Optical Mineralogy Textbook

[Ermengardi Atkisson]

Thistextbook presents the fundamental concepts and application of optical mineralogy in a very simple, systematic, and comprehensive way. The book is organized into 2 parts: Part I deals with the theory and techniques, and Part II provides a description of the optical properties of common minerals. The book is written in a lucid manner so that students are able to understand the realization behind the concepts in optics and the methods employed to elicit information about the interior of mineral crystals. All the subject fundamentals and related derivations are discussed in an easy and comprehensive way to make the students strong in the basics of optical mineralogy. The key features lie in the illustrations, examples, and questions at the end of each chapter to provide students with practical usage insights into optical mineralogy. The book benefits students who are taking introductory courses in optics to characterize rock minerals.

Pramod Kumar Verma, a professor(Retd.) of Geology at the University of Delhi, received his Ph.D. in Geology from Harvard University, USA, in 1973. Dr. Verma has taught undergraduate and graduate-level courses in mineralogy and physical geochemistry during the last four decades. He was primarily a metamorphic petrologist but began his research in the regional geology of metamorphic rocks in the Eastern Himalayas. In addition to the Himalayas, his area of interest was in the petrology of the planetary interior. He published several research articles in leading international journals and edited two books on Eastern Himalayan Geology. Prof. Verma had held visiting assignments in USA, Germany, and Zambia. He was a fellow of the Mineralogical Society London. He also served as the president of the Earth System Sciences Section of the Indian Science Congress and the vice president of the Indian Geological Congress.

But because mineral optics can seem arcane, before we jump into the underlying fundamentals, we will begin with a video that gives some background and a broad overview without all the details of optical theory and microscopy. The video finishes up by discussing the most important aspect of optical mineralogy, which is viewing rocks and minerals in thin section. And everything that follows this box in Chapter 5 is building to that end.

One approach to PLM involves examining grain mounts, which are ground-up mineral crystals on a glass slide. The grains must be thin enough so that light can pass through them without a significant loss of intensity, usually 0.10 to 0.15 mm thick. We surround a small number of grains with a liquid called refractive index oil, and then place a thin piece of glass, called a cover slip, over the grains and liquid. The photo in Figure 5.3, below, shows garnet grains in a grain mount.

Grain mounts and refractive index oils are necessary for making some types of measurements, but are not the focus of this chapter. They were extensively used in the past, but are not much used today. For more detailed information about studying minerals in grain mounts consult an optical mineralogy textbook.

Minerals with metallic luster and a few others are termed opaque minerals. They will not transmit light even if they are thin-section thickness. So they always appear black when viewed with a microscope. Magnetite is an opaque mineral; the photo in Figure 5.4 contains several small black magnetite grains. For studying opaque minerals, transmitted light microscopy is of little use. Reflected light microscopy (RLM), a related technique, can reveal some of the same properties. As the name implies, when using RLM, the light source is above the sample and light reflects from the sample to our eye. RLM, although an important technique for economic geologists who deal with metallic ores, is not used by most mineralogists or petrologists. So we discuss it only briefly in this book. Figure 5.5 shows a view of ore from Butte, Montana, seen with a reflecting light microscope. It contains several opaque minerals: galena, sphalerite, pyrite, and chalcopyrite. They appear in various shades of black, gray, and light yellow.

Most minerals can be identified when examined with a petrographic microscope, even if unidentifiable in hand specimen. Optical properties also allow a mineralogist to estimate the composition of some minerals. For example, we can learn the magnesium-to-iron ratio of olivine, (Mg,Fe)2SiO4, based on optical properties. And we can also determine the albite and anorthite content of plagioclase feldspar.

Box 5-3 (below) summarizes the optical properties used for mineral identification and gives the properties of some common minerals. At the largest level, we divide minerals into opaque minerals and nonopaque minerals. Opaque minerals will not transmit light unless the mineral grains are much thinner than normal thin sections. We further divide nonopaque minerals into those that are isotropic (having the same properties in all directions) and those that are anisotropic (having different optical properties in different directions). We discussed these terms, isotropic and anisotropic, previously in Section 4.1, Chapter 4. Finally, we divide the anisotropic minerals into those that are uniaxial and those that are biaxial, and according to whether they have a positive or negative optic sign. We look at the details of these and other diagnostic properties below.

Besides mineral identification, the polarizing microscope reveals important information about rock-forming processes (petrogenesis). When we examine thin sections, distinguishing igneous, sedimentary, and metamorphic rocks is often easier than when we look at hand specimens. More significantly, we can identify minerals and distinguish among different types of igneous, sedimentary, and metamorphic rocks. The microscope allows us to see textural relationships in a specimen that give clues about when and how different minerals in an igneous rock formed. Microscopic relationships between mineral grains allow us to determine the order in which minerals crystallized from a magma, and we can identify minerals produced by alteration or weathering long after the crystals first formed. Similar observations are possible for sedimentary or metamorphic rocks. Only the microscope can give us such information, information that is essential if rocks are to be used to interpret geological processes and environments.

5.2 Light and the Properties of Light5.6 The spectrum of electromagnetic radiationBefore starting a discussion of optical mineralogy, it is helpful to take a closer look at light and its properties. Light is one form of electromagnetic radiation (Figure 5.6). Radio waves, ultraviolet light, and X-rays are other forms of electromagnetic radiation. All consist of propagating (moving through space) electric and magnetic waves (hence the term electromagnetic). The interactions between electric waves and crystals are normally much stronger than the interactions between magnetic waves and crystals (unless the crystals are magnetic). Consequently, this book only discusses the electrical waves of light. In principle, however, much of the discussion applies to the magnetic waves as well.

Because light waves have wavelengths two orders of magnitude greater than atom sizes or bond lengths, the interactions of light with crystals does not reveal information about individual atoms in a crystal. To obtain that kind of information, we must study crystals using shorter wavelength radiation, X-rays (Chapter 12).

In a vacuum, light velocity is 3 x 108 meters per second. Velocity is slightly less when passing through air, and can be much less when passing through crystals. When the velocity of light is altered as it passes from one medium (for example, air) to another (perhaps a mineral), the wavelength changes, but the frequency remains the same.

Visible light has wavelengths of 390 to 770 nm, which is equivalent to 3,900 to 7,700 , or 10-6.1 to 10-6.4 m. Different wavelengths correspond to different colors (Figure 5.8). The shortest wavelengths (violet light) grade into invisible ultraviolet radiation. The longest wavelengths, corresponding to red light, grade into invisible infrared radiation.

Light composed of multiple wavelengths appears as one color to the human eye. If wavelengths corresponding to all the primary colors are present with equal intensities, the light appears white. White light is said to be polychromatic (many colored), because it contains a range, or spectrum, of wavelengths. Polychromatic light can be separated into different wavelengths in many ways. When one wavelength is isolated, the light is monochromatic (single colored).

Besides wavelength (λ) and frequency (ν), an amplitude and a phase characterize all waves. Amplitude (A) refers to the height of a wave. Phase refers to whether a wave is moving up or down at a particular time. If two waves move up and down at same times, they are in phase; if not, they are out of phase.

When two waves with the same wavelength travel in the same direction simultaneously, they interfere with each other. Their amplitudes may add, cancel, or be somewhere between. The nature of the interference depends on the wavelengths, amplitudes, and phases of the two waves. Light waves passing through crystals can have a variety of wavelengths, amplitudes, and phases affected by atomic structure in different ways. They yield interference phenomena, giving minerals distinctive optical properties.

In Figure 5.9a, two in-phase waves of the same wavelength are going in the same direction. If we could measure the intensity of the two waves together, we would find that their amplitudes have added. When waves are in phase, no energy is lost; this is constructive interference. In contrast, Figure 5.9b shows two waves that are slightly out of phase, and Figure 5.9c shows two waves that are completely out of phase.

When light passes near an atom, perhaps in a crystal, the vibrating electric wave causes electrons orbiting the atom to oscillate. The oscillations absorb energy from the light, and the wave slows. As light passes from air into most nonopaque minerals, its velocity may decrease by a third or a half. Because the frequency of the light remains unchanged, we know that the wavelength must decrease by a similar fraction (because v = λν).

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