Definition of Mineral. Characteristics and Properties
Although it is difficult to formulate a succinct and universally acceptable definition for the word mineral, the following applies well to the subject coverage in this text:
A mineral is a naturally occurring solid with a highly ordered atomic arrangement and a definite (but not fixed) chemical composition. It is usually formed by inorganic processes.
A step-by-step analysis of this somewhat restrictive definition will aid in its understanding. At the end of the discussion of the implications of this definition there will be a brief overview of how a broader definition of mineral is now used in many research aspects of mineral and materials science.
The qualification naturally occurring distinguishes between substances formed by natural processes and those made in the laboratory. Industrial and research laboratories routinely produce synthetic equivalents of many naturally occurring materials, including valuable gemstones such as emeralds, rubies, and diamonds. Since the beginning of the twentieth century, mineralogic studies have relied heavily on the results from synthetic systems in which the products are given the names of their naturally occurring counterparts. Such practice is generally accepted, although it is at variance with the strict interpretation of naturally occurring.
In this textbook mineral means a naturally occurring substance, and its name may be qualified by synthetic if purposely produced by laboratory techniques. One might now ask how to refer to CaC03 (calcite) that sometimes forms in concentric layers in city water mains. The material is precipitated from water by natural processes but in a manmade system. Most mineralogists would refer to it by its mineral name, calcite, because humanity's part in its formation was inadvertent.
The qualification solid excludes gases and liquids. Thus, H20 as ice in a glacier is a mineral, but water is not. Likewise liquid mercury, found in some mercury deposits, must be excluded by a strict interpretation of the definition. However, in a classification of natural materials, such substances that otherwise are like minerals in chemistry and occurrence are called mineraloids and fall in the domain of the mineralogist.
A highly ordered atomic arrangement indicates an internal structural framework of atoms (or ions) arranged in a regular geometric pattern. Because this is the criterion of a crystalline solid, minerals are crystalline. Solids that lack an ordered atomic arrangement are called amorphous (e.g., glass). Several natural solids are amorphous. Examples are volcanic glass (which is not classified as a mineral because of its highly variable composition and lack of ordered atomic structure), limonite (a hydrous iron oxide), and allophane (a hydrous aluminum silicate); also several metamict minerals qualify— for example, microlite, gadolinite, and allanite (in metamict minerals the original crystallinity has been destroyed, to various degrees, by radiation from radioactive elements present in the original structure). They, with the liquids water and mercury, which also lack internal order, are classified as mineraloids.
The statement that a mineral has a definite chemical composition implies that it can be expressed by a specific chemical formula. For example, the chemical composition of quartz is expressed as Si02. Because quartz contains no chemical elements other than silicon and oxygen, its formula is definite. Quartz is, therefore, often referred to as a pure substance. Most minerals, however, do not have such well-defined compositions. Dolomite, CaMg(C03)2, is not always a pure Ca-Mg-carbonate. It may contain considerable amounts of Fe and Mn in place of Mg. Because these amounts vary, the composition of dolomite is said to range between certain limits and is, therefore, not fixed.
The chemical formula of a pure dolomite is given as CaMg(C03)2; such a formula is commonly referred to as an end-member composition. The formula of a variety of dolomite with variable Fe and Mn in its structure would be expressed as Ca(Mg,Fe,Mn)(C03)2 without specific subscripts for Mg, Fe, or Mn. The pure end-member formula and the one with greater chemical variability have a set of atomic ratios in common. In the end- member formula Ca : Mg : C03 is 1 : 1 : 2, and in the iron, and manganese-containing variety of dolomite, Ca : (Mg + Fe + Mn) : C03 is also 1:1:2. In other words, the overall atomic ratios in these formulas remain the same (fixed) even though one of the two dolomite types indicates a range in chemical composition (its chemistry is not fixed).
According to the traditional definition, a mineral is formed by inorganic processes. This is prefaced with usually and thus includes in the realm of mineralogy those organically produced compounds that answer all the other requirements of a mineral. The outstanding example is the calcium carbonate of mollusk shells. The oyster's shell and the pearl that may be within it are composed in large part of aragonite, identical to the inorganically formed mineral.
Although several forms of CaC03 (calcite, aragonite, vaterite) and monohydrocalcite, CaC03*H20, are the most common biogenic minerals (meaning "mineral formed by organisms"), many other biogenic species have been recognized. Opal (an amorphous form of Si02), magnetite (Fe304), fluorite (CaF2), several phosphates, some sulfates, Mn-oxides, and pyrite (FeS2), as well as elemental sulfur, are all examples of minerals that can be precipitated by organisms (see Lowenstam 1981).
The human body also produces essential minerals. Apatite, Ca5(P04)3(0H), is the principal constituent of bones and teeth. The body can also produce mineral matter (calculi) in the urinary system. Such calculi consist predominantly of calcium phosphates (such as hydroxylapatite, carbonate-apatite, and whitlockite), calcium oxalates that are very uncommon in the mineral world, and magnesium phosphates (see Gibson 1974).
However, petroleum and coal, frequently referred to as mineral fuels, are excluded. Although naturally formed, they have neither a definite chemical composition nor an ordered atomic arrangement. An exception is graphite, formed when coal beds have been subjected to high temperatures that drive off the volatile hydrocarbons and crystallize the remaining carbon.
Having evaluated in detail the implications of the definition of mineral that is most applicable to the subject and purposes of this text, it is instructive to discuss the broader definition accepted in much of ongoing mineral research. There is a great diversity in the scientific research venues that directly relate to mineral science.
Taken in part from R. J. Hemly (1999), this includes: (1) the synthesis of new "minerals" at high pressures and temperatures to simulate materials that are expected to reside in the Earth's mantle and core (Ultrahigh-Pressure Mineralogy 1998; see reference list at the end of this chapter; see also p. 111); (2) the investigations of the transition from the crystalline to the amorphous (noncrystalline, disordered) state by application of extremely high pressures or by electron beam irradiation; (3) research on microorganisms that cause mineral precipitation and dissolution and control the distribution of elements in diverse environments at and below the surface of the Earth (Geomicrobiology 1997); (4) the study of mineral surfaces and their involvement in controlling reactions that occur near the Earth's surface; and (5) the synthetic production of zeolitic structures with spaceous channels that can be used in industrial applications as molecular sieves, ion exchangers, and catalysts.
These wide-ranging research endeavors are part of mineral and materials science. With time, further results from such investigations will provide the mineralogist and earth scientist with a fuller understanding of the complex and heterogeneous makeup of the Earth and other planets.
Date added: 2022-12-31; views: 138;