This issue of the MRS Bulletin is devoted to Earth materials — a subject so broad and with such a long history, stretching back to man's first use of materials, that we can offer only a small sample of topics, only a partial review. As our ancestors fashioned the first crude tools from stone, they must have distinguished among materials on the basis of their physical properties. They classified materials by color and density and very early must have wanted more than Nature had provided. They began to synthesize and fashion their own materials. Today's nanoscale engineering of materials is part of this same quest—a continuous evolution of this early toolmaking.

Thus, Earth materials were the first raw materials—and the first subjects of study and classification in materials science. Probably one of the earliest materials scientists to make quantitative observations was Nicolas Steno (1638–1686, actually Nils Stensen, a Dane by birth), who was among the first to note the “law of constancy of angle” in comments which accompany his drawings of quartz and hematite crystals (De Solido intra Solidum Naturaliter Conteno Dissertationis Prodromus, 1669). Such observations were the first clues to understanding the internal structure of solids. Steno was also a keen geological observer. He is given credit for the first statement of the law of superposition of sedimentary strata. The first quantitative experiments on the origin of minerals grew out of studies of glass and ceramics. French mathematician René-Antoine Ferchault de Réaumur (1683–1757) tried to determine the composition of Chinese porcelain by melting minerals and rocks.

Nature is generous with complexity. The number of thermodynamic variables necessary to describe even simple stream chemistry can easily number to a hundred. Nevertheless, research on Earth materials remains exciting because of the vastness of geologic time and the huge scale of global processes. For example, even simple ion-exchange experiments have profound implications when considered in the context of global cycling of elements. Sodium exchange from seawater onto the 1.83 × 1016 g of river-borne clays removes 20–30% of the yearly sodium addition to the ocean. Research on Earth materials, although complex, is rewarding through the scale of the potential result.

The surface chemistry of minerals is important for understanding natural mineral transformations and also because surface reactions help control the migration and degradation rates of pollutants in natural waters. These pollutants range from organic herbicides and pesticides, which leak past reactive soil horizons into groundwaters, to acid rain and heavy-metal leaching from mine tailings, sewage sludge, or coal fly ash. The importance of characterizing mineral surface chemistry is clear when one considers that 0.5 to 2% of usable groundwater in the United States is thought to be contaminated.

This article reviews some simple surface chemistry of oxide and silicate minerals in water. We focus on the kinetics of mineral corrosion because this subject is interesting to both geochemists and materials scientists. The surface properties that make some solid oxides relatively inert to acid corrosion, for example, are also manifested in the rates of natural mineral weathering.

Man has walked on the moon, sent probes to or near the surfaces of Mars and Venus, and dispatched vehicles to fly near comets, asteroids, and the outer planets and their moons. In contrast, a journey to the center of the Earth remains as unattainable and fictional as in Jules Verne's day. We know the interior of our planet from several types of evidence: direct sampling of rock brought to the surface by geologic process from depths no greater than 200 km, remote sensing especially by seismology (the study of natural or anthropogenic sound waves passing through the Earth), inferences from the chemistry of meteorites and from other geochemical arguments, and laboratory and computational simulations of the conditions at depth. The laboratory study of Earth materials at high temperatures and pressures is the subject of this review. In a sense, the basic question of deep earth geophysics is a peculiar sort of inverse problem in materials science; rather than determining the properties of a given material, one seeks to find materials, under constraints of natural elemental abundances, which have properties consistent with seismological and other geophysical observations.

What are the “hard facts” about planet Earth as a material system? Its radius and mass are well constrained, as are pressure and density as a function of depth (see Figure 1). Its vertical temperature distribution is less well known, but it is clear that the interior is hot, with temperatures in the mantle reaching perhaps 3000 K and in the core perhaps 6000–7000 K.

As early as 1893, mineralogist W.C. Broegger recognized the first example of the transition from the crystalline to aperiodic (amorphous) state in minerals and defined the term “metamikte.” Metamict minerals were considered to be one of three classes of amorphous materials (porodine and hyaline being the other two), but distinguishable as phases that were originally crystalline, as evidenced by well-developed crystal faces. The amorphous state was determined on the basis of a characteristic conchoidal fracture and optical isotropy. There was no mention of radiation damage as a potential cause. In 1914, Hamberg first suggested that metamictization is a radiation-induced, periodic-to-aperiodic phase transition caused by alpha-decay of the constituent radioactive uranium and thorium. In the late 1930s, Stackelberg and Rottenbach tried to test this hypothesis directly by bombarding a thin slab of zircon with alpha particles. Although unsuccessful, the experiment must have been one of the first in which an “ion beam” was used to “modify” a material. After this early effort, there was little research on metamict minerals, and they remained a mineralogical curiosity.

R.C. Ewing's interest in this topic began in the early 1970s. Since then, there has been a continuing research program using modern analytical techniques on minerals that have received α-decay doses up to 1026 α-decay events/m3 over geologic time periods up to 109 years. As an example, electron diffraction patterns have shown that naturally occurring zirconolites (CaZrTi2O7) containing varying concentrations of thorium oxide (up to 19 wt% ThO2) are amorphized to different degrees depending on their age and the resulting α-decay event dose (Figure 1).

From the earliest days of extractive metallurgy, materials scientists and geoscientists have shared common ground. Experimental approaches, such as phase equilibrium and structural studies, are often similar, as are the questions asked in attempts to connect microscopic fundamentals to technologically desired or naturally observed bulk properties. The actual materials studied by both groups are often similar or even identical, such as silicate ceramics and glasses, magnetic oxides, and crystals based on the perovskite structure.

Nuclear magnetic resonance (NMR) was applied to solid-state physics shortly after the technique was invented in 1946. Even at the start, many of the samples placed in magnets in physics laboratories were large single crystals of naturally occurring minerals such as gypsum (CaSO4 · 2H2O) and fluorite (CaF2), perhaps borrowed from mineralogist colleagues. In the last 10 years, however, applications to both the earth and materials science have rapidly expanded because of improvements in both technological capabilities and basic theory. Only work on inorganic materials will be discussed here, although 13C NMR studies have proved very useful in characterizing the complex, often inseparable mixtures of large organic molecules found in soils, kerogen, and coal. I will not attempt to thoroughly review the broad and fast growing literature in inorganic applications. Instead, I have chosen examples, primarily from our recent studies, to illustrate the scope of what is and will become possible.

Several recent books clearly introduced the basic concepts of solid-state NMR, and applications to crystalline and glassy silicates as well as NMR at high temperature have been reviewed recently.

Glasses are a minor fraction of Earth materials. However, all igneous processes within the Earth involve a melt phase (magma), and our understanding of the physical and chemical features of these processes is contingent upon an adequate understanding of the melt phase itself. In particular, we wish to relate the behavior of such melts to their composition and structure in the range of physical and chemical conditions in which they occur. The physical conditions of many of these processes are particularly difficult to reproduce in the laboratory, and the ability to predict behavior under such conditions is the goal. From a geological viewpoint, the primary interest is in silicate melts because of the composition of the Earth's lithosphere and mantle. There are significant experimental difficulties in looking at the behavior of silicate melts, so much work has focused on glasses as melt analogues since spectroscopic data on silicate glasses and melts (Figure 1) are similar.

Silicate glasses are also of considerable industrial interest, and early work in the geological field borrowed heavily from materials science. However, the thrust to understand natural magmas led geologically based work into more complex chemical systems (e.g., hydrous alkali-alkaline-earth-aluminosilicates) and their behavior at high pressures.

The structure of glasses is a complex issue, and many different experimental techniques are needed to resolve the details of their structure. Here I will focus on the following techniques: IR (infrared), Raman, Mössbauer, EXAFS (extended x-ray absorption fine structure) and XANES (x-ray absorption near-edge spectroscopy) spectroscopies.