The Role of ε -Fe 2 O 3 Nano-Mineral and Domains in Enhancing Magnetic Coercivity: Implications for the Natural Remanent Magnetization

: A natural ε -Fe 2 O 3 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. state at the interface enhances the coercivity ﬁeld. TB: twin boundary and SF: stacking fault.


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 also 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]. The exsolution lamellae can enhance the remanent magnetization due to magnetic coupling at the contact layers. This has been proven through natural rock samples, synthetic experiments and thermodynamic calculations [1,[9][10][11][12]. However, the exact role of exsolution lamellae in enhancing the magnetic stability is still not clear.
Here, we report detailed reasons for explaining the natural remanent magnetic anomalies. A new magnetic nano-mineral of ε-Fe 2 O 3 (luogufengite, IMA 2016-005) was discovered in late Pleistocene basaltic scoria from Menan Volcanic Complex Idaho [13]. Luogufengite is a polymorph of maghemite and hematite with a large magnetic coercive field at room temperature [13,14]. In this paper, we found the luogufengite nano-minerals in other young basaltic rocks. In addition, the luogufengite-like nano-domains were found in natural permanent magnets (lodestone) and Fe-Ti oxides with high natural remanent magnetism. Our findings suggest that the unique magnetic properties of nano-minerals and nano-domains with high coercivity could explain the unusual remanent magnetization in igneous and metamorphic rocks. The observation can be an important contributor to constraints on the geomagnetic field in surface rocks of the Earth and other planets.

Samples
Three scoria samples and one olivine basalt with oxidized surface were collected for studying luogufengite nano-mineral: (1) Menan Volcanic Complex, Rexburg, Madison County, ID, USA [15], (2) Red Dome Lava Products Mine, Fillmore, UT, USA [16], (3) The Laguna del Maule Volcanic Field, San Clemente, Chile [17] and (4) Mauna Kea volcano, Hawaii County, HI, USA [18]. The scoria samples have a vesicular texture associated with the presence of external water during the explosive eruptions of basaltic lava (Figure 1a and Supplementary Materials Figure S1). Iron-bearing volcanic glass was oxidized to form reddish-brown iron oxide mixtures on the vesicle surfaces ( Figure 1a). Lodestone samples (Supplementary Materials Figure S2) were collected from a magnetite deposit near Cedar City, UT. Hematite lamellae and micro-precipitates in the magnetite host can be seen in thin section (Figure 1b). Fe-Ti oxides (ilmenite-magnetite series) were collected from the Skaergaard layered mafic intrusion in Eastern Greenland. The early stage of coarse ilmenite exsolution lamellae with fine-scale secondary lamellae was observed in the magnetite host of the Fe-Ti oxides (Figure 1c and Supplementary Materials Figure S3). Pleistocene basaltic scoria from Menan Volcanic Complex Idaho [13]. Luogufengite is a polymorph of maghemite and hematite with a large magnetic coercive field at room temperature [13,14]. In this paper, we found the luogufengite nano-minerals in other young basaltic rocks. In addition, the luogufengite-like nano-domains were found in natural permanent magnets (lodestone) and Fe-Ti oxides with high natural remanent magnetism. Our findings suggest that the unique magnetic properties of nano-minerals and nano-domains with high coercivity could explain the unusual remanent magnetization in igneous and metamorphic rocks. The observation can be an important contributor to constraints on the geomagnetic field in surface rocks of the Earth and other planets.

Samples
Three scoria samples and one olivine basalt with oxidized surface were collected for studying luogufengite nano-mineral: (1) Menan Volcanic Complex, Rexburg, Madison County, ID, USA [15], (2) Red Dome Lava Products Mine, Fillmore, UT, USA [16], (3) The Laguna del Maule Volcanic Field, San Clemente, Chile [17] and (4) Mauna Kea volcano, Hawaii County, HI, USA [18]. The scoria samples have a vesicular texture associated with the presence of external water during the explosive eruptions of basaltic lava (Figure 1a and Supplementary Materials Figure S1). Iron-bearing volcanic glass was oxidized to form reddish-brown iron oxide mixtures on the vesicle surfaces ( Figure 1a). Lodestone samples (Supplementary Materials Figure S2) were collected from a magnetite deposit near Cedar City, UT. Hematite lamellae and micro-precipitates in the magnetite host can be seen in thin section (Figure 1b). Fe-Ti oxides (ilmenite-magnetite series) were collected from the Skaergaard layered mafic intrusion in Eastern Greenland. The early stage of coarse ilmenite exsolution lamellae with fine-scale secondary lamellae was observed in the magnetite host of the Fe-Ti oxides (Figure 1c and Supplementary Materials Figure S3).

Enrichment of Luogufengite
The samples were carefully scratched off from the vesicles' surfaces on the collected basaltic scoria. These samples were placed in a 10 M NaOH solution at 80 °C for 2 days to remove silicate glass, following the previous procedures of synthetic ε-Fe2O3 [13,19]. After washing the powder with distilled water several times, the luogufengite crystals were collected by using a weak magnetic bar, separating non-magnetic minerals (hematite and silicate minerals) from the magnetic minerals. The luogufengite was further enriched by using an iron needle to pick up magnetized crystals with strong remanent magnetism. The luogufengite nano-crystals are preferentially attached to the iron needle due to their the remanent magnetic property. These magnetic enrichment steps were repeated 5-7

Enrichment of Luogufengite
The samples were carefully scratched off from the vesicles' surfaces on the collected basaltic scoria. These samples were placed in a 10 M NaOH solution at 80 • C for 2 days to remove silicate glass, following the previous procedures of synthetic ε-Fe 2 O 3 [13,19]. After washing the powder with distilled water several times, the luogufengite crystals were collected by using a weak magnetic bar, separating non-magnetic minerals (hematite and silicate minerals) from the magnetic minerals. The luogufengite was further enriched by using an iron needle to pick up magnetized crystals with strong remanent magnetism. The luogufengite nano-crystals are preferentially attached to the iron needle due to their the remanent magnetic property. These magnetic enrichment steps were repeated 5-7 times to enrich the concentration of luogufengite nano-mineral. However, the powder sample still contains other nano-minerals (nano-sized hematite, maghemite and valleyite) (Figure 2a).

Techniques
We acquired XRD results from powdered samples placed inside Kapton tubes. XRD data were collected on a 2-D image-plate detector (Rigaku, Tokyo, Japan) using a Rigaku Rapid II instrument (Mo-Kα radiation) in the Geoscience Department at the University of Wisconsin-Madison. Two-dimensional diffraction patterns were converted to conventional 2θ vs. intensity XRD powder patterns using the Rigaku 2DP software (Rigaku, Tokyo, Japan). The quantitative ratios of mineral phases were calculated with the Rietveld refinement method by using TOPAS 5 software (Bruker AXS, Madison, WI, USA). A pseudo-Voigt method was used for fitting the peak profiles. Scanning electron microscope (SEM) (Hitachi, Tokyo, Japan) analysis samples were mounted onto glass slides, polished and coated with carbon (~200 nm). All SEM images were obtained using a Hitachi S3400N variable pressure microscope with an X-ray energy-dispersive spectroscopy (EDS) (Thermo Fisher Scientific, Waltham, MA, USA) system in the Geoscience Department at the University of Wisconsin-Madison. The bright-field transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images and selected-area electron diffraction (SAED) patterns were obtained using a Philips CM200-UT microscope operated (Philips, Amsterdam, The Netherlands) at 200 kV in the Materials Science Center at the University of Wisconsin-Madison. TEM samples were prepared both by depositing a suspension of crushed grains on a lacy carbon-coated TEM Cu-grids. Ion milled TEM sample was prepared by using a Fischione 1050 ion milling system. Magnetic hysteresis loops were measured by using a superconducting quantum interference device (SQUID) MPMS3 magnetometer Design (Quantum Design, San Diego, CA, USA) in the Chemistry Department at the University of Wisconsin-Madison. The powder and rock samples were measured with applied magnetic fields between −2 T (Tesla) and 2 T at room temperature.

Luogufengite Nano-Mineral with Giant Coercive Field
Luogufengite was first discovered in a late Pleistocene basaltic scoria from the Menan Volcanic Complex, Idaho using synchrotron powder X-ray powder diffraction and high-resolution TEM [13]. Luogufengite is a dark brown nano-mineral of the Fe 2 O 3 polymorph [13,20]. Oxidation of Fe-bearing volcanic glass resulted in the formation of luogufengite nano-minerals on the vesicle surfaces of scoria associated with maghemite (γ-Fe 2 O 3 ) and hematite (α-Fe 2 O 3 ). Luogufengite is considered an intermediate phase between maghemite and hematite [21,22]. The phase transformations from maghemite to hematite via luogufengite are associated with size-dependent changes of structures from cubic closest packing (ABC) to doubled hexagonal packing (ABAC) to hexagonal closest packing (AB) (Supplementary Materials Figure S4) [21]. Synthetic ε-Fe 2 O 3 nano-crystals are a promising magnetic material for technological applications due to its large coercive field value at room temperature that is not observed in other simple metal oxide magnets [19,22]. The large coercivity of ε-Fe 2 O 3 is associated with the nonzero orbital momentum and large magneto-crystalline anisotropy in Fe 3+ polyhedral [14,22]. Recently, synthetic ε-Fe 2 O 3 was also identified in archeological black glazed Jian ware from China [23] and baked clay block from Spain [8].
A particle size of about 100 nm is the most suitable for a large coercivity to be present [19].   Figure S1). This observation suggests that the luogufengite is a widely distributed magnetic mineral in high-temperature volcanic rocks. The sizes and shapes observed in these other luogufengite nano-minerals are similar to that from Menan Volcanic Complex [13]. Luogufengite could be an important mineral for recording paleomagnetism of volcanic rocks. Its large magnetic coercivity allows for preservation of the original magnetic field during mineral formation and cooling.    Figure S1). This observation suggests that the luogufengite is a widely distributed magnetic mineral in high-temperature volcanic rocks. The sizes and shapes observed in these other luogufengite nano-minerals are similar to that from Menan Volcanic Complex [13]. Luogufengite could be an important mineral for recording paleomagnetism of volcanic rocks. Its large magnetic coercivity allows for preservation of the original magnetic field during mineral formation and cooling.  Figure S1). This observation suggests that the luogufengite is a widely distributed magnetic mineral in high-temperature volcanic rocks. The sizes and shapes observed in these other luogufengite nano-minerals are similar to that from Menan Volcanic Complex [13]. Luogufengite could be an important mineral for recording paleomagnetism of volcanic rocks. Its large magnetic coercivity allows for preservation of the original magnetic field during mineral formation and cooling.

Luogufengite-Like Nano-Domains in Lodestone and Fe-Ti Oxides
Many studies have revealed that exsolution lamellae are an important contributor to the unusual remanent magnetization in slow cooling igneous and metamorphic rocks [1,2,4,12]. However, the role of exsolution lamellae for the coercivity and remanent magnetization is not well understood. To understand these unusual magnetic properties, we examined lodestone and Fe-Ti oxide (ilmenite-magnetite series).
The lodestone sample analyzed (Figure 1b and Supplementary Materials Figure S2) is a natural permanent magnet. It has partially oxidized magnetite intergrown with hematite and maghemite [34,35]. Rietveld refinement analysis of a hematite-bearing lodestone shows 64.3(4) wt % of magnetite with 26.9(3) wt % of maghemite and 8.7(4) wt % of hematite (Figure 2b). Interestingly, TEM images show aligned nanoscale exsolution lamellae of hematite and magnetite (Figure 5a). HRTEM images show the interfaces {111} Mgt //(0001) Hem of the nano-lamellae (Figure 5b). Many previous studies reported that this is the common interface between cubic magnetite and rhombohedral hematite, followed by oxygen packing direction of both iron oxides [1,10]. Another interesting observation of the TEM image is that host magnetite often shows stacking faults and twin boundaries related to {101} planes at the [111]-zone-axis (Figure 6a,b).

Luogufengite-Like Nano-Domains in Lodestone and Fe-Ti Oxides
Many studies have revealed that exsolution lamellae are an important contributor to the unusual remanent magnetization in slow cooling igneous and metamorphic rocks [1,2,4,12]. However, the role of exsolution lamellae for the coercivity and remanent magnetization is not well understood. To understand these unusual magnetic properties, we examined lodestone and Fe-Ti oxide (ilmenitemagnetite series).
The lodestone sample analyzed (Figure 1b and Supplementary Materials Figure S2) is a natural permanent magnet. It has partially oxidized magnetite intergrown with hematite and maghemite [34,35]. Rietveld refinement analysis of a hematite-bearing lodestone shows 64.3(4) wt % of magnetite with 26.9(3) wt % of maghemite and 8.7(4) wt % of hematite (Figure 2b). Interestingly, TEM images show aligned nanoscale exsolution lamellae of hematite and magnetite (Figure 5a). HRTEM images show the interfaces {111}Mgt//(0001)Hem of the nano-lamellae (Figure 5b). Many previous studies reported that this is the common interface between cubic magnetite and rhombohedral hematite, followed by oxygen packing direction of both iron oxides [1,10]. Another interesting observation of the TEM image is that host magnetite often shows stacking faults and twin boundaries related to {101} planes at the [111]-zone-axis (Figure 6a,b).  To test the interface's effect on the magnetism of lodestone, we prepared three lodestone samples with different hematite concentrations (Supplementary Materials Figure S5). Magnetic hysteresis loops from the samples clearly suggest that the magnetic coercivity is proportional to the concentration of hematite (Figure 7). The lodestone sample containing 8.7(4) wt % hematite has a coercive field of 46.7 mT at room temperature. Pure magnetite from natural rocks produced a 17.5 mT field with a grain size of 37 nm and 13.3 mT with a 100 nm grain size at room temperature [31]. From this, we can see that the interface with ABAC packing sequence enhances the coercivity field of lodestone since hematite has very weak ferromagnetic properties.  To test the interface's effect on the magnetism of lodestone, we prepared three lodestone samples with different hematite concentrations (Supplementary Materials Figure S5). Magnetic hysteresis loops from the samples clearly suggest that the magnetic coercivity is proportional to the concentration of hematite (Figure 7). The lodestone sample containing 8.7(4) wt % hematite has a coercive field of 46.7 mT at room temperature. Pure magnetite from natural rocks produced a 17.5 mT field with a grain size of 37 nm and 13.3 mT with a 100 nm grain size at room temperature [31]. From this, we can see that the interface with ABAC packing sequence enhances the coercivity field of lodestone since hematite has very weak ferromagnetic properties. To test the interface's effect on the magnetism of lodestone, we prepared three lodestone samples with different hematite concentrations (Supplementary Materials Figure S5). Magnetic hysteresis loops from the samples clearly suggest that the magnetic coercivity is proportional to the concentration of hematite (Figure 7). The lodestone sample containing 8.7(4) wt % hematite has a coercive field of 46.7 mT at room temperature. Pure magnetite from natural rocks produced a 17.5 mT field with a grain size of 37 nm and 13.3 mT with a 100 nm grain size at room temperature [31]. From this, we can see that the interface with ABAC packing sequence enhances the coercivity field of lodestone since hematite has very weak ferromagnetic properties.  Ilmenite-magnetite series of Fe-Ti oxides (Figure 1c), the ilmenite exsolution lamellae in host magnetite are associated with oxidative exsolution during cooling [1,10]. Rietveld refinement analysis of the Fe-Ti oxide sample shows 79.2(5) wt % of magnetite with the 13.3(3) wt % of ilmenite and 7.5(5) wt % of spinel (Figure 2c). Similarly, TEM images of the Fe-Ti oxide sample also show aligned lamellae of ilmenite in host magnetite, displaying an interfacial relationship of 111 Mgt //(0001) Ilm (Figure 5c,d).
Previous studies have attributed natural remanent magnetization to the common interface between the (0001) planes of the rhombohedral oxide and the (111) planes of the cubic oxide [1,4,7]. Interestingly, the interface between cubic and rhombohedral oxides can produce luogufengite-like 2-D crystals or domains with a doubled hexagonal structure (ABAC packing sequence) (Figure 8a). In addition, stacking faults and twin boundaries in magnetite and maghemite can also generate luogufengite-like layer domains with the ABAC stacking sequence locally (Figure 8b,c). We suggest that the structure of luogufengite-like nano-domains at the interface or within a mineral could enhance the magnetic coercive field.
The multi-layered structure of lodestone and Fe-Ti oxides plays an essential role to enhance the coercivity and to preserve remanent magnetization (Figure 5a,c). The magnetic easy axis of luogufengite is reported to be the a-axis [22], which corresponds to the longitudinal axis of the multilayers of Figure 5a,c. Thus, the multi-layers are parallel to the external magnetic field that contributes to the coercive field strength [33,36]. Studies of synthetic magnetic materials have reported that the larger value of coercivity could be achieved at the interface between magnetically different layers where the multilayer materials were oriented along the magnetic easy axis [33,36,37]. Ilmenite-magnetite series of Fe-Ti oxides (Figure 1c), the ilmenite exsolution lamellae in host magnetite are associated with oxidative exsolution during cooling [1,10]. Rietveld refinement analysis of the Fe-Ti oxide sample shows 79.2(5) wt % of magnetite with the 13.3(3) wt % of ilmenite and 7.5(5) wt % of spinel (Figure 2c). Similarly, TEM images of the Fe-Ti oxide sample also show aligned lamellae of ilmenite in host magnetite, displaying an interfacial relationship of 111Mgt//(0001)Ilm (Figure 5c,d).
Previous studies have attributed natural remanent magnetization to the common interface between the (0001) planes of the rhombohedral oxide and the (111) planes of the cubic oxide [1,4,7]. Interestingly, the interface between cubic and rhombohedral oxides can produce luogufengite-like 2-D crystals or domains with a doubled hexagonal structure (ABAC packing sequence) (Figure 8a). In addition, stacking faults and twin boundaries in magnetite and maghemite can also generate luogufengite-like layer domains with the ABAC stacking sequence locally (Figure 8b,c). We suggest that the structure of luogufengite-like nano-domains at the interface or within a mineral could enhance the magnetic coercive field.
The multi-layered structure of lodestone and Fe-Ti oxides plays an essential role to enhance the coercivity and to preserve remanent magnetization (Figure 5a,c). The magnetic easy axis of luogufengite is reported to be the a-axis [22], which corresponds to the longitudinal axis of the multilayers of Figure 5a,c. Thus, the multi-layers are parallel to the external magnetic field that contributes to the coercive field strength [33,36]. Studies of synthetic magnetic materials have reported that the larger value of coercivity could be achieved at the interface between magnetically different layers where the multilayer materials were oriented along the magnetic easy axis [33,36,37].   (Figure 9a). The domains/interfaces play an essential role in enhancing the magnetic coercivity of lodestone and Fe-Ti oxides (Figure 9b). The interfaces in the multi-layer texture are also associated with the magnetic coupling combined with exchange-spring state between soft and hard ferromagnets that lead to enhancing the coercive field [38,39], although the volume ratio of   (Figure 9a). The domains/interfaces play an essential role in enhancing the magnetic coercivity of lodestone and Fe-Ti oxides (Figure 9b). The interfaces in the multi-layer texture are also associated with the magnetic coupling combined with exchange-spring state between soft and hard ferromagnets that lead to enhancing the coercive field [38,39], although the volume ratio of luogufengite-like nano-domains at the interface is minor (Figure 9c). For example, the magnetic coercivity of the exchange-coupled isotropic FePt(hard)-Fe 3 Pt(soft) nanocomposites exceeds the theoretical limit of non-exchange-coupled FePt by over 50% [40]. luogufengite-like nano-domains at the interface is minor (Figure 9c). For example, the magnetic coercivity of the exchange-coupled isotropic FePt(hard)-Fe3Pt(soft) nanocomposites exceeds the theoretical limit of non-exchange-coupled FePt by over 50% [40]. The paleomagnetism requires long relaxation time that retains the magnetization over geological time-scale. The relaxation time is generally proportional to the coercivity and saturation magnetization [41,42]. The relaxation times and magnetic properties can be also changed as a function of domain states and size [42]. The relaxation time of multi-domain grains increases with decreasing grain size, while that of single-domain increases with increasing grain size [41,42]. Thus, the nanoscaled multilayer structure of lodestone and Fe-Ti oxides can increase coercivity and help to preserve the saturated magnetization of rocks with the long relaxation time.
NASA's Mars Global Surveyor spacecraft observed the localized magnetic anomalies on the Mars surface [5]. Especially, Noachian crust (3.7-4.1 Ga) of the southern hemisphere of Mars shows the strongest remanent magnetism in some locations ~20 times greater than Earth, although Mars currently does not possess a core dynamo [43,44]. The candidate minerals responsible for the strong remanent magnetism on the Mars surface are magnetite, pyrrhotite, multidomain hematite, titanohematite and hemoilmenite [45,46]. We suggest that the luogufengite could be a magnetic phase in the basaltic crust on Mars. In addition, the interface, stacking faults and twinning boundary of magnetic minerals may contribute to preserving the natural remanent magnetization on Mars surface.

Conclusions
A natural ε-Fe2O3 nano-mineral (luogufengite) with large coercivity was found in young basaltic rocks. Our observations suggest that luogufengite is a widely distributed magnetic mineral in hightemperature volcanic rocks. We think that this nano-mineral can be an important indicator of paleomagnetism in volcanic systems. Luogufengite-like nano-domains were also observed at the interfaces in lodestone and Fe-Ti oxide with strong coercivity and remanent magnetization. The multilayer of nano-texture paralleled to the magnetic easy axis plays an important role in enhancing the magnetic coercivity. Stacking faults and twin boundaries can also produce luogufengite-like layer domains to help to increase the coercivity. These observations can provide an explanation for coercivity and strong remanent magnetization in slow cooling igneous and metamorphic rocks. We believe that this is a good example of using the nanostructure to describe distinctive mineral properties in natural systems. The observation of nano-minerals and nano-domains certainly helps us to better understand anomalous magnetic properties found on Earth and other planetary systems. The paleomagnetism requires long relaxation time that retains the magnetization over geological time-scale. The relaxation time is generally proportional to the coercivity and saturation magnetization [41,42]. The relaxation times and magnetic properties can be also changed as a function of domain states and size [42]. The relaxation time of multi-domain grains increases with decreasing grain size, while that of single-domain increases with increasing grain size [41,42]. Thus, the nano-scaled multilayer structure of lodestone and Fe-Ti oxides can increase coercivity and help to preserve the saturated magnetization of rocks with the long relaxation time.
NASA's Mars Global Surveyor spacecraft observed the localized magnetic anomalies on the Mars surface [5]. Especially, Noachian crust (3.7-4.1 Ga) of the southern hemisphere of Mars shows the strongest remanent magnetism in some locations~20 times greater than Earth, although Mars currently does not possess a core dynamo [43,44]. The candidate minerals responsible for the strong remanent magnetism on the Mars surface are magnetite, pyrrhotite, multidomain hematite, titanohematite and hemoilmenite [45,46]. We suggest that the luogufengite could be a magnetic phase in the basaltic crust on Mars. In addition, the interface, stacking faults and twinning boundary of magnetic minerals may contribute to preserving the natural remanent magnetization on Mars surface.

Conclusions
A natural ε-Fe 2 O 3 nano-mineral (luogufengite) with large coercivity was found in young basaltic rocks. Our observations suggest that luogufengite is a widely distributed magnetic mineral in high-temperature volcanic rocks. We think that this nano-mineral can be an important indicator of paleomagnetism in volcanic systems. Luogufengite-like nano-domains were also observed at the interfaces in lodestone and Fe-Ti oxide with strong coercivity and remanent magnetization. The multilayer of nano-texture paralleled to the magnetic easy axis plays an important role in enhancing the magnetic coercivity. Stacking faults and twin boundaries can also produce luogufengite-like layer domains to help to increase the coercivity. These observations can provide an explanation for coercivity and strong remanent magnetization in slow cooling igneous and metamorphic rocks. We believe that this is a good example of using the nanostructure to describe distinctive mineral properties in natural systems. The observation of nano-minerals and nano-domains certainly helps us to better understand anomalous magnetic properties found on Earth and other planetary systems.