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9612 results in Mineralogy, petrology and volcanology

Page 64 of 481

  • 26 - Sulfides and related minerals. Hydrothermal processes

  • from Part V - A systematic look at mineral groups
  • Hans-Rudolf Wenk, University of California, Berkeley, Andrey Bulakh, St Petersburg State University
  • Book:
    Minerals
    Published online:
    25 September 2020
    Print publication:
    04 January 2016, pp 361-376

  • Summary

    About 500 minerals belong to the sulfides and related arsenides. Among the most important sulfides are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS). Sulfides are of great industrial importance and are the major ores for copper, zinc, lead, mercury, bismuth, cobalt, nickel, and other nonferrous metals. Although pyrite is not used as an ore for iron, it is used to produce sulfuric acid and it is also an important gold ore, containing small fragments of native gold as inclusions. Many sulfides form by hydrothermal processes and some examples will be discussed.

    Crystal chemistry

    Sulfides are generally subdivided into three chemical classes: (a) simple sulfides that are salts of HS (e.g., sphalerite, ZnS); (b) salts of thioacids, which are oxygen-free acids with sulfur playing the role of oxygen (e.g., pyrargyrite, Ag3SbS3, is the silver salt of the sulfoacid H3SbS3); and (c) polysulfuric compounds (persulfides) that can be considered as salts of the polysulfuric acid H2S2, which contains the bivalent group (pyrite FeS2 is an example). The closest analogs to sulfides are arsenides and their complex compounds (arsenide–sulfides) such as realgar (AsS) and arsenopyrite (FeAsS).

    The structural properties of sulfides are determined by bonding between a metal and sulfur, which is highly ionizing. The S2− ionic radius is large (1.84 Å) compared with that of metals, and most sulfide structures do not correspond to simple close-packing of anions with cations in interstices (sphalerite (ZnS), galena (PbS), cinnabar (HgS), pyrrhotite (FeS), and chalcopyrite (CuFeS2) are exceptions), though close-packing is never ideal. The large sulfur ions easily become polarized, and complex anion groups may form due to covalent pairing. In general, sulfides display a great diversity of crystal structures with complex bonding (ionic–metallic–covalent) in which metallic bonding always plays a considerable role. Among the numerous structures are polyhedral types, types with isolated molecular groups (S2), and types in which sulfur is linked to bands and sheets (Table 26.1).

    A typical example of a polyhedral sulfide is galena, with the same structure as halite (NaCl), in which both cations and anions have a coordination number of 6. The structure can be viewed as close-packing of S2− with Pb2+ in octahedral interstices. Polyhedral structures are also found in sphalerite, wurtzite, pyrrhotite, nickeline, and chalcopyrite.

  • 20 - Phase diagrams

  • from Part IV - Mineral-forming processes
  • Hans-Rudolf Wenk, University of California, Berkeley, Andrey Bulakh, St Petersburg State University
  • Book:
    Minerals
    Published online:
    25 September 2020
    Print publication:
    04 January 2016, pp 284-292

  • Summary

    With the background in thermodynamics we are now ready to approach some phase diagrams to appreciate conditions under which minerals form. Most important are crystallization from a melt, phase transitions (relevant to the deep Earth), and solid solutions, including exsolution.

    Introduction

    Mineral (phase) equilibrium diagrams show the limits of stable existence of minerals at different conditions. They are plotted either on the basis of thermodynamic calculations and the phase rule, as illustrated in the previous chapter, or as a graphic representation of experimental results.

    Phase diagrams, as a rule, are plotted as a function of two variables such as:

  • • temperature T versus total pressure P (Figure 19.2);

  • • temperature T versus partial pressure p (Figure 19.3);

  • • oxidation–reduction potential Eh versus pH (Figure 19.5);

  • • temperature T versus composition of system X.

    • Sometimes three or four variables are used, especially for complex compositions. Be aware that most phase diagrams of geological systems assume idealized situations.

    The principles of interpretation of phase diagrams are straightforward. They allow us to determine, for example, which phase is in equilibrium at a certain temperature and pressure. Temperature–composition phase diagrams, however, which describe crystallization of a magma and subsolidus exsolution processes in minerals, deserve further elaboration.

    Diagrams for crystallization from a melt

    The binary system diopside (CaMgSi2O6)–anorthite (CaAl2Si2O8) (Figure 20.1) is a classic example and has direct application to our understanding of crystallization processes in basaltic magmas. There are two components (CaMgSi2O6 and CaAl2Si2O8) and one free parameter (temperature, m = 1) in this system. The solid lines correspond to the univariant equilibrium between two phases (p = c + 1 − f = 2 + 1 − 1 = 2 for equation 19.39), which are a mineral (anorthite or diopside) and a melt.

    At point E, where the two univariant lines meet, the number of degrees of freedom is zero, while the number of coexisting phases is three (p = c + 1 − f = 2 + 1 − 0 = 3): diopside, anorthite, and melt. E is called the eutectic point and corresponds to the lowest temperature at which the melt and solid phases can coexist.

  • Preface

  • Hans-Rudolf Wenk, University of California, Berkeley, Andrey Bulakh, St Petersburg State University
  • Book:
    Minerals
    Published online:
    25 September 2020
    Print publication:
    04 January 2016, pp xiii-xvi

  • Summary

    Minerals: Their Constitution and Origin is an introduction to mineralogy for undergraduate and graduate students in the fields of geology, materials science, and environmental science. It has been designed as a textbook for use in a one- or two-semester course but gives students a broader view and covers all aspects of mineralogy in a modern and integrated way. It provides detailed references to important publications on principles of crystallography and mineralogy. The book is not only descriptive but for interested readers derives basic principles such as aspects of symmetry theory, background on stereographic projection, X-ray diffraction and thermodynamics, based on general background from mathematics, physics and chemistry. The overall goal is to emphasize concepts and to minimize nomenclature. The text includes appendices covering identification of hand specimens and optical properties. With the broad approach, the book is not only a textbook for students but also a reference for teachers, researchers, collectors and anyone interested in minerals. The book is written in a modular fashion that permits instructors to select or omit some parts, depending on the level of the course, without compromising the continuity.

    Today mineralogy is not just part of a geology curriculum. The importance of mineralogy has broadened to a wide variety of disciplines, from igneous petrology to soils science, from archaeology to cement engineering, from materials science to structural geology. Our book provides an alternative to existing texts by focusing more tightly on concepts, at the expense of completeness, and by integrating geological processes and applications more closely with the discussions of systematic mineralogy.

    The book is divided into six parts:

    Part I deals with general concepts of crystal chemistry, bonding, chemical formulas, mineral classification and hand specimen identification.

    Part II introduces concepts of symmetry expressed in the morphology and structure of crystals. It then explores defects in crystal structures and the diversity of features observed during crystal growth.

    Part III centers on the physics of minerals. First it shows how to use X-ray diffraction to determine the structural features introduced in Part II. A chapter on physical properties is optional but is significant for modern mineral physics and geophysics.

  • 6 - Mineral identification of hand specimens

  • from Part I - Minerals as chemical compounds
  • Hans-Rudolf Wenk, University of California, Berkeley, Andrey Bulakh, St Petersburg State University
  • Book:
    Minerals
    Published online:
    25 September 2020
    Print publication:
    04 January 2016, pp 49-58

  • Summary

    With 5000 mineral species, you probably wonder about how you can indentify a mineral that you find in the field or in a collection. While we will go into much more detail later in the book (Chapter 11 about X-ray diffraction, Chapters 13 and 14 about optical properties, and Part V about individual mineral groups), it seems appropriate to introduce simple hand specimen identification early, based on such properties as morphology, color, hardness, and density. This way you get an intuitive feeling for mineral identities.

    Different scales

    All geologists must be able to identify – more or less by inspection – most of the common rock-forming minerals and certain important accessory and ore minerals in rocks of all kinds. Such ability is developed largely through practice and experience, involving repeated observations of characteristic simple physical properties. Some of these observations can be made directly, with the naked eye. However, because many of the mineral grains in rocks are small, commonly less than 1 mm in diameter, a high-quality hand lens (magnification 5× or 10×) is an indispensable tool for routine field and laboratory observations.

    Most hand specimen identification is based only upon the state of aggregation and on simple physical properties that can be determined by inspection or by some rapid and easily performed nondestructive tests. Many common minerals can be identified reasonably accurately in this fashion, even in the field. Note, however, that most of these properties are qualitative descriptions and often vary within a mineral species. In other cases, two distinct minerals may have very similar crystals and physical properties (e.g., proustite/pyrargyrite, quartz/phenakite), and can be told apart only after a detailed examination of their optical characteristics, chemical composition, or X-ray diffraction patterns.

    State of aggregation (including crystallographic form and habit)

    Some crystals, particularly those that have grown to large size in veins or cavities in otherwise finer grained rocks, have characteristic shapes and dimensions that reflect the processes by which they have been deposited. Others occur in amorphous-looking masses with characteristic surface textures. Highly subjective and somewhat fanciful terms are used to describe the state of aggregation of mineral bodies and the visible shapes of individual crystals.

Minerals
  • Their Constitution and Origin
  • 2nd edition
  • Hans-Rudolf Wenk, Andrey Bulakh
  • Published online:
    25 September 2020
    Print publication:
    04 January 2016
  • The new edition of this popular textbook, once again, provides an indispensable guide for the next generation of mineralogists. Designed for use on one- or two-semester courses, this second edition has been thoughtfully reorganised, making it more accessible to students, whilst still being suitable for an advanced mineralogy course. Additions include expanded introductions to many chapters, a new introductory chapter on crystal chemistry, revised figures, and an extended plates section containing beautiful colour photographs. Text boxes include historical background and case studies to engage students, and end-of-chapter questions help them reinforce concepts. With new online resources to support learning and teaching, including laboratory exercises, PowerPoint slides, useful web links and mineral identification tables, this is a sound investment for students in the fields of geology, materials science and environmental science, and a valuable reference for researchers, collectors and anyone interested in minerals.

Page 64 of 481