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9612 results in Mineralogy, petrology and volcanology

Page 50 of 481

  • 10 - Rock–Water Systems

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 231-256

  • Summary

    Real Problems

    We have now completed our survey of the thermodynamic principles required to model natural systems. It only remains to gain practice in formulating problems involving natural systems in thermodynamic terms. Quite often, that is the hardest part. Once the problem has been set up in terms of relevant reactions and components, the equations can be solved by anyone who has absorbed the previous chapters. However, choosing the appropriate components and setting up the relevant balanced reactions only comes from experience. In this chapter we explore a few situations that have been investigated by thermodynamic methods.

    Is the Sea Saturated with Calcium Carbonate?

    If you have ever been to Florida or the Bahamas, you may be aware that there are vast areas adjacent to the coasts where the sea bottom at shallow levels is a white mud, which turns out to be made of almost pure aragonite. Carbonate muds extend well out to the deep sea as well; in fact, a fairly large proportion of the sea bottom is composed of calcium carbonate. There are also countless calcitic atolls and reefs throughout the tropical zones of the world. Given this amount of contact between the sea and calcium carbonate, both calcite and aragonite, plus the fact that there are vigorous oceanic currents stirring things up constantly, plus the fact that things have not changed drastically for millions of years, you would think that there would be little doubt that the system consisting of the oceans plus their bottom sediments must have reached equilibrium by now. If these were the only factors involved, perhaps they would have, but the situation is quite a bit more complicated. Why would anyone want to know? Reactions involving carbonate in the oceans are fundamental to an understanding of the global CO2 cycle, which in turn is linked to climate change and other things we would like to understand.

    How Do You Tell If a Solution Is Saturated?

    To explore this problem further, we must first find out how to determine whether a solution is saturated, undersaturated, or supersaturated with a given mineral or compound. One answer would be to just observe the solution in contact with the mineral.

  • 12 - Phase Diagrams

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 293-342

  • Summary

    What Is a Phase Diagram?

    A phase diagram in the general sense is any diagram that shows what phase or phases are stable as a function of some chosen system variable or variables. Therefore, the Eh–pH, log f O2–pH, and activity–activity diagrams we have been looking at are a kind of phase diagram. However, if you mention the subject of phase diagrams to a petrologist, a metallurgist, or a ceramic scientist, they will immediately think of a particular type of diagram that is of great usefulness in these subjects. In these sciences, the compositions of phases and their relationships during phase changes, particularly solid→liquid and liquid→solid changes, are of particular importance, so diagrams that depict this information as a function of temperature and pressure have come to be the subject of “phase diagrams.”

    Thermodynamics and Phase Diagrams

    Though it is true that phase relations can always be described in terms of the thermodynamic principles and equations we have been discussing, and that any phase diagram can in principle be calculated given the appropriate data, the emphasis in this chapter changes from one of calculating what we want to know from numbers in tables of data, to one of simply representing experimentally derived facts in diagrammatic form. The reason for this is that once we get into systems more complex than a single component, and especially when high-temperature melt phases are involved, the calculations are often not possible because the data are not available, or, even if they are available, the results are not very accurate, because they are very sensitive to small inaccuracies in the data. Therefore, in this book, although we will show the relationship between functions such as G and our diagrams, this will be in an illustrative rather than a quantitative way.

    Phase Diagrams as Models

    Metallurgists and ceramicists quite often deal with simple two- and three-component systems and use phase diagrams to represent their experimental results on the phase relations in these systems. The diagrams therefore truly represent their systems. Petrologists, on the other hand, are interested in the origins of natural rocks, which commonly have 10 or more important components.

  • 1 - What Is Thermodynamics?

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 1-10

  • Summary

    Introduction

    Thermodynamics is the branch of science that deals with relative energy levels and transfers of energy between systems and between different states of matter. Because these subjects arise in virtually every other branch of science, thermodynamics is one of the cornerstones of scientific training. Various scientific specialties place varying degrees of emphasis on the subject areas covered by thermodynamics – a text on thermodynamics for physicists can look quite different from one for chemists, or one for mechanical engineers. For chemists, biologists, geologists, and environmental scientists of various types, the thermodynamics of chemical reactions is of course a central concern, and that is the emphasis to be found in this book. Let us start by considering a few simple reactions and the questions that arise in doing this.

    What Is the Problem?

    Some Simple Chemical Reactions

    A chemical reaction involves the rearrangement of atoms from one structure or configuration to another, normally accompanied by an energy change. Let's consider some simple examples.

  • • Take an ice cube from the freezer of your refrigerator and place it in a cup on the counter. After a few minutes, the ice begins to melt, and it soon is completely changed to water. When the water has warmed up to room temperature, no further change can be observed, even if you watch for hours. If you put the water back in the freezer, it changes back to ice within a few minutes, and again there is no further change. Evidently, this substance (H2O) has at least two different forms, and it will change spontaneously from one to the other depending on its surroundings.

  • • Take an egg from the refrigerator and fry it on the stove, then cool to room temperature. Again, all change seems now to have stopped – the reaction is complete. However, putting the fried egg back in the refrigerator will not change it back into a raw egg. This change seems not to be reversible. What is different in this case?

  • 5 - Getting Data

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 110-136

  • Summary

    Introduction

    We have had quite enough theoretical discussion for now. Let's see how to get some numbers into our equations so as to be able to calculate something useful. Welcome to the world of experimental thermochemistry.

    In this chapter we will have a look at a few of the ways in which the thermodynamic parameters we have derived are measured; i.e., where the numbers in the tables and databases come from. A deep knowledge of this subject is not necessary in order to use thermodynamics to model chemical, geological, or environmental systems, in the same sense that a knowledge of a composer's life and times is not necessary to enjoy his or her music. But it does enrich the experience, and in the case of using thermodynamics, such knowledge does serve to make the user conscious of the many reasons why his or her data might be incorrect.

    Thermochemical data are produced for the most part by dedicated scientists, who devote a good part of their lives to tracking down elusive sources of error, and devising ever-improved methods for determining nature's fundamental parameters as defined by thermodynamic theory. When determined by independent methods and/or independent laboratories, the results are often satisfyingly in agreement, but almost as often they are not, meaning that there is some source of error, and identifying it can take a lot of discussion (perhaps argument would be a better term) and a long time.

    Beginning with the establishment of the Geophysical Laboratory in Washington in 1905, scientists primarily interested in geological processes have contributed to our knowledge of these thermodynamic properties, especially, as might be expected, those of the rock and soil-forming minerals, liquid–solid phase relationships of these minerals, and aqueous solutions such as sea water and hydrothermal solutions. In addition to the measurement of mineral and solution properties, collecting these data into databases for use by computer programs and the testing of these databases for internal consistency have become increasingly important.

    The experimental part and the self-consistent database part of using thermodynamics in the Earth sciences today have both become large and complex subjects, each worthy of separate study.

Thermodynamics of Natural Systems
  • Theory and Applications in Geochemistry and Environmental Science
  • 3rd edition
  • Greg Anderson
  • Published online:
    28 May 2018
    Print publication:
    27 April 2017
  • Thermodynamics deals with energy levels and energy transfers between states of matter, and is therefore fundamental to all branches of science. This new edition provides an accessible introduction to the subject, specifically tailored to the interests of Earth and environmental science students. Beginning at an elementary level, the first four chapters explain all necessary concepts via a simple graphical approach. Throughout the rest of the book, the author emphasizes the importance of field observations and demonstrates that, despite being derived from idealized circumstances, thermodynamics is crucial to understanding ore formation, acid mine drainage, and other real-world geochemical and geophysical problems. Exercises now follow each chapter, with answers provided at the end of the book. An associated website includes extra chapters and password-protected answers to additional problems. This textbook is ideal for undergraduate and graduate students studying geochemistry and environmental science.

  • 4 - The Second Law of Thermodynamics

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 66-109

  • Summary

    Introduction

    The first sentence of Gibbs’ (1961a) classic memoir “On the Equilibrium of Heterogeneous Substances” is

    The comprehension of the laws which govern any material system is greatly facilitated by considering the energy and entropy of the system in the various states of which it is capable.

    Given the fact that virtually all of equilibrium thermodynamics can be derived by doing exactly that, as Gibbs did, this must rank as one of the world's great understatements. In this chapter, we begin to explore what Gibbs was referring to. By considering “the laws which govern any material system,” we should be able to find out the answers to the questions we posed in Chapter 1.

    The Entropy Paradox

    In this chapter we introduce a thermodynamic parameter called entropy. Entropy is often described in the following way:

    The entropy of an isolated system continuously increases during a spontaneous, irreversible process until it reaches a maximum value at equilibrium.

    However, if at the same time we consider the fact that in classical thermodynamics entropy is defined only in equilibrium states we have a paradox, stated for example by Tisza (1966, pp. 120–121):

    The nature of this problem is illustrated by the following paradox: How are we to give a precise meaning to the statement that entropy tends toward a maximum, whereas entropy is defined only for systems in equilibrium? Thus in an isolated system, the entropy is constant, if it is defined at all.

    The discussion of constraints and metastable equilibrium states in this chapter will provide an answer to this paradox.

    The Problem Restated

    Having taken a couple of chapters to get our terminology settled and to get used to discussing energy changes in systems, we must now get back to our main problem – what determines whether chemical processes will go or not go? Our method of determining this might be considered a bit simple-minded – we will simply determine the “chemical energy” differences between equilibrium states. Processes can take place spontaneously if they are in the direction of lowering the chemical energy. They cannot take place spontaneously in the opposite direction.

  • 7 - Solutions

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp 150-176

  • Summary

    Introduction

    If the world were made of pure substances, our development of the thermodynamic model would now be complete. We have developed a method, based on measurements of heat flow, that enables predictions to be made about which way reactions will go in given circumstances. But one of the reasons why the world is so complex is that pure substances are relatively rare, and strictly speaking they are nonexistent (even “pure” substances contain impurities in trace quantities). Most natural substances are composed of several components, and the result is called a solution. Therefore, we need to develop a way to deal with components in solution in the same way that we can now deal with pure substances – we have to be able to get numerical values for the Gibbs energies, enthalpies, and entropies of components in solutions. We will then be able to predict the outcome of reactions that take place entirely in solution, such as the ionization of acids and bases, and reactions that involve solids and gases as well as dissolved components, such as whether minerals will dissolve or precipitate. Our thermodynamic model will then be complete.

    In this chapter we have a look at how to deal with dissolved substances – solutes. When we mix two substances together, sometimes they dissolve into one another, like sugar into coffee or alcohol into water, and sometimes they do not, like oil and water. In the former case, if we thought about it at all, we would probably expect that the properties of the mixture or solution would be some kind of average of the properties of the two separate substances. This is more or less true for some properties, but decidedly not true for the most important one, Gibbs energy.

    After making sure we understand how to express the composition of solutions, we begin by considering properties of ideal solutions, which are, as you might expect, the simplest possible properties that solutions might have. As you might also expect, no real solutions are in fact ideal, although some come fairly close.

  • Preface

  • Greg Anderson, University of Toronto
  • Book:
    Thermodynamics of Natural Systems
    Published online:
    28 May 2018
    Print publication:
    27 April 2017, pp xiii-xvi

  • Summary

    Why another book on thermodynamics?

    I wrote the first edition of this book because Thermodynamics in Geochemistry (Anderson and Crerar, 1993) was not suitable for the teaching I was doing at the time, a second-year geochemistry course for geology specialists and geological engineers. I wanted something shorter and less detailed. Later I wanted to write something more suitable for my students and other graduate students, resulting in the second edition.

    But then the first edition went out of print, and several people told me that it was better for teaching than the second edition, which was seen as more of a reference book despite the fact that it includes just about all of the first edition. So the objective with this edition is to make a book more like the first edition available again, that is, one more suitable for a first introduction to thermodynamics, though with many improvements. Several chapters and appendices of the second edition have been cut, and others made shorter and more concise. Because thermodynamic potentials are the central concept in chemical thermodynamics, the first four chapters extend the ball-in-valley analogy to introduce these, as well as exact and inexact differentials, considerably improving the introduction to thermodynamic theory. There is also an increased emphasis on the importance of field observations in the application of thermodynamics to natural systems.

    The three editions of this book and Thermodynamics in Geochemistry have several features in common, besides the fact that they have an Earth science point of view, and regarding which they differ from other books on thermodynamics. These differences include the following points.

  • • Presentation of thermodynamics as a model or idealization of real systems.

  • • Emphasis on the significance of the fact that thermodynamics consists of single-valued continuous functions.

  • • An approach to entropy which avoids discussion of cycles and the entropy of the Universe: Gibbs, not Carnot/Clausius.

  • • Emphasis on the importance of metastable equilibrium states and thermodynamic constraints.

  • • Emphasis on the fact that time is not a thermodynamic variable.

    • The presentation is exclusively classical, meaning that statistical mechanics is barely mentioned; only equilibrium states and macroscopic variables are considered. Some consider this approach obsolete, believing that classical thermodynamics has largely been superseded by the greater explanatory power of statistical mechanics.

  • Sub-decadal variations in outlet glacier terminus positions in Victoria Land, Oates Land and George V Land, East Antarctica (1972–2013)

  • A.M. Lovell, C.R. Stokes, S.S.R. Jamieson
  • Journal:
    Antarctic Science / Volume 29 / Issue 5 / October 2017
    Published online by Cambridge University Press:
    26 April 2017, pp. 468-483
    • Article
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  • Recent work has highlighted the sensitivity of marine-terminating glaciers to decadal-scale changes in the ocean–climate system in parts of East Antarctica. However, compared to Greenland, West Antarctica and the Antarctic Peninsula, little is known about recent glacier change and potential cause(s), with several regions yet to be studied in detail. In this paper, we map the terminus positions of 135 glaciers along the coastline of Victoria Land, Oates Land and George V Land from 1972–2013 at a higher temporal resolution (sub-decadal intervals) than in previous research. These three regions span a range of climatic and oceanic conditions and contain a variety of glacier types. Overall, from 1972–2013, 36% of glaciers advanced, 25% retreated and the remainder showed no discernible change. On sub-decadal timescales, there were no clear trends in glacier terminus position change. However, marine-terminating glaciers experienced larger terminus position changes compared with terrestrial glaciers, and those with an unconstrained floating tongue exhibited the largest variations. We conclude that, unlike in Greenland, West Antarctica and the Antarctic Peninsula, there is no clear glacier retreat in the study area and that most of the variations are more closely linked to glacier size and terminus type.

  • The influence of föhn winds on Glacial Lake Washburn and palaeotemperatures in the McMurdo Dry Valleys, Antarctica, during the Last Glacial Maximum

  • M.K. Obryk, P.T. Doran, E.D. Waddington, C.P. Mckay
  • Journal:
    Antarctic Science / Volume 29 / Issue 5 / October 2017
    Published online by Cambridge University Press:
    17 March 2017, pp. 457-467

  • Large glacial lakes, including Glacial Lake Washburn, were present in the McMurdo Dry Valleys, Antarctica, during the Last Glacial Maximum (LGM) despite a colder and drier climate. To address the mechanism capable of generating enough meltwater to sustain these large lakes, a conceptual model was developed based on the warming potential of infrequent contemporary föhn winds. The model suggests that föhn winds were capable of generating enough meltwater to sustain large glacial lakes during the LGM by increasing degree days above freezing (DDAF) and prolonging the melt season. A present-day relationship between infrequent summer föhn winds and DDAF was established. It is assumed that the Taylor Dome ice core record represents large-scale palaeoclimatic variations for the McMurdo Dry Valleys region. This analysis suggests that because of the warming influence of the more frequent föhn winds, summer DDAF in the McMurdo Dry Valleys during the LGM were equivalent to present-day values, but this enhanced summer signal is not preserved in the annually averaged ice core temperature record.

  • Relative palaeointensity and reservoir effect on Lake Esmeralda, Antarctica

  • M.A. Irurzun, M.A.E. Chaparro, A.M. Sinito, C.S.G. Gogorza, H. Nuñez, N.R. Nowaczyk, H.N. Böhnel
  • Journal:
    Antarctic Science / Volume 29 / Issue 4 / August 2017
    Published online by Cambridge University Press:
    15 March 2017, pp. 356-368

  • Four cores from the bottom sediments of Lake Esmeralda, Vega Island, Antarctica (60°48'S, 57°37'W) were studied. Analysis of rock magnetics indicates that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo-single-domain (titano-) magnetite with a small proportion of paramagnetic and antiferromagnetic minerals. The magnetic grain size of the samples is in the range of 1–5 μm and the variation of the interparametric ratios is less than one order of magnitude. Demagnetization of the natural remanent magnetization shows a stable remanent magnetization in most of the samples. Thus, the samples fulfil the necessary conditions to calculate relative palaeointensity (RPI) and the curves obtained correlated with global models enabling dating of the cores. The 250 cm of sediment recovered spans the last 10 200 yr bp. Finally, some samples with high organic matter content were dated by accelerator mass spectrometry 14C. By comparison with the age defined by the RPI curves, a reservoir effect of c. 5200 years is suggested for this region of Vega Island.

  • Inhabitant or visitor? Unexpected finding of Aglaophenia (Cnidaria, Hydrozoa) in Antarctic waters

  • Joan J. Soto Àngel, Álvaro L. Peña Cantero
  • Journal:
    Antarctic Science / Volume 29 / Issue 4 / August 2017
    Published online by Cambridge University Press:
    10 March 2017, pp. 331-339

  • Benthic hydrozoans are one of the most speciose and characteristic taxa from the Antarctic region, with a high number of endemic species, but diversity at the genus level is low and some families with world wide distribution are unrepresented. This is the case of the family Aglaopheniidae. A new species to science of the genus Aglaophenia Lamouroux has been found in the eastern end of the Weddell Sea, at depths of 65–116 m, within the material obtained by the German Antarctic expedition ANT XV/3. This finding constitutes a new record for the Weddell Sea fauna, the first evidence of the genus for the Polar Regions, and even the family Aglaopheniidae from Antarctic waters. The material has been accurately examined and described. Literature concerning the species of Aglaophenia from the sub-Antarctic and other close areas has been reviewed and, as a result, a checklist of 20 species, with their corresponding distribution, is given. The material examined does not agree with any of the species and therefore it is described as a new species. Some possible scenarios for the presence of an aglaopheniid in Antarctic waters are discussed (e.g. alien species, relict, global climate change, microhabitat).

  • A review of recent discoveries of exceptionally preserved fossil fishes from the Gogo sites (Late Devonian, Western Australia)

  • John A. Long, Kate M. Trinajstic
  • Journal:
    Earth and Environmental Science Transactions of The Royal Society of Edinburgh / Volume 108 / Issue 1 / March 2017
    Published online by Cambridge University Press:
    20 July 2018, pp. 111-117
    Print publication:
    March 2017

  • Significant new material of Late Devonian Gogo Formation fish fossils is still surfacing. Collecting in the past decade has uncovered the first Gogo shark fossils (Gogoselachus plus another new undescribed taxon), the first acanthodian (Halmacanthodes ahlbergi), the first coelacanth, as well as the first placoderm embryos. Recent studies have elucidated the nature of placoderm claspers, pelvic girdles, synarcuals and embryos, the structure of their teeth, a description of well-preserved muscles in placoderms, and how muscles attach to bones. Molecular biomarkers have also been identified in Gogo fossils. There are now five basal ray-fin fishes in the fauna, including one undescribed new taxon. The lungfish fauna from Gogo is the most diverse known for any Devonian site, with 10 genera and 12 species. The dermal skeleton and endocast of the dipterid Rhinodipterus kimberleyensis have been described in detail from CT scans; and the ontogenetic stages of neurocranium formation in Griphognathus. New specimens of the tetrapodomorph fish Gogonasus andrewsae have shed further light on its endocranium, pectoral girdle and fin. Through their exceptional preservation of both hard and varied kinds of soft tissues, the Gogo fishes remain crucial for resolving key debates on the diversification, physiology, biomechanics and phylogenetic relationships of early gnathostomes.

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