Comp. by: DShiva Date:15/2/07 Time:14:54:18 Stage:1st Revises File Path://spiina1001z/womat/production/ PRODENV/0000000005/0000001725/0000000022/000000A296.3D Proof by: QC by: MAGNETIC PROXY PARAMETERS 525 of terrestrial, synthetic, and lunar ilmenites. Earth and Planetary practice one should look for parameters that describe the essential parts Science Letters, 26: 377–386. of the system. Because these parameters can only describe the system Smirnov, A.V., and Tarduno, J.A., 2002. Magnetic field control of the in an approximate fashion they are termed “proxy parameters” or low-temperature magnetic properties of stoichiometric and cation- “proxies.” There exists a myriad of proxies including the chemical deficient magnetite. Earth and Planetary Science Letters, 194: analysis of inorganic or organic compounds, the number and types 359–368. of living and fossil biota, and the physical properties of a sample, such Snowball, I., and Torii, M., 1999. Incidence and significance of magnetic as grain-size distribution, porosity, magnetic properties, etc. Basically, iron sulphides in Quaternary sediments and soils. In Maher, B.A., and each physical, chemical, or biological parameter that is measurable Thompson, R. (eds.), Quaternary Climates, Environments and reasonably quickly at reasonable cost may serve as proxy to describe Magnetism. Cambridge: Cambridge University Press, pp. 199–230. a complete system. One should realize that up to over several thou- Strangway, D.W., Honea, R.M., McMahon, B.E., and Larson, E.E., sands of data points are rapidly gathered in a case study. Combinations 1968. The magnetic properties of naturally occurring goethite. of two or more of those parameters may yield valuable proxies as well. Geophysical Journal of the Royal Astronomical Society, 15: Another important condition for a parameter to serve as proxy is that 345–359. its meaning must be understood. This pertains to the analytical point Syono, Y., 1965. Magnetocrystalline anisotropy and magnetostriction of view as well as to its functioning in the natural system. In the fol- of Fe3O4–Fe2TiO4 series—with special application to rocks mag- lowing, we outline the merit and potential of the so-called mineral- netism. Japanese Journal of Geophysics, 4:71–143. magnetic proxies. Taylor, R.M., and Schwertmann, U., 1974. Maghemite in soils and its From the analytical viewpoint, the assets of mineral-magnetic or origin. I. Properties and observations on soil maghemites. Clay rock-magnetic proxy parameters are manifold. They include sensitivity, Minerals, 10: 289–298. rapidity, ease of sample preparation, comparatively modestly priced Thorpe, A.N., Minkin, J.A., Senftle, F.E., Alexander, C., Briggs, C., instrumentation for data acquisition, nondestructiveness, and in parti- Evans, H.T. Jr., and Nord, G.L. Jr., 1977. Cell dimensions and cular the ability to sense grain-size variation in the ultrafine grain-size antiferromagnetism of lunar and terrestrial ilmenite single crystals. range. Magnetic properties refer to bulk sample properties in contrast Journal of Physics and Chemistry of Solids, 38:115–123. to (electron) microscopical techniques where often only a small frac- Tucker, P., 1981. Low-temperature magnetic hysteresis properties of tion of a sample can be analyzed. Therefore, mineral-magnetic para- multidomain single-crystal titanomagnetite. Earth and Planetary meters are well suited as proxy parameters. As drawbacks should be Science Letters, 54: 167–172. mentioned that some of the parameters are nonunique, and that their Uyeda, S., 1958. Thermo-remanent magnetism as a medium of palaeo- scales are most often utilized in a relative sense. Often ratios of several magnetism, with special reference to reverse thermo-remanent parameters are used to reduce nonuniqueness. The meaning of a ratio magnetism. Japanese Journal of Geophysics, 2:1–123. may change depending on the absolute values of numerator and deno- van der Lugt, W., and Poulis, N.J., 1961. The splitting of the nuclear minator; hence, may vary among case studies. Proper interpretation of magnetic resonance lines in vivianite. Physica, 27: 733–750. a mineral-magnetic data set therefore requires expert training and a Verwey, E.J.W., 1939. Electronic conduction of magnetite (Fe3O4) and feel for potential variability in the data cloud. its transition point at low temperatures. Nature, 44: 327–328. Verwey, E.J.W., and Haayman, P.W., 1941. Electronic conductivity and transition point of magnetite (“Fe3O4”). Physica, 8: 979–987. Magnetism Walz, F., 2002. The Verwey transition—a topical review. Journal of Mineral-magnetic parameters refer to the concentration and properties Physics: Condensed Matter, 14: R285–R340. of iron oxides (and some iron sulfides), minerals that occur in trace Weiss, P., and Forrer, R., 1929. La saturation absolue des ferromagné- amounts in any rock, soil, or even organic tissue. In quantum tiques et les lois d’approche en fonction du champ et de la tempéra- mechanics, electrons are assigned orbital and spin magnetic moments. ture. Annales de Physique,10e série, 12: 279–374. For magnetism, the spin moment is most important: each electron can Weiss, P., and Renger, K., 1914. Die anfängliche Permeabilität von be regarded as a microscopic magnet. In most compounds electrons act Eisen und Magnetit in Funktion der Temperatur und die Abhängig- independently and consequently a very low-magnetic moment is mea- keit der Umwandlungspunkte von der Feldstärke. Archiv für sured in applied magnetic fields and no permanent or remanent Electrotechnik, 2: 406–418. magnetic moment can exist. This is the case in diamagnetic and Zergenyi, R.S., Hirt, A.M., Zimmermann, S., Dobson, J.B., and paramagnetic substances. Diamagnetism—occurring in compounds Lowrie, W., 2000. Low-temperature magnetic behavior of ferrihy- with no unpaired electron spins—is characterized by a small but nega- drite. Journal Geophysical Research, 105: 8297–8303. tive magnetic moment in applied fields. It is a property of all matter because all paired electrons yield a diamagnetic moment. Paramagnet- — — Cross-references ism characteristic of compounds with unpaired electron spins yields a positive magnetic moment that outweighs diamagnetic Iron Sulfides moments very quickly. Magnetic Mineralogy, Changes due to Heating By virtue of exchange interaction, a property described also by Magnetic Proxy Parameters quantum mechanics, the outermost electrons of transition elements Rock Magnetism may act collectively in some compounds, provided that their orbitals Rock Magnetism, Hysteresis Measurements sufficiently overlap. For the first row transition elements, this situation occurs in metal oxides. In nature, iron is the most important element and the collective electron behavior is termed ferromagnetism (sensu lato). Several classes of ferromagnetism are distinguished: ferromag- MAGNETIC PROXY PARAMETERS netism, ferrimagnetism, and antiferromagnetism (Figure M48). Ferromagnetic and ferrimagnetic materials have very large magnetic moments in applied fields, several orders of magnitude larger than a Introduction typical paramagnet. Antiferromagnetic materials ideally would have As a rule, a natural system is complex and the result of the interplay of a net magnetic moment of zero but in reality usually a weak moment numerous constituting parameters. To fully describe such systems, ide- persists. Collective spin behavior gives rise to the existence of perma- ally each (independent) parameter must be known. However, their nent or remanent magnetization, i.e., magnetic moments that remain mere number would imply a substantial analytical operation and in after removal of an external magnetic field. The magnetic properties Comp. by: DShiva Date:15/2/07 Time:14:54:19 Stage:1st Revises File Path://spiina1001z/womat/production/ PRODENV/0000000005/0000001725/0000000022/000000A296.3D Proof by: QC by: 526 MAGNETIC PROXY PARAMETERS of ferromagnets (sensu lato) are dependent on mineral type, tem- mineral; it is one of the endmembers of the titanohematite solid-solution perature, grain size, and strain state of the grain. Subtle variations in series (the other endmember is ilmenite, FeTiO3). Goethite magnetic properties can thus be interpreted in terms of changing con- (a-FeOOH), pyrrhotite (Fe7S8), and greigite (Fe3S4) are often referred centration, grain size, or strain state of the magnetic materials. In a to as minor magnetic minerals. Some copper minerals show magnetic geoscientific context, in turn, these can be interpreted along the lines properties as well but their occurrence is restricted to certain iron of changes in catchment area, in climate, in diagenesis, or in oxidation types. Metallic iron is very rare in terrestrial environments but occurs degree when dealing with a series of lava flows. This is exploited prominently in meteorites. Magnetic minerals can be distinguished from when utilizing magnetic properties as proxy parameters. each other by their Curie point temperature, the temperature above which collective behavior is lost and a paramagnetic substance re- Magnetic minerals mains. When dealing with antiferromagnetic minerals the temperature at which collective behavior is lost, is referred to as Néel point tem- – In nature, magnetic minerals are largely restricted to iron titanium oxi- perature. In practice Curie and Néel temperature are used inter- des that may contain minor amounts of substituted aluminum, chro- changeably. When cooling below the Curie or Néel temperature, mium, manganese, and magnesium. The most important