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9612 results in Mineralogy, petrology and volcanology

Page 53 of 481

  • 8 - Meteorites: Link between Cosmo- and Geochemistry

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 110-120

  • Summary

    It is as if Nature had taken a sample at each step of the planetary formation process, had kept it intact somewhere for 4.5 billion years, and had finally sent it to us from the sky so that we could study it in our laboratories

    C J Allegre

    INTRODUCTION

    From the point of view of a present-day observer in the Solar System, the time scale of the universe can be split into two major subdivisions—presolar and solar—as shown schematically in Figure 8.1. The presolar period begins with the big bang at time to (as measured from the present) and ends with the isolation from the Interstellar Medium (ISM) at time tt of a small portion of the dust and gas in the ISM to form the Solar System. The immediately following solar era begins at time ti with the formation of the solar nebula as a separate entity, and ends at the present time. Within these two broad time zones, one can distinguish discrete events and protracted processes even if some of the processes are yet unfamiliar. For example, the presolar era includes the cosmological or primordial synthesis of hydrogen (H) and helium (He) immediately after the Big Bang, formation of galaxies, synthesis of heavier elements from H and He in the interior of stars, and its termination in the material destined to form the Solar System. The solar era comprises the collapse of the isolated gas cloud into a central massive body (the potential sun) and a thin disc around the central object, condensation of the material in the disc into chemical compounds at tc, aggregation and accretion of the chemical species into the planets and other objects of the solar system at ta, and subsequent internal evolution of individual planets to the present time.

    As geologists recognized the immensity of geologic and, hence, the solar time from observations of large scale geologic structures and processes, astronomers and astrophysicists relied on observations of large scale structures in the universe, like stars, star clusters, and galaxies to infer an even longer presolar time scale. The best known of such indirect dating methods is the mutual recession of galaxies, as evidenced by their spectral red shift. The most precise result, so far, for the Big Bang, as based on the rate of expansion of the universe, is 13.4 ±1.6 Ga (Lineweaver, 1999).

  • 2 - Nuclear Transformations

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 12-22

  • Summary

    The discovery of radioactivity provided an additional (and continuous) heat source for earth's interior and revolutionized studies of earth history by permitting quantitative dating.

    C H Langmuir and W Broecker (2012)

    INTRODUCTION

    Certain nuclides with a specific combination of protons and neutrons may transform spontaneously or be transformed by external agents to another nuclide with a different combination of neutrons and protons. We will consider such transformations as relevant to radiometric dating in the present chapter.

    SPONTANEOUS NUCLEAR TRANSFORMATIONS

    Radioactivity

    The spontaneous transformation or decay of a potentially unstable nuclide to a more stable nuclide is called radioactivity. The energy released by the decay is carried mainly by particles and radiation. The decaying nuclide and its product nuclide are customarily labelled parent (p) and daughter (d), respectively. If d is radioactive, it decays to another nuclide until a stable d is produced. The nuclear decay process obeys the following conservation laws of Physics: (1) Mass/energy, (2) Electric charge, (3) Linear momentum, (4) Angular momentum, and (5) Nucleon number (Kaplan, 1955; Beiser, 1973; Leighton, 1959).

    It was noted in the first chapter that stable nuclides define a narrow band in a Z vs N plot (Figure 1.3) corresponding to the greatest stability of Z/N ratio as a function of N. Unstable nuclei that deviate from the path of stability eventually transform into stable nuclei by different decay modes and rates. A diagonal section across the path of the stability valley will contain isobaric nuclides with the most stable isobar in or close to the path. Neutron-rich isobars will fall below the path and proton-rich isobars above it. Unstable nuclides heavier than 209Bi along the path are also neutron-rich. Although nuclides decay in many modes, the modes most common and relevant to radiogenic isotope geochronometry are few (Dalrymple, 1991), and are shown in Figure 2.1.

    Beta (β-) decay

    Neutron-rich nuclei below the stability zone reach stability by converting one of their neutrons into a proton, an electron (β-), and an antineutrino (v), and then emitting the latter two. The electron created in the nucleus just before its emission is called a beta or b particle to distinguish it from an orbital or extra-nuclear electron.

  • 4 - Isotopics

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 33-39

  • Summary

    The beginning of wisdom is to call things by their right names.

    Confucius

    INTRODUCTION

    The basic measurement in isotope geochemistry is the quantitative variation of the relative and absolute numbers of isotopes of an element caused by natural physical and chemical processes and created artificially in the laboratory. This chapter deals with various topics related to this end.

    ISOTOPIC ABUNDANCE

    The fundamental unit of chemistry is the atom or molecule. It is, therefore, necessary to be able to measure and express the number of atoms or molecules in a chemical system (natural or artificial). However, the numbers of atoms, or atomic groups (molecules), even in a very small chemical system are far too numerous to be counted. For example, just one microgram of sodium chloride (NaCl) will contain ∼1016 sodium chloride molecules, each with one sodium and one chlorine atom. Chemists have, thus, developed a practical method to count such enormous numbers of chemical units by simply weighing them. This is based on the fundamental atomic theory that equal numbers of any atom will be contained in one gram atomic weight (atomic weight expressed in grams) of its element. This ‘equal number’, measured experimentally, is 6.023 × 1023, and is called the Avogadro's number after its discoverer.

    The fundamental unit of isotope chemistry is the isotope or isotopic molecule. Numbers of isotopes of an element will also be far too numerous to count, like atoms in a chemical system. So, like the gram atomic weight in chemistry, it is more convenient to use, in isotope chemistry, the unit ‘mole’ (gram molecular weight) defined as the amount of material which contains as many particles as there are carbon (C) atoms in exactly 12 gram of pure 12C. This number is the Avogadro's number, 6.023 × 1023. This definition emphasizes that the mole refers to a fixed number of any type of identical items (e.g. mole of electrons, mole of molecules, or even a mole of men). The product of mole and Avogadro's number gives the number of a given isotope in a chemical system, and the product of mole and atomic weight of the isotope gives the mass of that isotope in atomic mass units in the system. The definition of a mole permits the calculation of the value of the unified atomic mass unit, u.

  • 6 - Mass Spectrometry and Isotope Geochemistry

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 80-95

  • Summary

    Much of the most promising modern Earth Science depends on the measurements and observations that can no longer be obtained with a hammer, Brunton compass, and hand lens alone. Although much that is important is still being done and will continue to be done in this way by first class minds, new expensive and frequently unavailable instrumentation is called for

    Preston Cloud.

    INTRODUCTION

    Basic analytical measurements in isotope geochemistry are absolute and relative abundances of isotopes of an element available in a solid, liquid, or gaseous form. X-ray Fluorescence (XRF) Spectrophotometer, Atomic Absorption Spectrophotometer (AAS), Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES), and Electron Probe Micro Analyzer (EPMA) are commonly used for elemental analysis (Potts, 1987). These instruments rely on the excitation of orbital electrons and detection of the resulting emission of radiation, or absorption of external radiation, characteristic of each element. Isotopes of an element have the same electronic structure (Chapter 1), and, hence, are not distinguished by the above instruments. Mass sensitive instruments, called mass spectrometers, have become the preferred instruments for isotope geochemistry (De Laeter, 1998). Many types of mass spectrometers have been developed over the last century, but most of them consist of three main components:

  • 1. An ion source for production, acceleration, and collimation of ions of different isotopes of an element(s);

  • 2. A mass analyzer for separation of ions according to their mass/charge (m/e) ratio in space or time; and

  • 3. A detector for the collection and measurement of mass-separated ions, sequentially or simultaneously.

    • PRINCIPLES OF MASS SPECTROMETRY

    Figure 6.1 is the schematic of a very old design of a mass spectrometer (Dempster, 1918) showing all the three basic components. Consider a beam of two singly charged isotopic ions ‘a’ and ‘b’ of mass ma and mb (ma + δm), respectively, produced with negligible kinetic energy in front of a stack of metal plates with narrow rectangular slits, known as the ion source. The beam is accelerated by falling through a potential of V volts distributed between the plates of the ion source, and collimated to emerge as a narrow rectangular beam from the final slit of width, ws, called the Source Slit (SS).

  • Grey petrel population on Campbell Island 14 years after rodent eradication

  • Graham C. Parker, Kalinka Rexer-Huber, David Thompson
  • Journal:
    Antarctic Science / Volume 29 / Issue 3 / June 2017
    Published online by Cambridge University Press:
    19 December 2016, pp. 209-216

  • Populations of grey petrels have declined globally due to both incidental capture in commercial fisheries and predation by introduced mammals at breeding sites. In the New Zealand region, grey petrels only breed on Campbell and Antipodes islands. Rats were successfully eradicated from Campbell Island in 2001. We assessed the spatial extent and conducted the first quantitative population estimate of the grey petrel population on Campbell Island and surrounding islets. There was an estimated c. 96 pairs (95% CI: 83, 109) of breeding grey petrels from the four colonies. Since work was conducted during the middle of the chick-rearing stage, this is an underestimate of the breeding population. The Campbell Island grey petrel breeding population remains small. Our study provides a baseline for future population estimates of grey petrels on Campbell Island.

  • Preface

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp xv-xvii

  • Summary

    Over the past two decades, many curriculum changes have occurred in geology, Earth science, and environmental science programs in universities. Many of these have involved the compression of separate one-semester courses in mineralogy, optical mineralogy, and petrology into a single-semester offering that combines mineralogy and petrology, commonly called Earth Materials. Such a course is a challenge to the instructor (or a team of instructors) and the students. This is especially so when few, if any, textbooks for such a one-semester course have been available.

    This text, Earth Materials, is an introduction to mineralogy and petrology in which both subjects are covered with a roughly even balance. To keep this textbook reasonably short and applicable to a one-semester course, we decided against providing a shallow survey of everything and instead concentrated on what we consider the most fundamental aspects of the various subjects.

    In the writing of this text, we assumed that the students who enroll in an Earth materials course would have previously taken an introductory physical geology course, as well as a course in college-level chemistry.

    Coverage

    Basic aspects of mineralogy must precede the coverage of petrology. This sequence is obvious from the chapter headings. After a brief, general introduction in Chapter 1, minerals and rocks are broadly defined in Chapter 2. That is followed by three chapters that relate to various mineralogical aspects and concepts. Chapter 3 covers the identification techniques that students must become familiar with to recognize unknown minerals in the laboratory and in the field. It also includes discussion of two common instrumental techniques: X-ray powder diffraction and electron beam methods. Chapter 4 covers the most fundamental aspects of crystal chemistry, and Chapter 5 is a short introduction to basic aspects of crystallography. Chapter 6 covers optical mineralogy. This subject is included so that instructors who plan to introduce thin sections of rocks in their course can give their students quick access to the fundamentals of optical mineralogy and the optical properties of rock-forming minerals.

    The sequencing of subsequent systematic mineralogy chapters is completely different from that most commonly used in mineralogy textbooks. In these chapters, minerals are discussed in groups based first on chemistry (native elements, oxides, silicates, and so on) and, subsequently, for the silicates, on structural features (layer, chain, and framework silicates, and so on).

  • 3 - How Are Minerals Identified?

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 37-60

  • Summary

    The ability to recognize common minerals in hand specimens is basic to much that follows in this text. The tests that you will become familiar with are exactly the same as those used worldwide by field and research geologists, be it in the field, in the research laboratory, or in the home office. Your course instructor may discuss the various techniques for the unique identification of a mineral (or several minerals as in a rock) in lectures, but personal expertise can be gained best by hands-on work that you will likely do in the laboratory that accompanies your course. There you will have small test samples that you will be allowed to scratch (to test hardness), to hold in your hand (“to heft” the specimen so as to assess its average specific gravity), and to evaluate their reaction to dilute HCl, and so on. Even better is that you will probably also see much larger, better mineral hand specimens in which you can observe other properties such as crystal form, habit, cleavage, range of color, and state of aggregation. You will learn to combine these observations and develop the skills to identify unknown minerals correctly.

    Once you know what mineral you are dealing with, or which several minerals, as most common rocks contain a mix of minerals, you will be well prepared for understanding how rocks are classified and the conditions under which different rock types (igneous, sedimentary, and metamorphic) are formed. For example, once you have identified all four major minerals in a specific, relatively coarse-grained, light-colored rock as (1) quartz, (2) two types of feldspar, and (3) a mica, you can conclude that you are dealing with a granite. Such knowledge of common rock-forming minerals is basic to much of what is presented in this text. The four later chapters that deal with the systematic mineralogy of igneous rocks (Chapter 7); sedimentary rocks (Chapter 11); metamorphic rocks (Chapter 14); and ore deposits, coarse-grained pegmatites, and quartz veins (Chapter 16) give detailed information on the diagnostic properties that allow us to identify minerals.

    We introduce you to various observations and tests that can be made of minerals in hand specimens. In many instances a combination of the results of several of these leads to the identification of the mineral at hand.

  • 9 - How Do Igneous Rocks Form?

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 241-278

  • Summary

    Igneous rocks are the most abundant rocks in the Earth's crust, formed by the crystallization of melts that have risen from the planet's interior. The crust itself has formed over an extended period of time, in fact billions of years, by just such crystallization. The outer half of the planet is normally solid, so it is natural to wonder where magmas come from. Where does this melting take place, and why do normally solid rocks become molten? What composition do rocks have in the source region? Is melting partial or total? What magma compositions are formed when these rocks melt? What makes magma rise toward the surface, and how fast does it travel? As magma ascends and begins to cool, it becomes a mixture of liquid and crystals that can separate from one another and change the composition of the magma. Eventually, magma cools and solidifies to form igneous rock. The processes leading to the formation of this rock are of great importance because they have controlled the differentiation of our planet. The composition of the crust is very different from that of the planet as a whole, and life as we know it would be very different if these processes had not taken minor elements from the Earth's interior and concentrated them in the crust. In this chapter we look into each of these igneous rock-forming processes. We leave to Chapter 10 a discussion of the diversity and classification of igneous rocks.

    Igneous rocks are those formed by the solidification of molten rock. This molten material, which we call magma, is formed at depth in the Earth and rises toward the surface, where it cools and solidifies, either beneath the surface, where it usually has time to crystallize, or on the surface as volcanic rocks, where cooling may be rapid enough to form glass. We classify igneous rocks on the basis of the minerals they contain, which reflect the composition of the magma. Magma compositions are determined in the source region by the type of rock that undergoes partial melting, but they can be modified during ascent and solidification, especially in large magma chambers, where solidification can take thousands of years. Throughout geologic time, the rise of magmas from the mantle has slowly generated the Earth's crust, whose composition is therefore determined by the composition of magmas.

  • 13 - Sedimentary Rock Classification, Occurrence, and Plate Tectonic Significance

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 385-414

  • Summary

    In the previous chapter, we looked at how sediment is formed, transported, deposited, and lithified to form rock. In this chapter, we discuss the classification of sedimentary rocks, which fall into three main groups: (1) the siliciclastic sedimentary rocks, which include mudrocks (shale), sandstones, and conglomerates; (2) the biogenic sedimentary rocks, which include limestones, dolostones, cherts, and coals; and (3) the chemical sedimentary rocks, which include evaporites, phosphorites, and iron-formations. We learn how each type can be identified and examine the textures of the most important types in photomicrographs of petrographic thin sections. Correct identification of sedimentary rocks is the first step to working out the geologic history of an area, because these rocks preserve a record of the source of sediment, its mode of transport, and its site of deposition, which in many cases is determined by plate tectonic processes. We examine the geologic record preserved in some specific sedimentary rock sequences. Sedimentary rocks are the source of all fossil fuels, most iron ore, material used for fertilizers, much of the world's gold ore, and many other placer mineral deposits. Most of the world's construction material comes from sediments and sedimentary rocks in the form of building stones and raw material (lime and aggregate) for making cement. Porous sedimentary rocks form important aquifers both for agricultural use and for sustaining urban areas. In this chapter, we indicate with which sedimentary rocks each of these economically important resources is associated. We also discuss the plate tectonic setting in which each sedimentary rock is formed.

    Sedimentary rocks are formed from the lithification of sediment (Fig. 12.1). They are classified into three main groups based on the type of sediment from which they are formed. Those formed from the detritus of weathered rocks are called siliciclastic because most of their minerals are silicates. Those formed from sediment derived from organisms are described as biogenic, and those formed by chemical precipitation are referred to as chemical sedimentary rocks.

  • 11 - Sedimentary Rock-forming Minerals and Materials

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 333-352

  • Summary

    This chapter presents the systematic descriptions of uniquely sedimentary rock-forming minerals and materials. These are mainly the newly formed minerals that result from interactions of preexisting minerals with the atmosphere or precipitation from the oceans. Detrital minerals such as quartz, feldspar, and the micas, which were discussed in Chapter 7, are the major constituents of sedimentary rocks such as sandstones, arkoses, and graywackes. By studying hand specimens of these minerals in the laboratory while referring to the appropriate specimen descriptions in this chapter, you will further develop your mineral identification skills.

    In chapters 7, 9, and 10, we address many aspects of minerals and rocks of igneous origin. They are the result of crystallization from a liquid magma over a wide range of high temperatures (~1400 to 600°C) in an environment in which oxygen gas (O2) is nonexistent and in which H2O is generally a minor constituent. This chapter deals with minerals that (1) are newly formed as a result of chemical reactions (under atmospheric conditions) that slowly destroy earlier minerals (e.g., feldspars) and recombine various ions in solution in water to form clay minerals, and oxides or hydroxides, and (2) those that are chemical precipitates in depositional basins such as carbonates, evaporite minerals, and Precambrian iron-formations. However, we must also mention those minerals that have survived the physical and chemical weathering processes while exposed to the atmosphere; these are known as detrital minerals.

    We systematically treat 13 sedimentary rock-forming minerals. These include ice, the solid form of H2O; a hydroxide; a clay mineral (representative of the large clay mineral group); two polymorphs of CaCO3; four more carbonates; two halides; and two sulfates. Clearly, this group of common sedimentary minerals is totally different from the igneous rock-forming minerals discussed in Chapter 7.

    We close with discussions of three sedimentary materials: chert and agate, phosphorite, and soil.

Page 53 of 481