Fe2o3 Nano-Mineral and Domains in Enhancing Magnetic Coercivity: Implications for the Natural Remanent Magnetization

minerals

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

Seungyeol Lee ID and Huifang Xu * Department of Geoscience, NASA Astrobiology Institute, University of Wisconsin—Madison, Madison, WI 53706, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-608-265-5887

Received: 27 January 2018; Accepted: 26 February 2018; Published: 2 March 2018

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.

Keywords: luogufengite; lodestone; Fe-Ti oxide; coercivity; natural remanent magnetism

1. Introduction Numerous studies have reported strong remanent magnetization from igneous and metamorphic rocks in the Earth’s crust [1–4]. 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 [1,9–12]. 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-Fe2O3 (luogufengite, IMA 2016-005) was discovered in late

Minerals 2018, 8, 97; doi:10.3390/min8030097 www.mdpi.com/journal/minerals Minerals 2018, 8, 97 2 of 12

Minerals 2018, 8, x FOR PEER REVIEW 2 of 12 Pleistocene basaltic scoria from Menan Volcanic Complex Idaho [13]. Luogufengite is a polymorph of maghemitePleistocene basaltic and hematite scoria from with aMenan large magneticVolcanic Complex coercive Idaho field at[13]. room Luogufengite temperature is a [ 13polymorph,14]. In this paper,of maghemite we found and the hematite luogufengite with a nano-mineralslarge magnetic coercive in other field young at room basaltic temperature rocks. In[13,14]. addition, In this the luogufengite-likepaper, we found nano-domains the luogufengite were nano-minerals found in natural in other permanent young basaltic magnets rocks. (lodestone) In addition, and the Fe-Ti oxidesluogufengite-like with high natural nano-domains remanent were magnetism. found in natural Our findings permanent suggest magnets that (lodestone) the unique and magnetic Fe-Ti propertiesoxides with of nano-mineralshigh natural remane and nano-domainsnt magnetism. withOur findings high coercivity suggest couldthat the explain unique the magnetic unusual remanentproperties magnetization of nano-minerals in igneous and nano-domains and metamorphic with rocks.high coercivity The observation could explain can be the an importantunusual contributorremanent tomagnetization constraints on in theigneous geomagnetic and metamorphi field in surfacec rocks. rocksThe observation of the Earth can and be other an important planets. contributor to constraints on the geomagnetic field in surface rocks of the Earth and other planets. 2. Samples 2. Samples Three scoria samples and one olivine basalt with oxidized surface were collected for studying luogufengiteThree nano-mineral:scoria samples and (1) Menanone olivine Volcanic basalt Complex, with oxidized Rexburg, surface Madison were collected County, for ID, studying USA [15 ], (2)luogufengite Red Dome Lava nano-mineral: Products Mine,(1) Menan Fillmore, Volcanic UT, Co USAmplex, [16], Rexburg, (3) The Laguna Madison del County, Maule ID, Volcanic USA [15], Field, San(2) Clemente, Red DomeChile Lava Products [17] and Mine, (4) Mauna Fillmore, Kea UT, volcano, USA [16], Hawaii (3) The County, Laguna del HI, Maule USA [Volcanic18]. The Field, scoria samplesSan Clemente, have a vesicular Chile [17] texture and (4) associated Mauna Kea with volc theano, presence Hawaii of externalCounty, waterHI, USA during [18]. theThe explosive scoria eruptionssamples of have basaltic a vesicular lava (Figure texture1 associateda and Supplementary with the presence Materials of external Figure water S1). during Iron-bearing the explosive volcanic 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). 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 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 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 thin section (Figure 1b). Fe-Ti oxides (ilmenite-magnetite series) were collected from the Skaergaard layeredlayered mafic mafic intrusion intrusion in in Eastern Eastern Greenland. Greenland. TheThe earlyearly stage of of coarse coarse ilmenite ilmenite exsolution exsolution lamellae lamellae withwith fine-scale fine-scale secondary secondary lamellae lamellae was was observed observed inin thethe magnetite host host of of the the Fe-Ti Fe-Ti oxides oxides (Figure (Figure 1c 1c andand Supplementary Supplementary Materials Materials Figure Figure S3). S3).

Figure 1. (a) A thin section of a scoria sample from the Menan volcanic Complex, Idaho under Figure 1. (a) A thin section of a scoria sample from the Menan volcanic Complex, Idaho under reﬂected reflected light mode. The reddish Fe-oxides are coated on the vesicles’ surfaces. Groundmass contains light mode. The reddish Fe-oxides are coated on the vesicles’ surfaces. Groundmass contains platy platy labradorites. (b) Lodestone section showing hematite (Hem) precipitates (bright) in host labradorites. (b) Lodestone section showing hematite (Hem) precipitates (bright) in host magnetite magnetite (Mgt). (c) A BSE image of a Fe-Ti oxides section showing the early stage of coarse ilmenite (Mgt). (c) A BSE image of a Fe-Ti oxides section showing the early stage of coarse ilmenite (Ilm) lamellae (Ilm) lamellae and the late stage of ilmenite exsolution lamellae with spinel (Spn) in host magnetite. and the late stage of ilmenite exsolution lamellae with spinel (Spn) in host magnetite. 3. Experimental Methods 3. Experimental Methods 3.1. Enrichment of Luogufengite 3.1. Enrichment of Luogufengite The samples were carefully scratched off from the vesicles’ surfaces on the collected basaltic scoria.The samplesThese samples were carefullywere placed scratched in a 10 M off NaOH from thesolution vesicles’ at 80 surfaces °C for 2 ondays the to collectedremove silicate basaltic ◦ scoria.glass, These following samples the previous were placed procedures in a 10 of M synthetic NaOH ε solution-Fe2O3 [13,19]. at 80 AfterC for washing 2 days the to remove powder silicatewith glass,distilled following water theseveral previous times, proceduresthe luogufengite of synthetic crystals wereε-Fe 2collectedO3 [13,19 by]. using After a washingweak magnetic the powder bar, withseparating distilled non-magnetic water several minerals times, the (hematite luogufengite and silicate crystals minerals) were collected from the by magnetic using a minerals. weak magnetic The bar,luogufengite separating was non-magnetic further enriched minerals by using (hematite an iron and needle silicate to pick minerals) up magnetized from the crystals magnetic with minerals. strong Theremanent luogufengite magnetism. was further The luogufengite enriched by nano-crystal using an irons are needle preferentially to pick upattached magnetized to the iron crystals needle with strongdue remanentto their the magnetism. remanent magnetic The luogufengite property. Thes nano-crystalse magnetic areenrichment preferentially steps were attached repeated to the 5–7 iron

Minerals 2018, 8, 97 3 of 12 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).

3.2. 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.

4. Results and Discussion

4.1. 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 Fe2O3 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-Fe2O3) and hematite (α-Fe2O3). 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-Fe2O3 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-Fe2O3 is associated with the nonzero orbital momentum and large magneto-crystalline anisotropy in Fe3+ polyhedral [14,22]. Recently, synthetic ε-Fe2O3 was also identified in archeological black glazed Jian ware from China [23] and baked clay block from Spain [8]. Rietveld refinement analysis of the treated scoria sample from Menan complex shows 75.3(5) wt % luogufengite, 15.5(3) wt % hematite and 9.2(5) wt % valleyite (IMA 2017-026) [24] (Figure 2a). It is difficult to obtain pure luogufengite due to coexisting hematite intergrowths that form with luogufengite [19,21,22]. TEM images show that the size of luogufengite ranges from ~10 to 120 nm (Figure 3). The average size of natural luogufengite particles is ~40 nm in size (Figure 3). The luogufengite nano-mineral generally converts to the hematite polymorph once the crystal size exceeds Minerals 2018, 8, 97 4 of 12

a value of ~120 nm [21]. However, large ε-Fe2O3 nanocrystals, with sizes up to ~200 nm, can be stable withinMinerals a silica 2018, 8 matrix, x FOR PEER [19,21 REVIEW]. In HRTEM images, the pseudo-hexagonal twin relationships and stacking4 of 12 faults along the b-direction are frequently observed in the luogufengite nano-minerals, producing complicatedproducing complicated magnetic properties magnetic andproperties large coercive and large ﬁeld coercive (Figure field3 d–f(Figure and 3d–f Supplementary and Supplementary Materials FigureMaterials S1) [ 14Figure,21,25 S1)]. [14,21,25].

FigureFigure 2. 2.Powder Powder XRD XRD patterns patternsof of ((aa)) aa treated Fe-oxides Fe-oxides sample sample of of scoria scoria from from Menan Menan complex, complex, Idaho; Idaho; (b)(b a) lodestone a lodestone sample sample from from magnetite magnetite deposit, deposit, Utah; Utah; (c) a ( Fe-Tic) a Fe-Ti oxides oxides sample sample with magnetite-ilmenitewith magnetite- seriesilmenite from series Skaergaard from Skaergaard layered maﬁc layered intrusion, mafic East intrusion, Greenland. East PercentagesGreen land. ofPercentages minerals were of minerals calculated bywere the Rietveld calculated method by the withRietveld structure method models with structure [13,24,26 models–30]. [13,24,26–30].

The magnetic hysteresis loop of the treated sample (containing 75.3(5) wt % luogufengite) The magnetic hysteresis loop of the treated sample (containing 75.3(5) wt % luogufengite) showed showed a saturation remanence of 11.3 emu g−1 with a coercive field of 0.17 tesla (T) at room · −1 a saturationtemperature remanence (Figure 4). of The 11.3 magnetic emu g phasewith of a valle coerciveyite (~9.2 field wt of 0.17%) also tesla influenced (T) at room the temperaturehysteresis (Figureloop 4[24]). The (Figure magnetic 4). This phase coercivity of valleyite of treated (~9.2 wt sample %) also is influencedsignificantly the high hysteresis compared loop to [24 magnetic] (Figure 4). Thisminerals coercivity in the of earth treated crust sample [1,31]. isSynthetic significantly luogufengite high compared had a saturation to magnetic remanence minerals of 15 in emu the earthg−1 −1 crustand [ 1a, 31coercive]. Synthetic field of luogufengite 2.0 T at room had temperature a saturation [19]. remanence The coercivity of 15 emuof luogufengite·g and a coerciveis strongly field ofdependent 2.0 T at room on temperatureparticle size, [morphology,19]. The coercivity magneto-cr of luogufengiteystalline anisotropy is strongly and dependent cation substitution on particle size,[19,22,25,32]. morphology, A rod-like magneto-crystalline morphology oriented anisotropy along and the cationa-axis (magnetic substitution easy [19 axis),22, 25has,32 an]. enlarged A rod-like morphologycoercive field oriented [21–33]. along A particle the asize-axis of (magneticabout 100 nm easy is axis)the most has suitable an enlarged for a large coercive coercivity field [to21 be–33 ]. A particlepresent [19]. size of about 100 nm is the most suitable for a large coercivity to be present [19].

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Figure 3. Hand specimens and its TEM image of iron-oxide nano-minerals (a) Luogufengite nano- Figure 3. Hand specimens and its TEM image of iron-oxide nano-minerals (a) Luogufengite minerals of treated samples from vesicles’ surfaces on the scoria, Idaho, (b) Lugufengite (Luo) and nano-mineralsmaghemiteFigure 3.of Hand(Mgh) treated specimens from samples Fillmore and fromits scoria, TEM vesicles’ imag Utah,e of surfaces( ciron-oxide) Luogufengite on nano-minerals the scoria, nano-miner Idaho, (a) Luogufengiteals ( bin) Lugufengitethe matrix nano- of (Luo) minerals of treated samples from vesicles’ surfaces on the scoria, Idaho, (b) Lugufengite (Luo) and and maghemiteamorphous SiO (Mgh)2 from from brown Fillmore surface scoria,of a Hawaii Utah, basalt, (c) Luogufengite (d–f) High resolution nano-minerals TEM images in thewith matrix the of maghemite (Mgh) from Fillmore scoria, Utah, (c) Luogufengite nano-minerals in the matrix of amorphousfast Fourier SiO 2transformfrom brown (FFT) surface patterns of of a luogufengi Hawaii basalt,te grains (d –showingf) High the resolution twinning TEM (TW) images boundaries with the amorphous SiO2 from brown surface of a Hawaii basalt, (d–f) High resolution TEM images with the fast Fourierand stacking transform fault (SF) (FFT) along patterns the b-direction. of luogufengite grains showing the twinning (TW) boundaries fast Fourier transform (FFT) patterns of luogufengite grains showing the twinning (TW) boundaries and stacking fault (SF) along the b-direction. and stacking fault (SF) along the b-direction.

Figure 4. Magnetic hysteresis curves of the treated sample of scoria from Menan complex, Idaho. Figure 4. Magnetic hysteresis curves of the treated sample of scoria from Menan complex, Idaho. Figure 4. Magnetic hysteresis curves of the treated sample of scoria from Menan complex, Idaho. Luogufengite nano-minerals were found in other young basaltic rocks from Red Dome in Luogufengite nano-minerals were found in other young basaltic rocks from Red Dome in Fillmore, Utah, Laguna del Maule volcano field, Chile and on the surface of basaltic lava from Mauna Fillmore, Utah, Laguna del Maule volcano field, Chile and on the surface of basaltic lava from Mauna LuogufengiteKea volcano, HI nano-minerals (Figure 3b,c and were Supplementary found in other Materials young Figure basaltic S1). rocksThis observation from Red Domesuggests in that Fillmore, Utah, LagunaKea volcano, del MauleHI (Figure volcano 3b,c and ﬁeld, Supplementary Chile and Materials on the Figure surface S1). ofThis basaltic observation lava suggests from Mauna that Kea thethe luogufengite luogufengite is isa awidely widely distributed distributed magneticmagnetic mineral inin high-temperaturehigh-temperature volcanic volcanic rocks. rocks. The The volcano,sizessizes HI and and (Figure shapes shapes 3observedb,c observed and Supplementaryin in these these other other luogufengiteluogufengite Materials nano-minerals Figure S1). Thisare are similar similar observation to to that that from suggestsfrom Menan Menan that the luogufengiteVolcanicVolcanic Complex is Complex a widely [13]. [13]. distributed Luogufengite Luogufengite magnetic could could bebe mineral an important in high-temperature mineralmineral for for recording recording volcanic paleomagnetism paleomagnetism rocks. The sizes and shapesof volcanicof volcanic observed rocks. rocks. inIts Its theselarge large othermagnetic magnetic luogufengite coercivity coercivity allo nano-mineralsws for preservationpreservation are similar of of the the tooriginal original that from magnetic magnetic Menan field field Volcanic Complexduringduring [13 mineral]. mineral Luogufengite formation formation couldand and cooling. cooling. 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.

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4.2. Luogufengite-Like Luogufengite-Like Nano-Domains in Lodestone and Fe-Ti Oxides Many studies have revealed that exsolution lamellae are an important important contributor to the unusual unusual remanent magnetization in in slow cooling igneous and metamorphic rocks [[1,2,4,12].1,2,4,12]. However, the role of of exsolution exsolution lamellae lamellae for for the the coercivity coercivity and and remanent remanent magnetization magnetization is not is not well well understood. understood. To understandTo understand these these unusual unusual magnetic magnetic properties, properties, we examined we examined lodestone lodestone and Fe-Ti and oxide Fe-Ti (ilmenite- oxide magnetite(ilmenite-magnetite series). series). The lodestone sample analyzed (Figure 11bb and Su Supplementarypplementary Materials Figure S2) is a natural natural permanent magnet.magnet. It It has has partially partially oxidized oxidized magnetite magnetite intergrown intergrown with with hematite hematite and maghemite and maghemite[34,35]. [34,35].Rietveld Rietveld reﬁnement refinement analysis analysis of a hematite-bearing of a hematite-bearing lodestone lodestone shows shows 64.3(4) 64.3(4) wt % wt of magnetite% of magnetite with with26.9(3) 26.9(3) wt % wt of maghemite% of maghemite and 8.7(4) and 8.7(4) wt % wt of hematite% of hematite (Figure (Figure2b). Interestingly, 2b). Interestingly, TEM imagesTEM images show showaligned aligned nanoscale nanoscale exsolution exsolution lamellae lamellae of hematite of hematite and magnetite and magnetite (Figure (Figure5a). HRTEM 5a). HRTEM images images show showthe interfaces the interfaces {111}Mgt {111}//(0001)Mgt//(0001)Hem ofHem the of nano-lamellae the nano-lamellae (Figure (Figure5b). Many 5b). previous Many previous studies reported studies reportedthat this isthat the this common is the interface common between interface cubic betw magnetiteeen cubic andmagnetite rhombohedral and rhombohedral hematite, followed hematite, by followedoxygen packing by oxygen direction packing of both direction iron oxides of both [1 ,iron10]. Anotheroxides [1,10]. interesting Another observation interesting of theobservation TEM image of theis that TEM host image magnetite is that host often magnetite shows stacking often shows faults stac andking twin faults boundaries and twin related boundaries to {101} related planes to at{101} the planes[111]-zone-axis at the [111]-zone-axis (Figure 6a,b). (Figure 6a,b).

Figure 5. TEM images with the selected-area electron diffraction (SAED) patterns: (a) Bright-field Figure 5. TEM images with the selected-area electron diffraction (SAED) patterns: (a) Bright-ﬁeld TEM TEMimage image of lodestone of lodestone showing showing the aligned the aligned hematite hemati (Hem)te (Hem) exsolution exsolution within within the host the magnetite host magnetite (Mgt) (Mgt) with the (111)Mgt//(0001)Hem interface, (b) High-resolution image of the magnetite-hematite with the (111)Mgt//(0001)Hem interface, (b) High-resolution image of the magnetite-hematite interface interfacefrom Figure from5 a,Figure (c) Bright-ﬁeld 5a, (c) Bright-field TEM image TEM ofimage Fe-Ti of oxide Fe-Ti sampleoxide sample showing showing aligned aligned ilmenite ilmenite (Ilm) (Ilm) exsolution lamellae within host magnetite with the (111)Mgt//(0001)Ilm interface. Its SAED shows exsolution lamellae within host magnetite with the (111)Mgt//(0001)Ilm interface. Its SAED shows the thehexagonal hexagonal twin twin relationship relationship of ilmenite of ilmenite phases phases and (d and) High-resolution (d) High-resolution image ofimage the magnetite-ilmenite of the magnetite- ilmeniteinterface interface from Figure from5c. Figure 5c.

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Figure 6. (a) High-resolution TEM image of magnetite with its FFT pattern showing the <110> Figure 6. (a) High-resolution TEM image of magnetite with its FFT pattern showing the <110> displacements along [111] zone-axis. The projected displacements are associated with <111> stacking displacements along [111] zone-axis. The projected displacements are associated with <111> stacking Figurefaults (SF). 6. ( a(b) )High-resolution High-resolution TEM imageimage ofof magnetite magnetit ewi withth its itsFFT FFT pattern pattern showing showing twinning the <110> (TW) b faultsdisplacementsboundaries (SF). ( )along High-resolution along [1ത01] [111] direction zone-axis. TEM that image The is related projected of magnetite to (111) displacements twinning with its boundaries. FFT are associated pattern showing with <111> twinning stacking (TW) boundariesfaults (SF). along (b) High-resolution [101] direction TEM that image is related of magnetite to (111) twinning with its FFT boundaries. pattern showing twinning (TW) boundariesTo test the alonginterface’s [1ത01] directioneffect on that the ismagnetism related to (111) of lodestone, twinning boundaries.we prepared three lodestone samples withTo testdifferent the interface’s hematite effectconcentrat on theions magnetism (Supplementary of lodestone, Materials we preparedFigure S5). three Magnetic lodestone hysteresis samples withloops differentTo from test the hematitethe interface’s samples concentrations effect clearly on thesuggest (Supplementary magnetism that theof lodestone, Materialsmagnetic we Figurecoercivity prepared S5). three Magneticis proportional lodestone hysteresis samples to the loops with different hematite concentrations (Supplementary Materials Figure S5). Magnetic hysteresis fromconcentration the samples of clearlyhematite suggest (Figure that 7). The the magneticlodestone coercivitysample containing is proportional 8.7(4) wt to % the hematite concentration has a loops from the samples clearly suggest that the magnetic coercivity is proportional to the ofcoercive hematite field (Figure of 46.77). The mT lodestoneat room temperature. sample containing Pure magnetite 8.7(4) wt %from hematite natural has rocks a coercive produced field a of concentration of hematite (Figure 7). The lodestone sample containing 8.7(4) wt % hematite has a 46.717.5 mT mT at roomfield temperature.with a grain Puresize of magnetite 37 nm and from 13.3 natural mT rockswith a produced 100 nm agrain 17.5 mTsize fieldat room with a coercive field of 46.7 mT at room temperature. Pure magnetite from natural rocks produced a graintemperature size of 37 [31]. nm andFrom 13.3 this, mT we with can see a 100 that nm the grain interface size with at room ABAC temperature packing sequence [31]. From enhances this, we the can 17.5coercivity mT field field ofwith lodestone a grain since size hematite of 37 nm has andvery 13.3weak mT ferromagnetic with a 100 properties. nm grain size at room seetemperature that the interface [31]. From with this, ABAC we can packing see that sequence the interface enhances with ABAC the coercivity packing sequence field of lodestoneenhances the since hematitecoercivity has field very of weak lodestone ferromagnetic since hematite properties. has very weak ferromagnetic properties.

Figure 7. Magnetic hysteresis curves of three lodestone samples with different hematite concentrations at room temperature. Figure 7. Magnetic hysteresis curves of three lodestone samples with different hematite Figure 7. Magnetic hysteresis curves of three lodestone samples with different hematite concentrations concentrations at room temperature. at room temperature.

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

Minerals 2018, 8, x FOR PEER REVIEW 8 of 12 Minerals 2018, 8, 97 8 of 12 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 analysisof the Fe-Ti of the oxide Fe-Ti sample oxide sample shows 79.2(5)shows 79.2(5) wt % of wt magnetite % of magnetite with thewith 13.3(3) the 13.3(3) wt % wt of % ilmenite of ilmenite and and7.5(5) 7.5(5) wt % wt of % spinel of spinel (Figure (Figure2c). Similarly, 2c). Similarly, TEM imagesTEM images of the Fe-Tiof the oxide Fe-Ti sample oxide sample also show also aligned show alignedlamellae oflamellae ilmenite of in ilmenite host magnetite, in host displaying magnetite, an interfacialdisplaying relationshipan interfacial of 111 relationshipMgt//(0001) Ilmof (Figure111Mgt//(0001)5c,d). Ilm (Figure 5c,d). Previous studies have attributed natural remanent magnetization to the common interface between the (0001) planes of the rhombohedral oxide oxide and and the the (111) (111) planes planes of of the the cubic cubic oxi oxidede [1,4,7]. [1,4,7]. Interestingly, the interfaceinterface betweenbetween cubiccubic andand rhombohedralrhombohedral oxides can produce luogufengite-likeluogufengite-like 2-D crystals or domains with a doubled hexagonal structure (ABAC packing sequence) sequence) (Figure (Figure 8a).8a). In addition, stacking faults and twin boundaries in magnetite and maghemite can also generate luogufengite-like layer domains with the ABAC stac stackingking sequence locally (Figure 8b,c).8b,c). We suggest that thethe structurestructure of of luogufengite-like luogufengite-like nano-domains nano-domai atns the at interfacethe interface or within or within a mineral a mineral could enhance could enhancethe magnetic the magnetic coercive field.coercive field. The multi-layered multi-layered structure structure of of lodestone lodestone and and Fe-Ti Fe-Ti oxides oxides plays plays an anessential essential role role to enhance to enhance the coercivitythe coercivity and andto topreserve preserve remanent remanent magnetization magnetization (Figure (Figure 5a,c).5a,c). The The magnetic magnetic easy easy axis axis of luogufengite is is reported to be the aa--axisaxis [[22],22], which corresponds to the longitudinal axis of the multilayers of Figure 55a,c.a,c. Thus, Thus, the the multi-layers multi-layers are are parallel parallel to to the external magnetic field field that contributes toto the the coercive coercive field field strength strength [33, 36[33,36]. Studies]. Studies of synthetic of synthetic magnetic magnetic materials materials have reported have reportedthat the larger that the value larger of coercivityvalue of coercivity could be achievedcould be achieved at the interface at the betweeninterface magneticallybetween magnetically different differentlayers where layers the where multilayer the multilayer materials materials were oriented were alongoriented the along magnetic the magnetic easy axis easy [33,36 axis,37 ].[33,36,37].

Figure 8.8. StructureStructure models models showing showing the doubledthe doubled hexagonal hexagonal closest closest packing packing of ABAC of stacking ABAC sequence:stacking (sequence:a) Crystal ( interfacea) Crystal between interface cubic between magnetite cubic andmagnetit rhombohedrale and rhombohedral hematite/ilmenite, hematite/ilmenite, (b) The (111) (b) twin The (111)boundary twin ofboundary magnetite/maghemite of magnetite/maghemite and (c) The and stacking (c) The fault stacking of magnetite/maghemite fault of magnetite/maghemite along the along[111] direction.the [111] direction.

Figure 9 illustrated the nano-sized multilayer structure structure of lodeston lodestonee and Fe-Ti oxides for understanding thethe coercivity coercivity and and remanent remanent magnetism. magnetism. The luogufengite-likeThe luogufengite-like domains domains can be createdcan be createdfrom interfaces, from interfaces, twinning twinning boundary boundary and stacking and st faultsacking along faults the along [111] the direction [111] direction (oxygen (oxygen packing packingdirection) direction) (Figure 9(Figurea). The 9a). domains/interfaces The domains/interfaces play an essentialplay an essential role in enhancing role in enhancing the magnetic the magneticcoercivity coercivity of lodestone of lodestone and Fe-Ti and oxides Fe-Ti (Figureoxides (Figure9b). The 9b). interfaces The interfaces in the in multi-layer the multi-layer texture texture are arealso also associated associated with with the magneticthe magnetic coupling coupling combined combined with with exchange-spring exchange-spring state state between between soft andsoft andhard hard ferromagnets ferromagnets that that lead lead to enhancingto enhancing the the coercive coercive ﬁeld field [38 [38,39],,39], although although the the volumevolume ratioratio of

Minerals 2018, 8, 97 9 of 12 Minerals 2018, 8, x FOR PEER REVIEW 9 of 12 luogufengite-like nano-domains at at the interface is minorminor (Figure(Figure 99c).c). For example, the magnetic coercivity of thethe exchange-coupledexchange-coupled isotropicisotropic FePt(hard)-FeFePt(hard)-Fe33Pt(soft) nanocomposites exceeds the theoretical limit of non-exchange-coupled FePtFePt byby overover 50%50% [[40].40].

Figure 9. (a) Schematic diagram of multi-layer texture of lodestone and Fe-Ti oxides based on TEM observations, ( b) Schematic diagram of M-H hysteresis loops of individual oxide minerals with the 3 4 magnetic hysteresis loops ofof lodestonelodestone andand Fe-TiFe-Ti oxides,oxides,( c(c)) Normalized Normalized coercive coercive ﬁeld field of of FePt/Fe FePt/Fe3OO4 core-shell nanoparticles as a function ofof thethe FeFe33O44 mole fraction. The The exchange-spring state state at the interface enhances the coercivity ﬁeld.field. TB:TB: twin boundaryboundary andand SF:SF: stackingstacking fault.fault.

The paleomagnetism requires long relaxation time that retains the magnetization over geological The paleomagnetism requires long relaxation time that retains the magnetization over time-scale. The relaxation time is generally proportional to the coercivity and saturation 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 magnetization [41,42]. The relaxation times and magnetic properties can be also changed as a of domain states and size [42]. The relaxation time of multi-domain grains increases with decreasing function of domain states and size [42]. The relaxation time of multi-domain grains increases with grain size, while that of single-domain increases with increasing grain size [41,42]. Thus, the nano- decreasing grain size, while that of single-domain increases with increasing grain size [41,42]. Thus, scaled multilayer structure of lodestone and Fe-Ti oxides can increase coercivity and help to preserve the nano-scaled multilayer structure of lodestone and Fe-Ti oxides can increase coercivity and help to the saturated magnetization of rocks with the long relaxation time. preserve the saturated magnetization of rocks with the long relaxation time. NASA’s Mars Global Surveyor spacecraft observed the localized magnetic anomalies on the NASA’s Mars Global Surveyor spacecraft observed the localized magnetic anomalies on the Mars Mars surface [5]. Especially, Noachian crust (3.7–4.1 Ga) of the southern hemisphere of Mars shows surface [5]. Especially, Noachian crust (3.7–4.1 Ga) of the southern hemisphere of Mars shows the the strongest remanent magnetism in some locations ~20 times greater than Earth, although Mars strongest remanent magnetism in some locations ~20 times greater than Earth, although Mars currently currently does not possess a core dynamo [43,44]. The candidate minerals responsible for the strong does not possess a core dynamo [43,44]. The candidate minerals responsible for the strong remanent remanent magnetism on the Mars surface are magnetite, pyrrhotite, multidomain hematite, magnetism on the Mars surface are magnetite, pyrrhotite, multidomain hematite, titanohematite and titanohematite and hemoilmenite [45,46]. We suggest that the luogufengite could be a magnetic phase hemoilmenite [45,46]. We suggest that the luogufengite could be a magnetic phase in the basaltic crust in the basaltic crust on Mars. In addition, the interface, stacking faults and twinning boundary of on Mars. In addition, the interface, stacking faults and twinning boundary of magnetic minerals may magnetic minerals may contribute to preserving the natural remanent magnetization on Mars surface. contribute to preserving the natural remanent magnetization on Mars surface.

5. Conclusions

A natural ε-Fe2O3 nano-mineral (luogufengite) with large coercivity was found in young basaltic A natural ε-Fe2O3 nano-mineral (luogufengite) with large coercivity was found in young basaltic rocks. Our Our observations observations suggest suggest that that luogufengite luogufengite is a iswidely a widely distributed distributed magnetic magnetic mineral mineral in high- in high-temperaturetemperature volcanic volcanic rocks. rocks. We think We think that thatthis thisnano-mineral nano-mineral can canbe an be animportant important indicator indicator of ofpaleomagnetism paleomagnetism in volcanic in volcanic systems. systems. Luogufengite-like Luogufengite-like nano-domains nano-domains were were also alsoobserved observed at the at theinterfaces interfaces in lodestone in lodestone and andFe-Ti Fe-Ti oxide oxide with withstrong strong coercivity coercivity and andremanent remanent magnetization. magnetization. The Themultilayer multilayer of nano-texture of nano-texture paralleled paralleled to the to the magnetic magnetic easy easy axis axis plays plays an an important important role role in in enhancing enhancing the magnetic coercivity. Stacking faults and twin boundaries can also produce luogufengite-like layer domains toto help help to to increase increase the coercivity.the coercivity. These Th observationsese observations can provide can prov an explanationide an explanation for coercivity for andcoercivity strong and remanent strong magnetizationremanent magnetization in slow cooling in slow igneous cooling and igneous metamorphic and metamorphic rocks. We believe rocks. thatWe thisbelieve is a goodthat this example is a ofgood using example the nanostructure of using the to describenanostructure distinctive to describe mineral propertiesdistinctive in mineral natural 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.

Minerals 2018, 8, 97 10 of 12 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.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/3/97/s1, Figure S1: Images of hand specimen and the TEM images, Figure S2: Hand sample of lodestone, Figure S3: BSE image of ilmenite-magnetite series of Fe-Ti oxides, Figure S4: A size-dependent phase map of iron (III) oxide polymorphs of maghemite, luogufengite and hematite, Figure S5: Three powder XRD patterns of lodestone with different hematite concentrations, Table S1: Atomic coordinates of luogufengite. Acknowledgments: This research was funded by the National Science Foundation and NASA Astrobiology Institute (NNA13AA94A). Authors thank Franklin Hobbs and the anonymous reviewers for their constructive suggestions. Author Contributions: Huifang Xu conceived the idea and selected the samples; Seungyeol Lee conducted the experiments and drafted the manuscript. Seungyeol Lee and Huifang Xu analyzed the data and worked on manuscript writing. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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