The Role of ε-Fe2O3 Nano-Mineral and Domains in Enhancing Magnetic Coercivity: Implications for the Natural Remanent Magnetization

Seungyeol Lee,Huifang Xu

doi:10.3390/min8030097

Seungyeol Lee, Huifang Xu

Open Access

https://doi.org/10.3390/min8030097

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Abstract

A natural ε-Fe2O3 nano-mineral (luogufengite) has been discovered in young basaltic rocks around the world. Transmission electron microscopy (TEM) observed euhedral or subhedral luogufengite nano-minerals with crystal sizes ranging from 10 to 120 nm in the basaltic rocks. The magnetic property of treated scoria sample (containing 75.3(5) wt % luogufengite) showed a saturation remanence of 11.3 emu g−1 with a coercive field of 0.17 tesla (T) at room temperature. Luogufengite-like nano-domains were also observed in natural permanent magnets (lodestone) and Fe-Ti oxides (ilmenite-magnetite series) with strong remanent magnetization. The structure of luogufengite-like domains (double hexagonal close-packing) is associated with the interfaces between the (111) plane of cubic magnetite and the (0001) plane of rhombohedral hematite or ilmenite. Stacking faults and twin boundaries of magnetite/maghemite can also produce the luogufengite-like domains. The nano-domains oriented along the magnetic easy axis play an essential role in enhancing the magnetic coercivity of lodestone and Fe-Ti oxide. We conclude that the luogufengite nano-minerals and nano-domains provide an explanation for coercivity and strong remanent magnetization in igneous, metamorphic rocks and even some reported Martian rocks. These nano-scaled multilayer structures extend our knowledge of magnetism and help us to understand the diverse magnetic anomalies occurring on Earth and other planetary bodies.

Highlights

Numerous studies have reported strong remanent magnetization from igneous and metamorphic rocks in the Earth’s crust [1,2,3,4]
Oxidation of Fe-bearing volcanic glass resulted in the formation of luogufengite nano-minerals on the vesicle surfaces of scoria associated with maghemite (γ-Fe2 O3 ) and hematite (α-Fe2 O3 )
Our observations suggest that luogufengite is a widely distributed magnetic mineral in highrocks

Summary

Introduction

Numerous studies have reported strong remanent magnetization from igneous and metamorphic rocks in the Earth’s crust [1,2,3,4]. The Mars Global Surveyor spacecraft found similar unusual remanent magnetization on the Martian crust [5,6]. The preservation of strong remanent magnetization requires high magnetic stability and coercivity. The problem is that natural remanent magnetization cannot be explained by the properties of individual magnetic minerals only because none of them are high coercivity phases [1,4,7,8]. To understand the remanent magnetic anomalies of rocks, we need to identify the mechanisms that influence their magnetic properties. Previous studies have attributed these properties to fine exsolution microstructures related to local redox conditions and slow cooling history of rock [1,2,4]

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