Background about metals and intermetallics

After the discussion of some of the most common but also the most complex minerals in Chapter 21, we now examine some chemically and structurally very simple compounds that are rare and form only under unusual conditions. In this chapter we discuss minerals of native elements such as graphite (C), diamond (C), copper (Cu), gold (Au), and silver (Ag) and solid solutions of these elements (for instance, Au–Ag and Au–Cu) (Table 22.1). There are also intermetallic compounds with ordered crystal structures that differ from the end members. Often, these intermetallic compounds form during cooling as a result of ordering of high-temperature solid solutions of the same compositions. Some intermetallic minerals are awaruite (FeNi3) and auricupride (Cu3Au), both occurring in serpentinites.

For many years it was generally assumed that only a few elements could exist in nature in the native form because of their chemical inertness. Just a few decades ago no mineralogist would have imagined that elements such as aluminum, cadmium, and silicon could form in nature. Yet recent investigations with electron microscopes and electron microprobes have led to the discovery of minerals whose existence was thought to be impossible in rocks and ores. Certainly they occur under unique conditions and are found in infinitesimally small quantities and tiny grains, which generally cannot be seen, even with an optical microscope.

Overview

The color of a mineral is our perception of the wavelengths of light that are either reflected or transmitted through the material and that reach our eye. Color is one of the most striking features of minerals and is most readily observed (e.g., Loeffler and Burns, 1976). There are many reasons why a mineral displays a particular apparent color, all related to the interaction of light with the crystal. Light may be transmitted, absorbed, scattered, refracted, or reflected by a crystal (Nassau, 1980). As we will see, however, color is generally not a bulk property determined by the general structure, as for example is the refractive index, but rather depends on the trace elements present, or on mineral defects. For example, a mineral with the general composition Al2O3 may be white (as corundum, Plate 1e), red (as ruby, Plate 1f), or blue (as sapphire, Plate 1g), with only very minor differences in composition. The same is true for quartz, basically SiO2, which can be colorless-transparent (Plate 2a), brown (as smoky quartz, Plate 2b), black (as morion), purple (as amethyst, Plate 2c), yellow (as citrine, Plate 7c, 20c), pink (as rose quartz, Plate 20d), or green (as chrysoprase). A summary of different causes of color in minerals is given in Table 15.1.

Absorption

If white light is transmitted through a crystal without absorption, the crystal appears clear and colorless. If some wavelengths are preferentially absorbed, the combination of the remaining spectrum is perceived as color. For example, in the corundum variety ruby, the colors violet, green, and yellow are preferentially absorbed, leaving a spectrum composed largely of blue and red that gives rise to the typical dark-red ruby color.

We have already discussed the interaction of electromagnetic radiation and atoms in Chapters 11 and 13.

Overview

Mineralogy relies on a quantitative characterization of minerals and many different techniques are available. We have discussed X-ray diffraction (see Chapter 11) and analyses with the petrographic microscope (see Chapters 13 and 14). In this chapter we will describe briefly some other methods. Reading it you will not become an expert, but at least you will have an idea of how to pursue more in-depth studies. Some introductory books that give you an introduction into advanced analytical methods are given in “Further reading” at the end of this chapter. You may want to skip the chapter for now and return to it after knowing more about mineral systems (Part V), in order to better appreciate the discussion of examples.

The petrographic microscope furnishes some immediate answers to mineralogical inquiries but there are obviously limitations. One of them is the spatial resolution, which is limited by the wavelength of visible light. For high-resolution imaging, electron microscopes provide unique opportunities. Two types of electron microscopes are applied. The scanning electron microscope (SEM) studies crystal surfaces at high magnification. It can be used to determine morphology and compositional variations on a very fine scale (<1 μm) (e.g., Reed, 2010). The transmission electron microscope (TEM) is used to investigate microstructures and defects, such as dislocations, antiphase boundaries, exsolution lamellae, and microtwins. Over 50 years, since its first use for mineralogical studies, the TEM has revolutionized mineralogy by documenting that many minerals are heterogeneous on a very fine scale. It has also led to the discovery of many new minerals that exist only as submicroscopic particles. Taking advantage of its high resolution it is possible to use the TEM for imaging individual atoms. There are other methods for imaging at the microscopic scale. Atomic force microscopy and X-ray tomography provide three-dimensional images with resolutions reaching the nanometer scale.

What is mineralogy?

The answer to the question posed above may seem obvious: mineralogy is the study of minerals. From introductory geology classes you may know that all rocks and ores consist of minerals. For instance, quartz, biotite, and feldspar are the main minerals of granites; and hematite and magnetite are the major minerals of iron ores. At one point mineralogy was well defined as dealing with those naturally occurring elementary building blocks of the Earth that are chemically and structurally homogeneous. This simple definition of a mineral has changed over time. As the definition of “mineral” has become more vague the boundaries of “mineralogy” have opened and increasingly overlap other sciences.

In this book we take a broad view of mineralogy. Minerals are naturally occurring, macroscopically homogeneous chemical compounds with regular crystal structures. Traditionally also included are homogeneous compounds that do not have a regular structure such as opal (a colloidal solid), natural liquid mercury, and amorphous mineral products formed by radioactive decay, known as metamict minerals. Rocks, ores, and mineral deposits, which are studied in petrology and geochemistry, will also be discussed in order to emphasize the geological processes that are of central interest to all who study Earth materials.

Other materials are more peripheral but nevertheless have similar properties and obey the same laws as the minerals mentioned above. For example, ice is mainly the object of glaciology, planetary and soils science. Apatite (a major constituent of bones and teeth), carbonates (which form the skeletons of mollusks), and oxalates and urates (of which human kidney stones are composed) are studied in medicine and in a specific branch of mineralogy called “biomineralogy”. Crystals growing in concrete, known as cement minerals, are not natural products but they are of enormous economic importance. Other artificial compounds with a crystal structure occur as a result of industrial transformations in natural conditions.

Introduction

In the previous chapter we have become familiar with the extraordinary regularity of the internal structure of crystals. The local balancing of bonding forces between atoms leads to a periodic repetition of elementary units. We have seen that these unit cells and the corresponding lattice arrays are diagnostic for specific minerals and have classified them according to their symmetry. Symmetry emerged as a central feature of minerals and crystals. In Chapter 7, we recognized seven crystal systems, ranging from the highest cubic symmetry to the lowest triclinic symmetry. In this chapter we will look at symmetry more formally, particularly in view of the possible symmetries that are present in the external morphology of crystals.

A characteristic feature of crystals is “directionality”: specific directions in crystals are inherently different, and these differences are implicit in the lattice structure. Take, for example, a cubic crystal of galena (Figure 8.1a,e): if we view it along a crystal axis (e.g., [100]), we observe a 4-fold symmetry (Figure 8.1b); if the crystal is viewed along a body diagonal (e.g., [111]), it displays 3-fold symmetry (Figure 8.1c); if the cube is viewed along the bisectrix of two faces (e.g., [110]), two different mirror planes are observed (Figure 8.1d). If the growth velocity were isotropic, or equal in all directions, crystals would occur as spheres. Instead they display a regular morphology with planar surfaces. The largest faces are indicative of directions in which the growth velocity is slowest. A material displaying directionality is called anisotropic.

Chemical composition of the Earth

On the Earth, only minerals located in the crust or suspended in the atmosphere and hydrosphere (oceans, lakes, and rivers) are accessible to direct investigation. Except for a few locations, where upper mantle material has been juxtaposed with the crust, the mineralogical compositions of the deeper zones can only be inferred from indirect evidence provided by studies of gravity, inertia, seismic wave propagation, magnetism, phase stability, and the general abundance of elements.

Particularly important for determining the structure of the Earth's interior are seismic waves. Their velocities increase with the elastic stiffness and the density of the material they pass through. Therefore one can draw conclusions about the material on the basis of travel times. Furthermore (as was discussed briefly in Chapter 12), there are two types of seismic waves. Longitudinal (or P waves) pass through solids as well as liquids, whereas transverse waves (S waves) pass only through solid material. This distinction can be used to identify regions of melt within the Earth.

Figure 38.1 shows a profile of the average longitudinal and transverse wave velocity from the surface to the center of the Earth, as well as the average increase in density and temperature, based on a model by Anderson and Hart (1976). The pressure is also indicated on the left. There is no linear relationship with depth because pressure depends on density which is a function of mineral composition. Note that the wave velocity graph shows both gradual changes (attributed to the general increase of velocity with pressure) and abrupt discontinuities, which are thought to be due to changes in composition or phase transformations. Particularly striking is the discontinuity at 2900 km, where transverse waves vanish and the velocity of longitudinal waves drops from 14 to 8 km/s. This discontinuity is attributed to the transition from the largely solid silicate mantle to the liquid iron core.