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Mineralogy
Richard J. Harrison

In the past year we have seen exciting new studies illustrating the fundamental importance of mineral behavior and its effect on geological processes on Earth and other planets. The central theme has been how the behavior of minerals is influenced by their microstructure — that is, the arrangement of structural and chemical heterogeneities at a micrometer-to-nanometer-length scale. Microstructure provides a record of the phase transformations that a mineral has undergone over its lifetime, allowing mineralogists to unravel its complex geological history. Recent interest has focused on how the response of microstructures to external stimuli (for example, a mechanical stress or a magnetic field) can profoundly effect physical properties. Described here are the results of three studies. The first relates to the origin seismic of attenuation in the lower mantle and how the presence of transformation twins can lead to dramatic changes in the seismic-frequency mechanical properties of perovskite. Two other studies address the origin of strong and stable remanent magnetization in the Martian crust and how nanometer-scale exsolution microstructures can lead to the generation of planetary-scale magnetic anomalies.

Figure 1. Image of the magnetization states of individual magnetite blocks in a natural intergrowth of magnetite and ulvöspinel. Figure courtesy of Richard Harrison.

Seismic-frequency anelasticity in perovskite


As seismologists develop a sharper picture of seismic attenuation in the lower mantle, there is now a clear need to understand the mineralogical and rock-mechanical processes responsible for the attenuation of seismic waves (B. Romanowicz and J.J. Durek, Earth's Deep Interior - Mineral Physics and Tomography from the Atomic to the Global Scale, American Geophysical Union, 2000). To investigate the microscopic origin of anelasticity in minerals it is necessary to determine their mechanical response in the low-frequency seismic range (millihertz-hertz) rather than the high-frequency ultrasonic range (Megahertz-Gigahertz) more conventionally used to measure the elastic properties of minerals. This is achieved using the technique of dynamical mechanical analysis, employed in either forced-torsional-oscillation or three-point-bend geometry.

Much recent work in this field has concentrated on the role of grain-boundary viscoelastic processes (grain-boundary sliding) as a source of seismic attenuation (for example, Webb and others, Physics of the Earth and Planetary Interiors, v. 115, p. 259-291).

Harrison and co-workers considered an alternative mechanism related to the motion of transformation domain walls in perovskite (for example, Physics of the Earth and Planetary Interiors, v. 134, p. 253-272). They investigated the seismic-frequency mechanical behaviour of single-crystal lanthanum aluminate, a structural analogue of lower-mantle magnesium silicate perovskite. A structural phase transition at 550 degrees Celsius leads to a loss of symmetry from cubic to rhombohedral, accompanied by a spontaneous shear lattice strain. A choice of energetically equivalent crystallographic orientations for the spontaneous strain leads to the formation of transformation twins separated by domain walls. When a dynamic stress in the seismic-frequency range is applied, the mechanical strain is accommodated by the lateral displacement of domain walls.

The result is a dramatic mechanical softening and a large relaxational peak in the mechanical loss spectrum. The frequency and temperature dependence of mechanical loss yields an activation energy for domain wall motion of 0.72 electron volts (72 kilojoules/mole), close to that observed for the diffusion of oxygen vacancies in perovskite. Given reasonable estimates for the activation volume of oxygen vacancy diffusion, it is possible that domain walls play a significant role in determining the seismic properties of perovskite in the lower mantle.

Nanoscale magnetic interactions in minerals

Since the first discovery of magnetic anomalies in the Martian crust (for example, Acuña and colleagues, Science, v. 284, p. 790-793), a debate has raged as to the mineralogical nature of the magnetic carriers (for example, G. Kletetschka and others, Physics of the Earth and Planetary Interiors, v. 119, p. 259-267. The anomalies were created more than 4 billion years ago, at a time when Mars still generated its own magnetic field. Now that the Martian geodynamo has stopped, the anomalies can only be explained by a remanent magnetization carried by iron-bearing minerals in the crust. The remanent anomalies are around 20 times stronger than those observed on Earth and, given their age, are extremely stable.

This combination of magnetic properties is difficult to generate in nature without the aid of fine-scale microstructures. It has long been proposed that solid-state processes such as subsolvus exsolution could generate stable single-domain (SD) magnetite particles from multi-domain (MD) grains via processes of grain subdivision. Harrison and co-workers (Proceedings of the National Academy of Sciences, v. 99, p. 16556-16561) used off-axis electron holography (a TEM technique that yields a two-dimensional vector map of the magnetic fields in a sample with nanometer resolution) to image the magnetization states of individual magnetite blocks in a natural intergrowth of magnetite and ulvöspinel (Fig. 1). Both single-domain and vortex states in individual magnetite blocks could be imaged directly, and detailed information about magnetostatic interaction fields was obtained. The magnetic microstructure was found to be dominated by the shape anisotropies of individual blocks and by magnetostatic interactions between neighboring blocks.

Lamellar magnetism

Arguably, the terrestrial magnetic anomalies most closely resembling those in the Martian crust are observed in igneous and metamorphic provinces in Sweden, Norway, Canada and the United States (for example, McEnroe and others, Journal of Geophysical Research, v. 106, p. 30523-30546), characterized by large remanence-dominated magnetic anomalies up to 1 billion years old. The rocks have high coercivities and high thermal stabilities, making them excellent analogies of the magnetic anomalies on Mars. This intriguing possibility has led to a comprehensive multidisciplinary study of their magnetic mineralogy (Robinson and others, Nature, v. 418, p. 517-520).

Common to all the rocks studied are grains of ilmenite-hematite containing multiple generations of exsolution lamellae, with thickness ranging down to unit-cell scale (1-2 nanometers). These minerals consist of canted antiferromagnetic hematite intergrown with paramagnetic ilmenite. The hematite-rich phase has a high Néel temperature but is only weakly magnetic. The ilmenite-rich phase has the potential to be strongly ferrimagnetic, but its Néel temperature is well below room temperature. The observed magnetic properties of the intergrowth present something of a paradox, since they exhibit both large magnetization and high Néel temperature.

This paradox has been resolved with the aid of Monte Carlo computer simulations (for example, Harrison and Becker, European Mineralogical Society Notes in Mineralogy, v. 3). Their work has laid the basis for the theory of "lamellar magnetism," in which a stable ferrimagnetic substructure is created by the characteristic arrangement of cations and spins at the coherent interfaces between hematite and ilmenite exsolution lamellae, a concept that has revolutionized thinking about the possible magnetic moments in exsolved phases.

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Harrison is a teaching fellow in the Department of Earth Sciences at the University of Cambridge, England. E-mail: rjh40@esc.cam.ac.uk.

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