour of natural magnetite/maghemite systems on annealing. Removal of stacking faults is reported to result in a magnetic softening of the grain assemblage. Annealing at 300–350 °C removes most of the lepidocrocite and the second maghemite type, type B, becomes prominent. Haematite formation sets in at slightly higher temperatures, yet the type B maghemite is in part thermally stable up to 600 °C enabling Thellier–Thellier experiments. This stability is also inferred from Arrhenius fitting that shows a comparatively high activation energy for the maghemite to haematite reaction. In Thellier–Thellier experiments the CRM showed a markedly downward convex Arai–Nagata plot while a second thermoremanent magnetization (TRM) showed perfect linear behaviour as expected. This feature may be used to recognize CRM in natural rocks.

Rock magnetic and paleointensity results from Mesozoic baked contacts of Armenia

“…Moreover, the only direct Thellier experiments with TCRM and TRM that have been published in the literature and performed on SD maghemite grains precipitated in lepidocrocite grains showed a drastic non-similarity of the AraiNagata diagrams of these two remanences (Gendler et al, 2005). Thus, we conclude that the strong similarity of the NRM-TRM plots and the good quality of the Arai-Nagata …”

Acquisition of chemical remanent magnetization during experimental ferrihydrite–hematite conversion in Earth-like magnetic field—implications for paleomagnetic studies of red beds

“…Özdemir and Dunlop (1993) studied the CRM accompanying the phase transformations lepidocrocite → maghemite → hematite in a field of 50 µT from 200 to 650 o C. They observed two CRM intensity peaks around 275 o C and 400 o C which were also detected by subsequent studies of Gendler et al (2005), where transformations lepidocrocite → maghemite and maghemite → hematite occur respectively, and CRM directions are always at a large angle to the applied field.…”

“…To further our understanding of CRM in red beds, the acquisition process of CRM in hematite has been simulated experimentally in the laboratory (Hedley, 1968;Bailey and Hale, 1981;Stokking and Tauxe, 1987,1990a, 1990bÖzdemir and Dunlop, 1993;Gendler et al, 2005). Two types of CRM are distinguished growth-CRM and alteration-CRM: the first involves growing of the magnetic minerals through the SP threshold; the second implies the alteration of an existing magnetic mineral to another magnetic mineral at a temperature below the magnetic ordering temperature.…”

Constraining the Origins of the Magnetism of Lepidocrocite (γ-FeOOH): A Mössbauer and Magnetization Study

“…Among these minerals, lepidocrocite (γ-FeOOH) has been less studied from a magnetism point of view than other iron-bearing minerals. It is commonly found in hydromorphic soils where there is seasonal alternation of reducing and oxidizing conditions, and has been shown to be a precursor of more magnetic phases such as maghemite or magnetite (eg., Fitzpatrick et al, 1985;Gehring and Hofmeister, 1994;Cornell and Schwertmann, 2003;Gendler et al, 2005;Till et al, 2014). Lepidocrocite is a ferric oxyhydroxide, orange in color, with an orthorhombic crystal structure (e.g., Ewing, 1935).…”