Iron Oxides: From Nature to Applications

By Damien Faivre and Richard B. Frankel

Compiling all the information available on the topic, this ready reference covers all important aspects of iron oxides.
Following a preliminary overview chapter discussing iron oxide minerals along with their unique structures and properties, the text goes on to deal with the formation and transformation of iron oxides, covering geological, synthetic, and biological formation, as well as various physicochemical aspects. Subsequent chapters are devoted to characterization techniques, with a special focus on X-ray-based methods, magnetic measurements, and electron microscopy alongside such traditional methods as IR/Raman and Mössbauer spectroscopy. The final section mainly concerns exciting new applications of magnetic iron oxides, for example in medicine as microswimmers or as water filtration systems, while more conventional uses as pigments or in biology for magnetoreception illustrate the full potential.
A must-read for anyone working in the field.

• Language: English
• Publisher: Wiley
• Release date: Apr 25, 2016
• ISBN: 9783527691388

Book preview

List of Contributors

Jean-Baptiste Abbé

CEA/CNRS/Aix-Marseille University

Biosciences and Biotechnologies Institute

UMR7265 Cellular Bioenergetics Laboratory

13108 Saint Paul les Durance

France

Javier Alonso

BCMaterials

Bizkaia Science and Technology Park

Building 500

48160 Derio

Spain

Amanda K. Andriola Silva

UMR 7057 CNRS/Université Paris Diderot

Laboratoire Matières et Systèmes Complexes

10 rue Alice Domon et Léonie Duquet

75205 Paris Cedex 13

France

Noam Aronovitz

Ben-Gurion University of the Negev

Department of Life Sciences

National Institute for Biotechnology in the Negev

POB 653

84105 Beer-Sheva

Israel

Subir K. Banerjee

University of Minnesota

Department of Earth Sciences

Institute for Rock Magnetism

100 Union Street SE

Minneapolis MN 55455

USA

Amanda S. Barnard

Commonwealth Scientific and Industrial Research Organisation

343 Royal Parade

3052 Parkville

VIC

Australia

Mathieu A. Bennet

Max Planck Institute for Colloids and Interfaces

Department of Biomaterials

Science Park Golm

Am Mühlenberg 1

14476 Potsdam

Germany

Lesley R. Brooker

University of the Sunshine Coast

School of Health & Sports Sciences

Faculty of Science, Health, Education & Engineering

Locked Bag 4

Maroochydore DC

Queensland 4558

Australia

Corinne Chaneac

Sorbonne Universités

UPMC Univ. Paris 06

CNRS Collège de France

Laboratoire de Chimie de la Matière Condensée de Paris

11 place Marcelin Berthelot

75005 Paris

France

Joanna F. Collingwood

University of Warwick

School of Engineering

Library Road

Coventry CV4 7AL

UK

Elodie C.T. Descamps

CEA/CNRS/Aix-Marseille University

Biosciences and Biotechnologies Institute

UMR7265 Cellular Bioenergetics Laboratory

13108 Saint Paul les Durance

France

Anne Duchateau

Sorbonne Universités

UPMC Univ. Paris 06

CNRS Collège de France

Laboratoire de Chimie de la Matière Condensée de Paris

11 place Marcelin Berthelot

75005 Paris

France

Stephan H. K. Eder

Ludwig-Maximilians-University Munich

Department of Earth and Environmental Sciences Geophysics

Theresienstr. 41

80333 Munich

Germany

Ana Espinosa

UMR 7057 CNRS/Université Paris Diderot

Laboratoire Matières et Systèmes Complexes

10 rue Alice Domon et Léonie Duquet

75205 Paris Cedex 13

France

Marjorie Etique

Sorbonne Universités

Université Pierre et Marie Curie

CNRS UMR 7590

Institut de Minéralogie

Physique des Matériaux et Cosmochimie. Museum National d'Histoire Naturelle

IRD 206. 4 place Jussieu

75005 Paris

France

and

ETH Zürich

Institute of Biogeochemistry and Pollutant Dynamics

Soil Chemistry group

Universitätstrasse

8092 Zürich

Switzerland

Damien Faivre

Max Planck Institute of Colloids and Interfaces

Department of Biomaterials

Science Park Golm

Am Mühlenberg 1

14476 Potsdam

Germany

M. Luisa Fdez-Gubieda

Universidad del País Vasco - UPV/EHU

Departamento de Electricidad y Electrónica

Barrio Sarriena s/n

48940 Leioa

Spain

and

BCMaterials

Bizkaia Science and Technology Park

Building 500

48160 Derio

Spain

Ana García-Prieto

Dpto. de Física Aplicada I

Universidad del País Vasco - UPV/EHU

Paseo Rafael Moreno Pitxitxi 2

48013 Bilbao

Spain

and

BCMaterials

Bizkaia Science and Technology Park

Building 500

48160 Derio

Spain

Florence Gazeau

UMR 7057 CNRS/Université Paris Diderot

Laboratoire Matières et Systèmes Complexes

10 rue Alice Domon et Léonie Duquet

75205 Paris Cedex 13

France

Haibo Guo

School of Materials Science and Engineering

Shanghai University

333 Nanchen Road

The Materials Building B, Room 345

Baoshan District

Shanghai 200444

China

Ali Abou-Hassan

Sorbonne Universités

UPMC Univ. Paris 06

CNRS Laboratoire PHENIX

4 place Jussieu

75005 Paris

France

Ann M. Hirt

ETH Zürich

Institut für Geophysik

NO H 31

Sonneggstrasse 5

8092 Zürich

Switzerland

Mike J. Jackson

University of Minnesota

Department of Earth Sciences

Institute for Rock Magnetism

100 Union Street SE

Minneapolis MN 55455

USA

Derk Joester

Northwestern University

Department of Materials Science and Engineering

2220 Campus Drive

Evanston IL 60208

USA

Young-Shin Jun

Washington University in St. Louis

Department of Energy

Environmental and Chemical Engineering

One Brookings Drive Campus

Box 1180

St. Louis MO 63130

USA

Jelena Kolosnjaj-Tabi

UMR 7057 CNRS/Université Paris Diderot

Laboratoire Matières et Systèmes Complexes

10 rue Alice Domon et Léonie Duquet

75205 Paris Cedex 13

France

France Lagroix

Université Paris Diderot, CNRS

Institut de Physique du Globe de Paris

Paleomagnetism Research Group

Sorbonne Paris Cité

1 rue Jussieu

75005 Paris

France

Byeongdu Lee

Argonne National Laboratory

X-ray Science Division

9700 South Cass Avenue

Argonne IL 60439

USA

Christopher T. Lefèvre

CEA/CNRS/Aix-Marseille University

Biosciences and Biotechnologies Institute

UMR7265 Cellular Bioenergetics Laboratory

13108 Saint Paul les Durance

France

Admir Masic

Massachusetts Institute of Technology

Department of Civil and Environmental Engineering

77 Massachusetts Avenue

Cambridge MA 02139

USA

Chiara Mastrippolito

Adamantio srl

Incubatore di Impresa dell'Università di Torino

Via Quarello 11/A

10135 Torino

Italy

Carlo Meneghini

University Roma Tre

Science Department

Via della Vasca Navale 84

00146 Rome

Italy

Jennyfer Miot

Sorbonne Universités

Université Pierre et Marie Curie

CNRS UMR 7590

Institut de Minéralogie

Physique des Matériaux et Cosmochimie. Museum National d'Histoire Naturelle

IRD 206. 4 place Jussieu

75005 Paris

France

Michal Neeman

Weizmann Institute of Science

Department of Biological Regulation

Rehovot 76100

Israel

Marco Nicola

Adamantio srl

Incubatore di Impresa dell'Università di Torino

Via Quarello 11/A

10135 Torino

Italy

Stefan Peiffer

University of Bayreuth

Department of Hydrology

Bayreuth Center of Ecology and Environmental Research (BayCEER)

Universitätsstraβe 30

95445 Bayreuth

Germany

R. Lee Penn

University of Minnesota

Department of Chemistry

207 Pleasant Street SE

Minneapolis MN 55455

USA

David Pignol

CEA/CNRS/Aix-Marseille University

Biosciences and Biotechnologies Institute

UMR7265 Cellular Bioenergetics Laboratory

13108 Saint Paul les Durance

France

Tanya Prozorov

Emergent Atomic and Magnetic Materials Group

Division of Materials Science and Engineering

Ames DOE Laboratory

332 Wilhelm Hall

Ames IA 50011

USA

Thomas B. Scott

University of Bristol

Interface Analysis Centre

School of Physics

HH Wills Physics Laboratory

Tyndall Avenue

Bristol BS8 1TL

UK

Jennifer A. Soltis

University of Minnesota

Department of Chemistry

207 Pleasant Street SE

Minneapolis MN 55455

USA

Neil D. Telling

Keele University

Institute of Science and Technology in Medicine

Guy Hilton Research Centre

Thornburrow Drive

Hartshill

Stoke on Trent ST4 7QB

UK

Sarah J. Tesh

University of Bristol

Interface Analysis Centre

School of Physics

HH Wills Physics Laboratory

Tyndall Avenue

Bristol BS8 1TL

UK

Tina Ukmar-Godec

Max Planck Institute of Colloids and Interfaces

Department of Biomaterials

Science Park Golm

Am Mühlenberg 1

14476 Potsdam

Germany

Peter Vach

Max Planck Institute of Colloids and Interfaces

Department of Biomaterials

Science Park Golm

Am Mühlenberg 1

14476 Potsdam

Germany

Fernando Vereda

University of Granada

Biocolloid and Fluid Physics Group

Department of Applied Physics

Faculty of Science Avenida de la Fuente Nueva S/N C.P.

18071 Granada

Spain

Moli Wan

University of Bayreuth

Department of Hydrology

Bayreuth Center of Ecology and Environmental Research (BayCEER)

Universitätsstraβe 30

D-95445 Bayreuth

Germany

Claire Wilhelm

UMR 7057 CNRS/Université Paris Diderot

Laboratoire Matières et Systèmes Complexes

10 rue Alice Domon et Léonie Duquet

75205 Paris Cedex 13

France

Raz Zarivach

Ben-Gurion University of the Negev

Department of Life Sciences

National Institute for Biotechnology in the Negev

POB 653

84105 Beer-Sheva

Israel

Foreword

Iron oxide and iron oxyhydroxide minerals comprise more than 5 wt% of the Earth's crust. Hematite (α-Fe2O3), the most abundant iron oxide in the crust, has been widely used by humans for millennia, mostly as durable pigments for artistic and personal adornment. Following the discovery, about 2000 years BCE, that it could be smelted to yield iron metal, hematite obtained economic significance as iron ore for the production of iron and, after the mid-nineteenth century, steel. Thus hematite played a significant role in the building of the modern, industrialized world. It is curious in this regard that the detritus of all corroded iron and steel exposed to molecular oxygen and water is rust, a hydrous variant of hematite. The corrosion process is catalyzed by bacterial respiration, that is, the transfer of electrons from the metal surface to molecular oxygen. An interesting example is the so-called rusticles on the hulk of the Titanic on the North Atlantic seafloor.

Hematite is antiferromagnetically ordered below 250 K; at 300 K it has a weak magnetic moment (0.02 μB). The second most abundant iron oxide, magnetite (FeO·Fe2O3), is the most magnetic crustal mineral (4.1 μB). A naturally magnetized piece of magnetite is known as a lodestone. The attraction between lodestone and pieces of iron was first described in the sixth century BCE in China and in the fourth century BCE on the Aegean coast of Asia Minor. The earliest reports of a lodestone navigation device date to the twelfth century CE in both Asia and Europe. In subsequent centuries, marine magnetic compasses were fashioned from an iron needle that had been stroked along its length with lodestone. Columbus carried such a compass on his voyages across the Atlantic. The iron needles slowly lost their magnetization and had to be regularly treated with the lodestone in order to restore their magnetization. The importance of the magnetic compass cannot be over emphasized. It allowed navigators to keep their heading over long distances in the open ocean, even when the sun and stars were obscured. In this sense, iron oxides facilitated the great voyages of discovery that commenced in the fourteenth century.

Iron oxides have a longer connection to the biosphere. Iron is essential for all life forms because many essential proteins have active sites that contain iron. However, it is difficult for organisms to obtain iron from the environment because ferrous iron spontaneously oxidizes to ferric when exposed to molecular oxygen, and ferric iron is very insoluble. In order to protect excess accumulated iron for future use, it is deposited as a ferric oxyhydroxide, ferrihydrite, inside the protein ferritin, a quasispherical protein shell of diameter 12 nm with an 8 nm storage pocket.

Magnetite has been reported in organisms as diverse as chitons, trout, honeybees, pigeons, turtles, lobsters, and magnetotactic bacteria. The latter deposit magnetosomes, nanoscale magnetite crystals in intracellular vesicles, arranged in chains. The chain of magnetosomes comprises a permanent magnetic dipole that causes a cell to be oriented in the geomagnetic field and thus keep its heading as it swims.

Iron Oxides provides a comprehensive look at the geochemistry, biochemistry, and synthesis of iron oxides, especially at the nanoscale. It also presents recent advances in experimental methods for their study. Finally, it looks forward to applications of iron oxide minerals in chemical catalysis, environmental remediation, and medicine.

January 2015

San Luis Obispo, CA, USA

Richard B. Frankel

Preface

Iron oxides are ubiquitous in Nature. They can be found in geological settings as different as the surface of Mars where they mostly account for the color of the red planet or for the acidic mine drainage on Earth where their presence can help to reduce pollution. Different types of iron oxides can also be biomineralized by organisms, which in turn are used for purposes as different as iron storage, magnetic, or mechanical properties. Iron oxides are not only widely present in the environment but also have a large variety of applications that make them irreplaceable, for example, from paintings to the reconstruction of past climate and to magnetic resonance imaging. Therefore, this scientific field has evolved as a multidisciplinary field between areas as diverse as geology, biology, chemistry, and even medicine.

As a graduate student, I early on considered the book by Cornell and Schwertmann as a must. I was studying the formation of magnetite with potential application for the search of life on Mars and as soon as I had any problem, I was able to find at least some hints for the answer in this book. I had to suffer since the book was not available in France for some time (no longer printed before reedition). Now that I have my own research group, I see my students still using this book on a nearly daily basis. Participating in conferences on the subject, I could also recognize how this book was widely used in the community. However, the last edition of the book appeared about a decade ago, and though some fields have not evolved much, some have dramatically changed. I therefore happily and positively answered the offer of Dr Reinhold Weber from Wiley-VCH to update the knowledge gained during these years in the field when we met at a conference from the German Society of Chemistry in 2014.

The book thus aims at presenting the different fields associated with iron oxides, and where those play a critical scientific role. In particular, the book starts by general overviews that cover the geological and the synthetic facets as well as the biological formation of dedicated phases in organisms such as limpets, chitons, and bacteria and also in humans. The second part of the book presents modern characterization techniques that are used to analyze iron oxides. Finally, the third part addresses some current and potential applications of iron oxides, with a particular emphasis on magnetic iron oxides, which are at the core of these applications because of their magnetic properties.

I thank the authors of the different chapters for accepting to take part in this adventure. I would like to particularly thank my past and present group members who provided several of the chapters. I also particularly appreciate R. Frankel for providing the foreword of the book. I also thank the editorial team at Wiley for their support in getting the chapters in time, formatting, and proofreading those materials. I acknowledge the support of several colleagues who reviewed the manuscripts and in particular of my close collaborator Dr. Jens Baumgartner who helped with numerous chapters. My wife Nathalie, apart from others support, provided several illustrations from the field.

Potsdam, February 2016

Damien Faivre

Chapter 1

Introduction

Damien Faivre

1.1 Iron Oxides: From Nature to Applications

As the name of the book Iron oxides: from nature to applications suggests, iron oxides are not only widespread in the environment, but also widely used by mankind in a variety of applications (Figure 1.1). Both this ubiquitous presence in nature and the utilization as tools have been established for centuries and are still valid today. The first illustrative examples of iron oxides certainly are compass needle or rust (Figure 1.2). Iron oxides are present in solid, liquid, and gaseous environments, with respective examples such as rocks, as mineral inclusion in swimming bacteria or in aerosols. Depending on the type of use, several sources of iron oxides exist. Applications range from the heavy steel production to medicine and art. The different aspects of mineral formation and their use as well as modern characterization techniques are reviewed in this book.

c01fgz001

Figure 1.1 Scheme of the iron oxide occurrences, sources, and applications.

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Figure 1.2 Images of agricultural machine left in a field for decades (a). A closer view clearly shows the presence of rust (b).

As a consequence of this omnipresence and significance in scientific and technological fields, a multidisciplinary interest has emerged with iron oxides at the center of its focus (Figure 1.3). The books in the collection by Cornell and Schwertmann were the most recent examples of efforts to summarize the knowledge on the subject [1–3]. Here, we focus on scientific aspects that have developed in the meantime and are therefore mostly not present in the book series published more than a decade ago. We also present some topics that were simply not addressed previously. This is particularly true for biological iron oxide formation, the role of which has only recently been recognized, as well as for the application aspects related to the iron oxides, which were not in the focus of the previous books.

c01fgz003

Figure 1.3 The iron oxides at the core of a multidisciplinary interest.

1.2 A Very Brief Overview of the Iron Oxides and How They Found Names

There are 16 iron oxides, hydroxides, or oxyhydroxides recognized so far, all called in short iron oxides (Table 1.1). Most of them were discovered and described at the beginning of the nineteenth century. In the table below, the compounds are simply classified based on their composition, that is, they are made from ferric, ferrous, or ferric and ferrous iron; and contain oxides (O), hydroxides (OH), or oxides and hydroxides.

Table 1.1 Summary of the different known iron oxides

The references to the minerals are discussed in the text since some mineral names have varied over time.

With the advancement of the characterization and synthesis techniques, some of them were only named or fully characterized after lively debates. For example, the first mineral listed below (wüstite) was initially called lozite [4], before the name Wüstit (in German) was given by Schenck et al. [5] in recognition of Fritz Wüst, the founding director of the Kaiser Wilhelm Institute of Iron Research in Düsseldorf (Germany) (which later became the Max Planck Institute of Iron Research). The case of maghemite is even more striking: if magnetite was long known, martite was presented as having an intermediate composition between Fe2O3 and Fe3O4, closer to hematite in composition but with an octahedral form similar to magnetite [23]. However, after the compound was obtained in the lab by oxidation of magnetite [24], it was called ferro-magnetic ferric oxide and its natural existence was questioned [25]. Wagner confirmed its natural occurrence and discussed that the name ferro-magnetic ferric oxide was too long, the name oxidized magnetite misleading as the mineral in question did not contain any ferrous iron and therefore he proposed maghemite, probably as a condensed form of magnetite and hematite [13]. This in turn was problematic to Winchell [26], who disliked the fact that the name maghemite suggested a magnetic hematite. This author argued that maghemite should be used in the case of hematite being deoxidized to the composition of magnetite while retaining its own space-lattice and becoming magnetic. Finally, Winchell proposed oxymagnetite [26], a name that did not become established in the community, where maghemite is now the name recognized by the International Mineralogy Association (IMA).

Another dispute, which is certainly more contemporary, concerns ferrihydrite. It is not related to the name, rather to the structure of the mineral, which was first reported by Towe and Bradley in 1967 [27] and named 4 years later by Chukhrov [20]. Despite its ubiquitous presence in environmental environments, its sole existence as nanometer-scaled materials had made its characterization difficult by traditional X-ray diffraction techniques based on long-range order analysis. About 10 years ago, Michel et al. proposed a structure based on 20% tetrahedrally and 80% octahedrally-coordinated iron and a P63mc space group [28] but structural research is ongoing [29]. This short introductory chapter hopefully illustrates the fact that iron oxide related research has been and remains a lively field of broad interest.

References

1. Cornell, R.M. and Schwertmann, U. (1996) The Iron Oxides: Structure, Properties, Reactions, Occurrence and Uses, VCH Publishers, Weinheim.

2. Cornell, R.M. and Schwertmann, U. (2003) The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, Wiley-VCH Verlag GmbH, Weinheim.

3. Schwertmann, U. and Cornell, R.M. (2007) Iron Oxides in the Laboratory, Wiley-VCH Verlag GmbH, Weinheim.

4. Brun, A. (1924, periode 5, tome 6) Arch. Sci. Phys. Nat., 244.

5. Schenck, R., Dingmann, T., Bökmann, J., Ebert, W., Kesting, W., Lepetit, G., Müller, J., and Pratje, W. (1927) Z. Anorg. Allg. Chem., 166, 113.

6. Natta, G. and Casazza, A. (1925) Rend. Accad. Lincei, 6, 495.

7. Natta, G. and Casazza, A. (1928) Gazz. Chim. Ital., 58, 344.

8. Dana, E.S. (1911) Descriptive Mineralogy, John Wiley & Sons, Inc., New York.

9. Trolard, F., Abdelmoula, M., Bourrié, G., Humbert, B., and Génin, J.M.R. (1996) C.R. Acad. Sci., Ser. IIa: Sci. Terre Planets, 323, 1015.

10. Birch, W.D., Pring, A., Reller, A., and Schmalle, H.W. (1993) Am. Mineral., 78, 827.

11. Svendsen, M.B. (1958) Naturwissenschaften, 45, 542.

12. MacKay, A.L. (1962) Min. Mag., 33, 270.

13. Wagner, P.A. (1927) Econ. Geol., 22, 845.

14. Lacroix, A. (1901) Minéralogie de la France et de ses Colonies, vol. 3, Ch. Béranger, Paris.

15. Glemser, O. and Gwinner, E. (1939) Z. Anorg. Allg. Chem., 240, 161.

16. Francombe, M.H. and Rooksby, H.P. (1959) Clay Miner. Bull., 4, 1.

17. Chukhrov, F.V., Zyvagin, B.B., Gorshkov, A.I., Ermilova, L.P., Korovushkin, V.V., Rudnitskaya, E.S., and Yakubovskaya, N.Y. (1976) Izvest. Akad. Nauk. S.S.S.R., Ser. Geol., 5, 5.

18. Fleischer, M., Pabst, A., Mandarino, J.A., and Chao, G.Y. (1977) Am. Mineral., 62, 1057.

19. Schrader, R. and Büttner, G. (1963) Z. Anorg. Allg. Chem., 320, 220.

20. Chukhrov, F.V., Zvyagin, B.B., Gorshkov, A.I., Ermilova, L.P., and Balashova, V.V. (1973) Izvest. Akad. Nauk. S.S.S.R., 23.

21. Fleischer, M., Chao, G.Y., and Kato, I. (1975) Am. Mineral., 60, 485.

22. Bigham, J.M., Carlson, L., and Murad, E. (1994) Mineral. Mag., 58, 641.

23. Sosman, R.B. and Hostetter, J.C. (1916) J. Am. Chem. Soc., 38, 807.

24. Welo, L.A. and Baudisch, O. (1925) Philos. Mag., 50, 399.

25. Gilbert, G. (1927) Econ. Geol., 22, 308.

26. Winchell, A.N. (1931) Am. Mineral., 16, 270.

27. Towe, K.M. and Bradley, W.F. (1967) J. Colloid Interface Sci., 24, 384.

28. Michel, F.M., Ehm, L., Antao, S.M., Lee, P.L., Chupas, P.J., Liu, G., Strongin, D.R., Schoonen, M.A.A., Phillips, B.L., and Parise, J.B. (2007) Science, 316, 1726.

29. Masina, C.J., Neethling, J.H., Olivier, E.J., Manzini, S., Lodya, L., Srot, V., and van Aken, P.A. (2015) RSC Adv., 5, 39643.

Part I

Formation, Transformation

Chapter 2

Geological Occurrences and Relevance of Iron Oxides

France Lagroix, Subir K. Banerjee and Mike J. Jackson

2.1 Introduction

Iron oxide minerals have been the focus of many review articles, books and book chapters (e.g., [1–4]) because of their widespread occurrence on Earth in extended ranges of thermodynamic and biogeochemical conditions. The environmental dependence of iron oxide formation, itself, enhances the relevance of studying iron oxides as indicators of present and past conditions (e.g., temperature, moisture, pH, and redox state). In this brief chapter we give a succinct and selective view of some key aspects of iron oxide mineral formation and transformations, and their relationship with geological and environmental conditions; for much more comprehensive reviews the reader is referred to the works listed above. We begin with an abridged summary of the origins and distribution of terrestrial iron, followed by a classification and cataloging of iron oxide minerals. The final sections cover processes and conditions of formation and transformation of these minerals, which we highlight in more detail for one particular occurrence, the loess/paleosol sequences in which iron oxides have proven to be extremely valuable paleoenvironmental indicators (e.g., [5–7]).

2.2 Elemental Iron: From the Universe to the Earth

As a result of nucleosynthetic processes in stellar interiors (e.g., [8, 9]), iron is disproportionately abundant in the universe. Despite the general exponential decrease in abundance with increasing atomic number Z, atoms of iron (Z = 26) outnumber those of all but a handful of lighter elements, and by mass Fe is the sixth most abundant element in the universe, after only H, He, O, C, and Ne [10]. Solar abundances of the elements closely reflect those of the presolar nebula, as do those of primitive chondrites except for depletion of light and volatile elements in the latter (e.g., [11]). Protoplanetary formation was influenced by the temperature gradient in the solar nebula: the condensed material that was available for accretion varied systematically in bulk chemical composition as a function of distance from the Sun, from volatile depleted for the inner planets to volatile rich for the outer ones (e.g., [12, 13]). Thus Mercury consists of ∼70% iron by mass, whereas Mars contains only about 20% [14]. The proportion of Fe in the Earth was increased by the loss of a significant fraction of the silicate mantle in the lunar fission event [15, 16], and the weak gravitational pull of the Earth and the consequent dissipation of light elements have further enriched the terrestrial iron concentration to the extent that it is, along with oxygen, one of the two most abundant elements in our planet by mass, representing 28–35% of Earth's total (e.g., [17–21]). The bulk chemical composition of the Earth is roughly FeMgSiO3 plus a few percent of other elements, which, apart from a deficit of oxygen, is essentially equal to that of an intermediate olivine.

2.3 Residency of Elemental Iron on Earth

Most of Earth's iron and siderophile elements are segregated in the core [17–19], as a result of some heterogeneity of initial accretion, along with one or more major density-driven planetary chemical differentiation events (e.g., [22, 23]). The inner core consists principally of iron, along with Ni and some lighter as well as heavier elements [24–26], in a state sufficiently solid (Figure 2.1) to transmit seismic shear waves [29, 30] and to be seismically anisotropic [31–33]. The liquid outer core is composed of iron alloyed with 10–20% of lighter elements such as S, O, K, or others [34]. Fractional crystallization at the top of the inner core as it freezes produces buoyant liquid enriched in these light elements, which helps to drive convection in the outer core and to power the geomagnetic dynamo [35, 36].

c02fgz001

Figure 2.1 Phase diagrams of Fe: red from Tateno et al. [27] and green from Anzellini et al. [28], both based on static pressure diamond-anvil experiments and fast synchrotron X-ray diffraction. Anzellini et al. [28] reinterpret the solid–liquid boundary of Tateno et al. as the onset of fast recrystallization rather than melting. Black curve is the geotherm of Anzellini et al. [28]; gray area shows uncertainty. UM, upper mantle; IC, inner core; bcc, body-centered cubic (alpha) iron; fcc, face-centered cubic (gamma) iron; hcp, hexagonal close-packed (epsilon) iron.

The iron-rich core represents nearly a third of the mass of the Earth and contains most of its iron, but iron nevertheless remains an important constituent of the silicate mantle and crust. After core formation, the primitive mantle had approximate atomic proportions of Mg:Fe:Si ∼ 8 : 1 : 8 [37, 38]. Further differentiation occurred with the formation of the crust, which is strongly enriched in lithophile elements including Si, Al, Ca, Na, and K [37, 38]. The tabulation of Wedepohl [39] places iron fourth among the eight major elements in the crust, which each constitute at least 1% of its mass: O (47.2%), Si (28.8%), Al (7.96%), Fe (4.32%), Ca (3.85%), Na (2.36%), Mg (2.20%), and K (2.14%). From such a set of ingredients, it follows naturally that silicates and oxides of Al, Fe, and other metals are the most abundant and important minerals in the crust.

Iron has an electron configuration of 1s² 2s²p⁶ 3s²p⁶d⁶ 4s² and readily forms cations by losing the two 4s electrons, either alone (ferrous, Fe²+) or together with one 3d electron (ferric, Fe³+). Other valence states are possible but far less common. The abundance of iron and its variable valence ensure that it is incorporated into a wide variety of minerals including sulfides, sulfates, and carbonates as well as oxides and silicates. Our principal focus here is on iron oxide, for which the diverse mineral occurrences are presented next.

2.4 Mineral Forms of Iron Oxides

Strictly speaking, iron oxides contain only Fe and O, where Fe is present in a divalent (ferrous) state, trivalent (ferric) state, or in a mixed-valence state. At present, there are only four known naturally occurring iron oxide minerals (Table 2.1). Magnetite (Fe3O4) contains both Fe²+ and Fe³+, with a stoichiometric 1 : 2 ratio. Hematite (α-Fe2O3) and maghemite (γ-Fe2O3) both have uniquely trivalent iron, whereas wüstite (FeO) is composed of uniquely divalent iron. It is worth mentioning the recent discovery of a high-pressure Fe4O5 phase [41] likely to reside in the upper mantle. Even though it has yet to be identified in a natural setting, its formation from the breakdown of magnetite, known to be present in the upper mantle, and its known recovery in ambient conditions lead to the conclusion that it is only a matter of time before Fe4O5 is identified in a natural rock sample [42, 43].

Table 2.1 Naturally occurring iron oxide minerals (s.l.)

The prime in δ′-FeOOH identifies the naturally occurring composition [40] which structurally differs from the synthetic form with composition denoted as δ-FeOOH.

Our discussion will however not be limited to the strict definition of iron oxides but instead inclusive of hydroxides and oxyhydroxides (Table 2.1) which are key phases in sedimentary environments. Naturally occurring oxyhydroxides are all ferric iron minerals. The FeOOH composition comprises four polymorphs based on the spatial arrangement of the octahedra. The most common are goethite (α-FeOOH) with a hexagonal close packing (hcp) of anions and lepidocrocite (γ-FeOOH) with cubic close-packed anions. The rarer polymorphs, akaganeite (β-FeOOH) and feroxyhyte (δ′-FeOOH), have body-centered cubic and hcp of anions, respectively. Ferrihydrite and schwertmannite are both poorly crystalline hydrated oxyhydroxides. Ferrihydrite occurs exclusively as nanoparticles [1], in either a somewhat more crystallized six-line form, so-called because it displays six lines in X-ray diffraction [44], or a more poorly crystalline two-line form which exhibits only two broad X-ray diffraction lines. Schwertmannite has the same structure as akaganeite but bears a sulfate complex in the tunnel structure instead of a chloride ion for akaganeite. Of the two naturally occurring hydroxides, green rust (GR), like magnetite, is a mixed-valence iron mineral with variable ferrous to ferric ratios ranging between 0.8 and 3.6. As a product of corrosion, GR has chloride ions (GR1) or sulfate ions (GR2) that bind the iron hydroxide layers, whereas in the naturally occurring GR observed in anoxic soils, OH− ions are found in the interlayers [45, 46]. Bernalite is a more recently identified ferric hydroxide (Fe(OH)3) mineral [47] with a perovskite structure.

Lastly, cation substitutions in iron oxides are common and lead to solid solution series such as magnetite–ulvöspinel (Fe3−xTixO4 where x is the mole fraction of ulvöspinel, 0 ≤ x ≤ 1) and hematite–ilmenite (Fe2−yTiyO3 where y is the mole fraction of ilmenite, 0 ≤ y ≤ 1) in which titanium substitutes for ferric iron (Ti⁴+ + Fe²+ ↔ 2Fe³+). These are geologically key series indicators of the petrologic origin of igneous and metamorphic rocks and therefore of interest to our discussion. Collectively, the above compositions will be referred to as iron oxides.

2.5 Occurrence and Geological Relevance of Iron Oxides

The geological occurrence of iron oxides is initiated through either crystallization of a melt, precipitation from a solution, alteration of a preexisting mineral phase, or transport as a detrital component. The former three processes leading to the formation of iron oxides are governed by thermodynamics and biogeochemistry: the site of mineral formation and that of the occurrence are one and the same, and therefore the geological relevance of these iron oxides, to a first order, lies in revealing the local conditions of their genesis. The geological relevance of iron oxides occurring as a detrital component can, for example, reveal the workings of erosional and transport processes at variable scales from local to global. Aeolian dust and its accumulation on continental surfaces, a dynamic geological environment with respect to iron oxides, will be the focus of Section 2.6.

2.5.1 Crystallization from Melt and Partial Melts

Temperature and pressure but most importantly oxygen fugacity, fO2, and melt composition dictate the iron oxide phase or phases to be crystallized. Here only wüstite, magnetite, hematite, and the iron-titanium oxides are relevant. Iron oxyhydroxides and hydroxides are not stable at melt temperatures and will be discussed in Sections 2.5.2 and 2.6. Maghemite is thermodynamically metastable and forms by topotactic oxidation of magnetite, as described in Section 2.6.

The conditions of crystal growth of iron oxides from a melt can be predicted from experimentally defined phase stability diagrams. The temperature–composition phase diagram of the Fe–O system at 1 atm (Figure 2.2; [48]) emphasizes the wide range of conditions under which magnetite may crystallize and explains the ubiquity of magnetite in the geological record. Moreover, when considering higher pressures, magnetite is known to persist up to ∼25 GPa at room temperature [49] before transforming to a denser polymorph phase. With increasing temperatures the transformation occurs at lower pressures; at 1000 K the threshold is about 15 GPa [50]. From first-principles density functional theory, it is suggested that the high-pressure transition of magnetite occurs gradually up to pressures of ∼60 GPa [51], extending the possible occurrence of magnetite in its high-pressure form down into the lower mantle. More recently, at high pressure and temperature, it has been shown that magnetite may also decompose at ∼9.5–11 GPa (95–110 kbar) and 973–1673 K (700–1400 °C) to hematite and Fe4O5 [42, 52].

c02fgy002

Figure 2.2 The temperature–composition phase diagram of the iron–oxygen system at a total pressure of 1 atm.

(Redrawn from [48]).

It has long been known that from coexisting rhombohedral and cubic iron-titanium oxide phenocrysts (Figure 2.3), oxygen fugacity and temperature of the crystallizing melt can be estimated if equilibrium is assumed [54]. Therefore iron oxides are relevant geothermo-oxybarometers (e.g., [55–58]) in addition to their well-known relevance as recorders of the geomagnetic field. The ubiquity of magnetite in the Earth's crust with respect to other iron oxides results from oxygen fugacities of melts being rarely low enough to form wüstite (examples of exceptions: subduction zones where carbonaceous sediment mixing with melt produces reducing conditions; serpentinization of peridotites) nor high enough to favor hematite formation (examples of exceptions: fumaroles, cooling lava lakes, i.e., near surface conditions) (Figure 2.4). However, in the mantle, fO2 is some five log units lower than the fayalite–magnetite–quartz (FMQ) buffer [60], thus making possible the formation of wüstite.

c02fgy003

Figure 2.3 Fe–Ti–O phase diagram at 1300 °C. TH, rhombohedral (hematite–ilmenite) solid solutions; TM, spinel (magnetite–ulvöspinel) solid solutions; wü, Fe1−xO wüstite phase; Fe, metallic Fe phase. Vertical lines represent Ti/(Ti + Fe) of magnetite, ulvöspinel, and ilmenite end members.

(Redrawn after Senderov et al. [53]).

c02fgy004

Figure 2.4 Solid-phase oxygen buffers of the system Fe–Si–O. IW, iron–wüstite; WM, wüstite–magnetite; MH, magnetite–hematite; QIF, quartz–iron–fayalite; FMQ, fayalite–magnetite–quartz, plotted from equations in Myers and Eugster [59].

It seems likely that iron oxides occur widely in the condensed planetesimals and planets of the solar system and beyond, but direct observational evidence is only available for lunar and martian rocks and sediments and for meteorites (including some meteorites of lunar and martian origins (e.g., [61])). These extraterrestrial iron oxides have received much attention due to their potential for retaining magnetic records of past planetary dynamo evolution [62–64], tectonic activity [65, 66], and large impact events [67, 68]. Mars rover measurements show that in many of the sampled basalts, more than 10% of the iron present is in titanomagnetite that is interpreted to be of primary igneous origin [69]. However the titanomagnetite content of martian meteorites and of synthetic ferrobasalts is typically much lower than in the in situ basalts, suggesting oxide crystallization in the latter under more oxidizing conditions than FMQ and/or at temperatures as low as 1000 °C ([70]; see also [71]).

2.5.2 Precipitation from Solution and Alteration/Transformation

In surficial and near-surface environments, iron oxides may form by precipitating out of a solution containing divalent or trivalent iron, or by the alteration of a precursor mineral phase via dissolution and reprecipitation, or through solid-state topotactic or pseudomorphic transformation. The low to very low solubility of iron oxides combined with the occurrence of most iron within trivalent iron oxides should result in a general immobility of iron in nature. However, geological occurrences such as iron hardpans in soils and development of iron ore deposits are two examples showing that, in reality, iron is dissolvable and mobile. At ambient temperatures and pressures, pH and Eh dominate in dictating the thermodynamic stability and hence the occurrence of iron oxides (e.g., [72–74]). From laboratory experiments, other factors such as relative humidity (e.g., [75]), rate of Fe²+ oxidation, concentration of other anions (e.g., chloride, phosphate), cations (aluminum, silica) or organic ligands, and particle size (or surface area) control the mineralogy of the precipitate or the end product of alteration. How these laboratory observations translate in nature is not straightforward. Figure 2.5 displays the higher stability of goethite with respect to ferrihydrite across a wide range of pH values, yet ferrihydrite is commonly found in nature, providing evidence of nonequilibrium reactions [76].

c02fgy005

Figure 2.5 Eh–pH relation for goethite and ferrihydrite at a Fe²+ activity of 10−4 M l−1 and at 100 kPa and 25 °C.

(Redrawn after Schwertmann [76]).

Ferrihydrite is a poorly crystalline and invariably nanometer-sized hexagonal ferric iron compound that is usually the first product of iron diagenesis in sediments. Being structurally unstable under ambient conditions, it can be called the mother of some of the more stable iron oxides in soils. The structure is composed of hcp anions (O and OH), but the long-range structure has, to this date, defied a convergence of opinion among mineralogists [77–79]. Michel et al. have claimed the presence of 30% of tetrahedrally coordinated ferric ions based on X-ray absorption fine structure. Recently, Guyodo et al. [80] have used the synchrotron technique X-ray magnetic circular dichroism (XMCD) to confirm the presence of 28% tetrahedrally coordinated ferric ions in six-line ferrihydrite. An ordered version of two-line ferrihydrite, ferrimagnetic at 300 K, has also been claimed to exist [78] but, so far as we know, without confirmation by other sources. Ferrihydrite sheds the attached water molecules to convert (within solution) into either goethite or hematite. Both ordered ferrihydrite and hydromaghemite [81] have been proposed as explanations of the magnetic red soil in Andalucia [82] in Spain. Another source may be nanoparticles of maghemite when they are produced as intermediate-stage product (Figure 2.6). As Navrotsky [83] has shown, nanophase maghemite may be more stable than hematite and, we believe, can be the source of the red magnetic soils, the color reflecting the presence of hematite along with nanophase maghemite.

c02fgy006

Figure 2.6 Common pathways of iron oxide formation and transformation.

(Reproduced from Schwertmann [76], with permission from Springer Science+Business Media).

Nanophase materials differ from bulk materials of the same composition in various ways: surface energies and processes dominate; quantum mechanical effects become significant; chemical reactivity may be much larger; and even the distinctions between amorphous, disordered, and crystalline solids become somewhat blurred (e.g., [3, 84–87]). Changes in specific surface area and surface energy as a function of particle size cause nanophase, poorly ordered materials to grow and become more crystalline (Figure 2.7). Aging experiments have shown the spontaneous growth of nanophase goethite into larger crystals [87, 88].

c02fgy007

Figure 2.7 Model phase diagram for a nanoparticle system where surface and bulk energy contributions to the total particle free energy change considerably with respect to one another as a function of particle size. For large crystallites the stable polymorph is α. With decreasing particle size the surface energy contribution increases to the point where the β polymorph, with lower surface energy per unit area, becomes favored. With further size reduction, eventually the β phase becomes unstable with respect to an amorphous structure having lower surface energy per unit area. All nanoparticles are metastable with respect to coarsening and adopting the alpha structure.

(Redrawn from Waychunas et al. [87], after Navrotsky [83]).

A comprehensive diagram of iron oxide formation and conversion is provided in Schwertmann [76] (Figure 2.6). The mechanisms of these pathways involve either (i) hydrolysis of trivalent iron-bearing salt solutions or oxidized divalent iron-bearing salt solutions, (ii) thermal decomposition of a solid phase in a dry state or via a solution (e.g., hydrothermal setting), or (iii) complete dissolution and reprecipitation. Generally, dehydration and dehydroxylation reactions occur between two minerals of the same crystal symmetry (ferrihydrite to hematite or goethite to hematite), whereas higher temperature or dissolution and reprecipitation are generally needed to induce a hexagonal to cubic (e.g., goethite to maghemite) or vice versa (maghemite to hematite) alteration. The role of iron-reducing bacteria in inducing the formation of iron oxide in aqueous and soil environments is nonnegligible; even though in nature differentiating abiotic from biotically mediated pathways may be difficult but perhaps not impossible. Till et al.'s [89] laboratory solid-state abiotic and aqueous biotic reduction (by Shewanella putrefaciens ATCC 8071) experiments of lepidocrocite to magnetite showed a distinct morphological difference of the end product magnetite. Inorganically produced magnetite nanocrystals displayed a porous nanostructure, while biogenically induced crystals were highly crystalline and euhedral, enabling a means of identifying abiotic versus biotic formation pathways in nature. A difficulty faced by magnetite in nanophase in nature is stability in an open system such as soil where translocation, oxidation, and reduction can vary in microscopic environments.

The relevance of iron oxides as indicators of past surficial environmental conditions on Earth holds true also for Mars [90], contributing to the exploration of suitable conditions for the development of life (specifically the past presence of water) [91, 92] and even as direct (albeit controversial) evidence of ancient life on Mars [93–98].

2.6 Iron Oxides in Continental Dust Deposits

Detrital iron oxide grains in sediment possess characteristics inherited from the conditions of their initial crystallization or precipitation, as well as characteristics imposed by the subsequent processes of weathering, erosion, transport, deposition, and possible postdepositional chemical alteration, as described in Section 2.5.2. Thus, like other detrital grains, they contain complexly integrated forensic information about their source rocks and depositional environment, and they provide a basis for reconstructing local, regional, and sometimes global phenomena. In this section we focus on one canonical example: the thoroughly studied but not yet completely understood evidence of Quaternary environmental and climatic change derived from iron oxides in aeolian sediments.

Continental dust sources are widely prevalent not only in deserts like the Sahara but also in the flat, wide basins carved out by glaciers of the Quaternary period, which now contain sandy and silty remnants of glacial moraines. Here we focus on the latter because they contain a variety of iron oxides that when incorporated in aeolian dust deposits provide a valuable record of Quaternary environmental and climatic change on the continents (e.g., [99, 100]). Additional dust sources are glacial lake beds where all the water has evaporated during warm and arid climates and now contribute dust for aeolian transport. If these source areas are situated in the paths of seasonal winds controlled by higher-level jet streams, dust is lifted and transported over distances of hundreds of kilometers or longer. Large lakes or river basins may act as dust sinks, but the finest dusts (submicrometer to tens of micrometers) can also cross distances as large as the Pacific or Atlantic oceans. Deposition of dust takes place when wind velocities decrease slowly or suddenly when the dust-laden wind encounters hills or mountains. Repeated depositions can lead to dust deposits as thick as hundreds of meters as in China and central Eurasia or tens of meters as in Alaska, Central United States, South America, and western Europe. Early German-speaking geologists called the deposited dust loess or loose deposits. Since loess deposits have been most widely studied, our discussion here will concentrate on them. Repeated climatic changes over the last millennium or longer have helped develop soils over loess deposits, as in China, with the aid of increased temperature and rainfall. When such soil layers are covered up by new loess deposits during much colder and drier climates, the buried soil layers are termed paleosols. When we discuss the iron oxide contents of such alternating sediment layers, models for survival or alteration of the original iron oxides (or of the cotransported silicate dust) will borrow heavily from our experience with modern soils developed over parent rocks or sediments during the last hundreds of years. Thick loess deposits (with intercalated paleosols) can be as old as several million years or more, as in China, and they provide an archive of environmental and climatic change over such long intervals, provided we can date these horizons with accuracy and precision. The first problem with accuracy arises from absolute age dating difficulty for loess/paleosol edifices because they are not physicochemically closed systems. Physical loss of material, diagenesis, and pedogenesis, as well as lack of carbon in loess layers, contribute to major age dating problems and are beyond the purview of the present discussion.

We begin our discussion of iron oxides in loess deposits with the mineralogy of the original transported material at a deposition site. Taking the Chinese Loess Plateau (CLP) as the canonical case of loess deposits, we find that the surface sediments in putative northwestern source areas like Tarim Basin, Taklamakan Desert, Hexi Corridor, and Mongolian Gobi contain quartz, feldspar, hornblende, mica, and other silicates but only traces of iron oxides as coarse-grained magnetite [7, 100]. Suggestions have been made that the magnetite is coarse (10–100 µm) and magnetically multidomained because the fine-grained magnetite has already been transported to the CLP. However, we can argue that the presence of coarse-grained magnetite in likely source areas combined with the observation that such magnetite is rarely seen in the CLP is in fact an argument in favor of local pedogenesis as the driver for creation of the common finer-grained magnetite and that their partly oxidized (maghemitized) products found in the CLP are produced in situ. Liu et al. [101] used wet magnetic separation followed by grain size separation of the remaining loessic silt material by settling it in a water column and applying Stokes' law. They found that magnetically pseudo-single-domain sized (1–10 µm) particles of magnetite and oxidized magnetite were present in loess and therefore of detrital origin. Similarly, Deng et al. [102] studied a long transect from northwest of the CLP to southeast of the CLP to conclude that the magnetic species (maghemite and oxidized magnetite) were formed not far from their present deposition site.

Although it is unlikely that single particles of hematite or goethite are present in source areas, these minerals may more likely be present as patches or stains on iron-poor silicates during stages of saltation when they have suffered occasional heavy rainfalls common in dry lands. Alteration of such poorly crystalline hematite or goethite (during the dry phase after monsoonal rainfall) in the CLP to produce submicrometer- to nanometer-sized magnetite or oxidized magnetite in the paleosol layers has not been addressed in a thorough manner yet. Bulk loess (without magnetic separation) from ocean sediments near the margins of North and South America displays a bimodal particle size distribution with a major peak near 30 µm and a smaller, asymmetric one near 10 µm with a tail extending into small grains significantly less than 2 µm [103]. The magnetic iron oxides that we will delve into here – magnetite, oxidized magnetite (meaning compositions corresponding to a solid solution between magnetite and maghemite), goethite, and hematite – have been found mainly within this grain size peak at ∼10 µm.

In the previous paragraph we have discussed the identities of iron oxides that are transported to the CLP from putative sources to the north and northwest of the sink areas. However, it is the identity and relative amounts of different iron oxides found in glacial loess layers and interglacial paleosol that provide information about changes in the climatic environment that is claimed to have led to their final size and composition. They are, therefore, our principal target here. Secondly, rapid analysis of composition, concentration, and particle size of such very small iron oxides has been made possible only via magnetic methods. The principles behind specific magnetic techniques that reveal the target properties are the subject of Chapter 15 in this volume. To highlight the connections between mineral occurrence and environmental conditions, we begin with the thermodynamic stability fields of iron oxides (goethite and hematite, commonly found in loess and the intercalated paleosol) by reference to Figure 2.5 [76] while recalling that such stability fields can be modified by size-controlled surface energy terms when the minerals occur as nanometer-sized (1–50 nm) particles, as indicated in Section 2.5.2 [85, 86].

Why mostly goethite, magnetite, oxidized magnetite, and hematite are prevalent in soils with near normal pH values can be answered by reference to Figure 2.6, from Schwertmann [76], whose work has clearly been influential, except in one major instance. Even though it is possible to show that ferrihydrite-to-hematite conversion can happen via dehydroxylation in the solid phase, Schwertmann et al. [104] have argued that in nature water has to be present during such a reaction and recrystallization to hematite. A requirement of the presence of water in soil during recrystallization of hematite, if generally true, weakens the proposition that the ratio of goethite to hematite in the B-horizon of any given soil can be used as a proxy for precipitation [105] (or precipitation minus evaporation [106]). Kampf and Schwertmann [107] have argued in the past that the ratio of goethite to hematite in temperate regions of Europe can be used as proxy for warm, wet conditions. The argument behind this depended on a bifurcation model of ferrihydrite aging leading to hematite in warm, dry regions, while goethite production would be limited and proportionately less. However, a prolific number of publications since 1990 have considered strongly ferrimagnetic magnetite and/or maghemite, or a solid solution of the two, as the main proxy for modern and Quaternary period rainfall. We have generally ignored in the past the weakly magnetic (antiferromagnetic) hematite or goethite as precipitation proxy (especially for high rainfall (>1000 mm per year)) perhaps due to the ease with which high susceptibility ferrimagnets (magnetite/maghemite) can be recognized in the field with simple susceptibility meters. The situation may be changing now, as some older and some new observations of a relationship between precipitation and production of antiferromagnetic hematite and goethite have come to the fore [108–110].

The observation by many rock magnetism practitioners that neoformed magnetite was present in soil A- and B-horizons was debated at first because of the perceived lack of a clear pathway (see Figure 2.6; [76]) to form cubic close-packed magnetite from hcp ferrihydrite, but repeated observations of nanophase magnetite and maghemite in topsoil in the United Kingdom [111–113] and United States [114, 115] begged the question of where from originates neoformed magnetite in soil. Inorganic production of nanophase magnetite [116–118] and later an observation of even biogenic magnetite magnetosomes in bacteria within a bog soil [119] led to a general supposition that along with ferrihydrite, goethite, and hematite, magnetite (and oxidized magnetite or maghemite) is also a member of iron-bearing minerals in topsoil. Additionally, Lovley [120, 121] and Sparks et al. [122] have thoroughly proven the production of nanophase extracellular magnetite in pure cultures by iron-reducing bacteria. However, the preservation of such extracellular magnetite is a question that has not yet been satisfactorily answered.

2.7 Concluding Remarks

Iron oxide minerals (s.l.) are almost ubiquitous in the rocks, sediments, and soils accessible on Earth's surface. Their mineralogy, concentrations, particle sizes, crystallinity, and other characteristics reflect their geological and environmental history, and they are therefore important sources of information about the evolution of coupled atmosphere–biosphere–hydrosphere–lithosphere systems. Moreover the same iron oxide mineral characteristics can be sensitively detected and quantified by magnetic measurements, which thereby provide valuable complements to isotopic, geochemical, and other characterization techniques.

Acknowledgments

This is contribution number 3682 of the Institut de Physique du Globe de Paris and 1507 of the Institute for Rock Magnetism (IRM). The IRM which is funded by the Instrumentation and Facilities Program of the National Science Foundation and by the University of Minnesota.

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