Читать книгу Earth Materials - John O'Brien - Страница 14

The term mineral is used in a number of ways. For example, the chemical elements, such as calcium, iron, and potassium, listed on your breakfast cereal box, your bottle of vitamin supplements or your bag of fertilizer are called minerals. Coal, oil, and gas are referred to as mineral resources. All of these fall under a broad use of the term mineral. In a stricter sense used by many, but not all geologists, minerals are defined by the following properties:

1 Minerals are solid, so do not include liquids and gases. Minerals are solid because the atoms in them are held together in fixed positions by forces called chemical bonds (Chapter 2).

2 Minerals are naturally occurring, meaning that they occur naturally within the Earth. This definition excludes synthetic solids that are produced only by technologies in laboratories or factories. It does include solid Earth materials that are produced by both natural and synthetic processes, such as natural and synthetic diamonds and the solid materials synthesized in high temperature and high pressure laboratory experiments that are thought to be analogous to real minerals that occur only in the deep interior of Earth.

3 Each mineral species has a specific chemical composition which may vary only within well‐defined limits; that is to say that each mineral possesses a chemical composition that can be expressed by a chemical formula. An example is common table salt or halite which is composed of sodium and chlorine atoms in a 1 : 1 ratio (NaCl). Chemical compositions may vary within well‐defined limits because minerals incorporate impurities, have atoms missing, or otherwise vary from their ideal compositions. In addition some types of atoms may substitute freely for one another when a mineral forms generating a well‐defined range of chemical compositions. For example, magnesium (Mg) and iron (Fe) may substitute freely for one another in the mineral olivine whose composition is expressed as (Mg,Fe)2SiO4. The parentheses are used to indicate the variable amounts of Mg and Fe that may substitute for each other in olivine group minerals (Chapter 3).

4 Every mineral species possesses a long‐range, geometric arrangement of constituent atoms or ions. This implies that the atoms in minerals are not randomly arranged. Instead minerals crystallize in geometric patterns so that the same pattern is repeated throughout the mineral. In this sense, minerals are like three‐dimensional wall paper. A basic pattern of atoms, a motif, is repeated systematically to produce the entire geometric design. This long range pattern of atoms characteristic of each mineral species is called its crystal structure. All materials that possess geometric crystal structures are crystalline materials. They are minerals, in the narrow sense, if they are naturally occurring, inorganic solids with a well‐defined chemical composition. Solid materials that lack a long‐range crystal structure are amorphous materials, where amorphous means without form and without a long‐range geometric order.

Many would add a fifth property that requires minerals to sometimes be formed by inorganic processes. It is certainly true that the vast majority of minerals conform to this property and that the vast majority of organically formed crystalline solids are not considered to be minerals. However, many solid Earth materials that form by both inorganic and organic processes are considered minerals, especially if they are important constituents of naturally formed rocks. For example, the mineral calcite is also precipitated as shell material by organisms such as clams, snails, and corals and is the major constituent of the rock limestone (Chapter 14).

Over 5500 minerals have been discovered to date (www.mindat.com) and each is distinguished by a unique combination of chemical composition and crystal structure. Strictly speaking, naturally occurring, solid materials that lack one of the properties described above are commonly referred to as mineraloids. Common examples include amorphous materials such as volcanic glass in which the atoms lack long‐range order and amber or ivory which are formed only by organic processes.