ISSN 1028�334X, Doklady Earth Sciences, 2013, Vol. 453, Part 1, pp. 1145–1149. © Pleiades Publishing, Ltd., 2013.Original Russian Text © N.S. Bortnikov, V.M. Novikov, N.M. Boeva, A.P. Zhukhlistov, E.A. Zhegallo, T.S. Gendler, L.V. Zaitseva, S.V. Soboleva, 2013, published in DokladyAkademii Nauk, 2013, Vol. 453, No. 3, pp. 305–309.

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The abundance of microbial forms (bacteria, bio�films, bacterial mats, glycocalyx) participating in theformation of iron oxide and hydroxide minerals is apeculiar feature of weathering crusts (CWs) in basaltfrom South Vietnam. Their maximal concentration isobserved in the zone of lateritic bauxite and cuirassformed above in the undersoil horizon [1, 2]. Cur�rently magnetite Fe3O4 is well�studied as a product ofiron biomineralization by modern organisms andincluded in the top ten by scientific and practicalimportance. Special attention has been paid to natu�ral, as well as synthetic, minerals formed with partici�pation of magnitotactic bacteria.

Finds of biogenic hematite Fe2O3 are poorlydescribed in the literature in comparison with not onlymagnetite, but goethite α�FeOOH and ferrihydrite2.5Fe2O3 ⋅ 4.5Н2О, which sometimes have a biogenicorigin and, similarly to hematite, contain trivalentiron. However, the two latter minerals containhydroxyl groups or water molecules, whereas anhy�drous hematite may be formed only in presence of oxi�dizing reagents. Attempts to explain its formation bydehydration of goethite under natural conditions arefruitless. The activity of iron bacteria is the major fac�tor of iron oxidation in the zone of hypergenesis. Ideason the likely participation of microorganisms in theformation of hematite in hypergene iron ores haveappeared in relation to the discovery of biogenic ferri�hydrite as a protohematite material [3].

Biogenic hematite was originally discovered byF.V. Chukhrov [3] in red ocher of the Moa deposit(Cuba). The mineral was represented by disklikeround particles with a size of 0.n–n μm considered as

relics of Gallionella bacterium. According to the elec�tron microscopic studies, hematite discs, in turn, werecomposed of small (0.0n μm) hexagonal sheets.Relicts of bacterial forms of hematite demonstrate thatits formation included the stage of ferrihydrite [3]. Weshould note that the disklike shape is very typical ofbiogenic hematite from ancient, as well as young, CWs[4]. New possibilities of the discovery and study of var�ious morphostructural varieties of biominerals andtheir properties have appeared due to construction ofmodern precise equipment.

We registered the presence of biogenic nanohema�tite in the contact zone between lateritic bauxite andcuirass in bauxite�bearing CW in basalt of South Viet�nam. The main morphological and structural pecu�liarities were studied using scanning (SEM) Cam�Scan�4 (Cambridge) and TESCAN VEGA IIXMU(Tescan) and transmitting (TEM) JEM 2100 (JEOL,Japan) microscopes. Microanalysis with TEM wasperformed using a device for X�ray energy�dispersiveanalysis X�Max (Oxford Instruments, United King�dom).

X�ray diffraction studies were performed on aD/MAX�2200 diffractometer (Rigaku, Japan) with aCu cathode (working mode 40 kV and 30 mA, startingangle 5° 2θ, final angle 65° 2θ, step of scanning0.02° 2θ). The thermal behavior of hematite duringheating was studied by the method of differentialgravimetry and scanning calorimetry on a STA 449F1Jupiter synchronous thermoanalyzer at a heating rateof 10°C/min (Netzsch, Germany). The magneticproperties of biogenic nanohematite were studied atthe Laboratory of Geomagnetism, Institute of Physicsof the Earth, Russian Academy of Sciences, on a vibro�magnetometer (OOO Orion, Borok). The constructionof this apparatus allows us to perform continuous suc�cessive measurements of hysteresis loops and thermo�magnetic curves within the ranges of magnetic fields of0–9000 Oe and temperatures of 20–750°C.

SEM observations demonstrate that the initialsource of hematite was represented by amorphous ironhydroxide with an admixture of aluminum, silicon,and organic material forming isometric, sinter uneven

The First Find of Biogenic Nanohematite in the Bauxite�Bearing Weathering Crust in Basalt from South Vietnam

Academician N. S. Bortnikova, V. M. Novikova, N. M. Boevaa, A. P. Zhukhlistova, E. A. Zhegallob, T. S. Gendlerc, L. V. Zaitsevab, and S. V. Sobolevaa

Received July 17, 2013

DOI: 10.1134/S1028334X13110159

a Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow, Russiab Borisyak Paleontological Institute, Russian Academy of Sciences, Moscow, Russiac Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russiae�mail: bns@igem.ru

GEOCHEMISTRY

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forms in rock (Fig. 1a). Its gradual transformation intocrystalline biogenic nanohematite was observed in onesample. Aging and cracking in the marginal areas ofamorphous aggregates result in the formation of indi�vidual ball�like forms and those exfoliating by parallelplains (Fig. 1b). On their periphery (with distance

from the protosource), we observe segregations ofindividual disclike and then fan�shaped growths rep�resented by complex twins of ellipsoidal hematite par�ticles (Fig. 1c). In the simple case, there are individu�als composed of four or six plates located 90° or 60°from each other. The variation of the chemical com�position of morphological types in the consideredseries is given in Table 1.

As is evident from the results of TEM, hematite isrepresented by two morphological varieties: ellipsoidsand particles of isometric irregular shape. Ellipsoidsform intergrowths, individual particles with a size of1 × 1.5 μm (Fig. 2a) and their half fragments (Fig. 2b).It is evident from images of the marginal areas of theseformation that they are entirely composed of numer�ous nanoparticles with a size of 20–30 nm. The size ofnanoparticles is slightly larger (30–40 nm) in the cen�tral part of the ellipsoid observed in the place of frac�ture (Fig. 2b).

According to the positions and dhkl of discretereflexes, selected�area electron diffraction patterns ofdistinguished areas of the considered morphologicaltypes demonstrated in Fig. 2 correspond to diffractionpatterns of hematite monocrystals, which is possibleonly at the same mutual orientation of nanoparticlesin crystallite. The presence of spatial reflections hkl indiffraction patterns provides evidence for the three�dimensional ordered structure of individual hematitenanoparticles. Hematite particles of isometric irregu�lar shape with a size up to 0.8–1.0 μm are character�ized by spot diffraction patterns of the selected areatypical of monocrystals, as well as ring patterns typicalof polycrystalline aggregates. The studied polycrystal�line hematite consists of nanoparticles with a size of10–20 nm. The main difference between polycrystal�line particles of isometric irregular and ellipsoidalshapes is that in the first case nanoparticles are com�pletely disoriented forming a true polycrystal, whereasin the second case, they have the same mutual orien�tation demonstrating diffraction peculiarities of themonocrystal and morphology unusual for monocrys�tals.

According to the data of energy�dispersive analysis,ellipsoidal individuals have the similar chemical com�position mainly represented by Fe and small amountsof Al, Si, and Mn. The empirical formula of the min�eral is the following: Fe1.84Al0.07Si0.06Mn0.03О3.

There is a wide maximum on X�ray diffraction pat�tern in the 2θ area of 10°–60°, which may be relatedto amorphous iron hydroxide or the first reflex 110 ofpoorly crystallized two�linear fine�dispersed ferrihy�drite with a particle size of 0.77–1.00 nm. Quite nar�row reflexes mostly corresponding to hematite and, toa lesser extent, to goethite and gibbsite are distin�guished against the background of this “pedestal.”Estimation of the relationships between goethite andhematite based on analysis of the intensity values ofwell�separated reflections 110 and 012 provides a valueof ≈0.14 [5, 6].

10 µm

5 µm

2 µm200 nm

Fig. 1. Amorphous iron hydroxide (a), ball�like biogenicnanohematite (b), and complex ellipsoidal twins of bio�genic nanohematite (c). Synthetic hematite is shown in theinset [14].

(a)

(b)

(c)

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THE FIRST FIND OF BIOGENIC NANOHEMATITE 1147

The thermal analysis supported the idea on the ori�gin of the studied hematite obtained by electronmicroscopic studies. The flattened endothermal effectwith a maximum at ≈100°C and a weight loss of 2.14%corresponds to the presence of amorphous ironhydroxide (initial source of hematite). The low�inten�sity exoeffect at ~615°C results from enlargement ofhematite particles upon heating. A small endoeffect at~680°C corresponds to the reversible polymorphtransformation α�Fe2O3 → γ�Fe2O3.

The flattened exothermal effect in the temperaturerange of 600–950°C is explained by decomposition ofthe organomineral phase. Carbon in the adsorbedorganic compounds, transformation of which is cata�lyzed by the hematite surface at high temperatures,interacts with oxygen in the crystal structure of themineral with the formation of СО2. This effect con�trols the loss of weight in the mentioned range (0.54%)[7]. At 942°C we observe an exothermal effect result�ing from transformation of hematite into magnetite.The analysis was carried out in oxygen and in an argonatmosphere. A redox reaction and the formation ofmagnetite are registered in presence of oxygen,whereas in the presence of argon, this effect is notobserved.

Measurements of the magnetic properties of thesamples in two mutually perpendicular directions inrelation to the magnetic field demonstrate their signif�icant differences from analogous properties of massivenatural and synthetic hematites. It is evident that themost typical specific peculiarities are demonstrated byhysteresis loops at room temperature for the initialstate and after a heating/cooling cycle in the magneticfield. These peculiarities are the following: (1) strongconstriction of symmetric loop in the origin area(“wasp�waists”) for the initial state, which was origi�nally registered for iron concretion from the lateriticbauxite deposit Baolok in South Vietnam [1]; (2) theexistence of a two�phase system with different coerciv�ities; (3) significant shifts of the loop in relation to theorigin to the left along the axis of the magnetic fieldand upwards along the magnetization axis after cool�ing in the magnetic field from Т = 750°C (by 173.5 and128.5 Oe, respectively). These indicative peculiaritiesare observed only under magnetization in one conven�tionally selected direction.

In this short paper we cannot provide a physicalexplanation for this phenomenon. Numerous investi�gations of synthesized magnetic nanostructures [8–10] have demonstrated that the combination of theabove�mentioned peculiarities of hysteresis loopbehavior in the studied object undoubtedly providesevidence for the formation of a magnetic two�phasenanostructure “ferro–antiferromagnetic” within theindividual crystallographic structure of hematite,which is unusual for hematite at room temperature[10]. This structure is anisotropic and layered, which isconsistent with the SEM data, with a strong unidirec�tional exchange anisotropy in the interface area (in

our case, these may be the boundaries of twins andcontacting monocrystalline nanoparticles).

The anisotropy of the magnetic properties of crys�tallized hematite is additionally evident from theabsence of a hysteresis loop shift in the perpendiculardirection and the difference of forms of thermomag�netic (TM) curves depending on the magnetic fielddirection. The TM curve for the direction without thehysteresis loop shift is smooth and single�phase with ashape similar to the curves of pure natural and syn�thetic hematites. However, in the case of magnetiza�tion in the perpendicular direction, the TM curve isrepresented by a series of bends in different tempera�ture ranges below the major Curie temperature forsuch hematite (ТC = 638°C). Previously [1] weobserved ТC lower than the standard value (675°C),which is most likely explained by small isomorphicsubstitutions of Fe by Al in the hematite structure.

Single�crystal aggregates composed of nanosizedparticles were mostly observed for synthetic com�pounds and minerals. Experiments on the synthesis ofhematite demonstrate the important (if not determin�ing) role of organic matter in the formation of struc�tural–morphological varieties and sizes of mineralindividuals. W. Fischer and U. Schwertmann [11]studied the formation of hematite from amorphoushydroxide of trivalent iron in an aqueous environmentat pH 6 and 70°С in the presence and absence ofammonium oxalate. As is evident from TEM data, inthe first case ellipsoidal forms composed of aggregatesof hexagonal hematite crystals with a size of 50–100 Åwere formed. By providing a matrix for iron hydroxide,oxalate accelerated the formation of crystallizationcenters of the mineral. In the absence of oxalate, smallamorphous particles stuck together in aggregates,which were later transformed into defect hematitemonocrystals. Thus, it was established that ammo�nium oxalate caused a change in the external form, aswell as the internal structure of the mineral [11].

In later experiments disklike and ellipsoidal formsof hematite synthesized with the participation oforganic compounds were composed of similarly ori�ented nanoparticles and characterized by diffractionpatterns typical of monocrystals of this mineral. It wasestablished that their formation occurred either by ori�ented attachment of nanoparticles with subsequenttransformation into the monocrystal or during the for�mation of mesocrystals as a result of segregation of pri�

Element composition in the genetic sequence of transforma�tion of amorphous iron hydroxide into hematite (SEM data)

Morphology of he�matite Al Si Fe O Total

Amorphous Fe�phase 8.88 9.96 31.58 49.58 100.00

Layered balls 8.29 8.92 32.13 50.66 100.00

Disc twins 11.44 11.70 72.79 24.07 100.00

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mary nanoparticles covered by organic components,with subsequent transformation (removal of thesecomponents) into crystals with the same mutual ori�entation of nanoparticles and further transformationinto the monocrystal [12, 13]. The formation of syn�thetic twins of ellipsoidal hematite similar to thoseobserved in nature [14] is an interesting fact (Fig. 1c).

Comparative analysis of the structural–morpho�logical peculiarities of natural biogenic nanohematite,as well as generalization of the known published dataon its laboratory synthesis allow us to consider CWs asa natural reactor, in which various morphologicaltypes and ultradispersed modifications of biomineralsof iron oxides and hydroxides with nonstandard mag�

200 nm

118−

208

104 214

110

114−

200 nm

024

012

110

50 nm

50 nm

Fig. 2. Electron microscopic images of ellipsoidal hematite particles. (a) The whole particle (diffraction pattern is demonstratedin the inset); (b) half fragment of the particle (magnified images of the marginal and central parts and diffraction pattern areshown in the inset).

(a)

(b)

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THE FIRST FIND OF BIOGENIC NANOHEMATITE 1149

netic properties are formed. Hydrohematite iron oresin CWs of ferruginous quartzite compose the richestdeposits. In addition to goethite, dispersed hematite isa chromophore mineral in natural mineral pigments[15]. There is no doubt that the considered peculiari�ties of biogenic nanohematite will be applied in nano�technologies in the future.

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research, project nos. 13�05�00765a,13�04�00933a.

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Translated by A. Bobrov