Mineralogy

Solids, Liquids and Gases

Matter may exist in three states, the solid, the liquid, and the gaseous. Most minerals are solid, but some materials considered here, such as petroleum and natural gas, are fluids. Liquids and gases are “fluids,” i.e. unlike solids they flow under the action of gravity: a gas entirely fills the space containing it, whereas a liquid may not, but may be bounded by an upper horizontal surface. Most pure substances can exist in all three states, and may be caused to pass from one to another by heating or cooling. At sufficiently high temperatures many minerals are melted to liquids, although some are chemically decomposed by heat before they reach their melting point.

Elements, Compounds and Mixtures

A pure substance is one that possesses characteristic and invariable properties; matter can thus be divided into mixtures and single (or pure) substances. Pure substances may be of two kinds, viz., elements and compounds.

Elements are substances which have not so far been split up into simpler substances by any chemical means. About ninety elements are at present known, but many are extremely rare and of little importance to the mineralogist. I t has been estimated that the crust of the earth is composed of 46.5% oxygen, 27.6% silicon, 8.1% aluminum, 5.1% iron, 3.6% calcium, 2.6% potassium, 2.8% sodium, and 2.1% magnesium. Thus, over 98% of the earth’s crust is composed of but eight elements, and most of the elements of economic value are absent from this list.

Compounds are pure substances made up of two or more elements. They are formed as a result of chemical change and are different from mere mixtures in the following ways:

1. The elements constituting a compound are combined in definite proportions by weight.
2. A compound cannot easily be split up, whereas the components of a mixture can usually be separated by mechanical means. These components may themselves be either elements or compounds.
3. The properties of a compound are often very different from those of the elements it contains, whereas a mixture usually possesses the properties of its constituents.
4. Heat is either given out or absorbed when a compound is formed; this does not in general occur when substances are merely mixed.

Minerals are compounds of their constituent elements, while rocks are mixtures of their component minerals. Thus, the mineral quartz is a compound (silica) of the elements silicon and oxygen, whereas the rock granite, as we have seen, is a mixture of several minerals, one of which is quartz.

Atoms

The chemical and physical behavior of substances is best explained in terms of an Atomic Theory of Matter. It is possible to break down the matter of an element into smaller and smaller particles, and at one stage of this process the particle is called an atom. The atoms of one element are all alike and differ from those of other elements. Chemical combination is the binding together of atoms, and hence a useful definition is:

An atom is the smallest part of an element that can enter into chemical combination with another element.

Atoms unite with one another in definite proportions, though an atom of one element may unite with different numbers of atoms of another element in two or more different compounds. For example, the carbon atom combines with the oxygen atom to form two different compounds: carbon monoxide, in which one atom of oxygen is joined to one atom of carbon, and carbon dioxide, which has two atoms of oxygen combined with each carbon atom. Again, iron combines with oxygen in the proportions of 1:1 (ferrous oxide) and 2:3 (ferric oxide).

Molecules

The particles of a substance in the gaseous condition are widely separated from each other and in a state of rapid, random motion. These freely moving particles are called molecules, and they may consist of single atoms, as in the gas helium, or of two or more atoms of the same element, as in hydrogen or oxygen or, in the case of compounds, of two or more atoms of different elements, e.g. steam, carbon dioxide.

When a gas condenses to a liquid the molecules are no longer separated in space but come together and, to a certain extent, lose their identity. When the liquid is frozen to a solid, the atoms arrange themselves in a fairly rigid pattern, and it is no longer possible to segregate anyone group of atoms from the rest. The term “molecule” is thus not really applicable to the solid state.

Symbols and Formulae

For convenience, an atom of every element is represented by an abbreviation called a symbol which is usually the first letter, or the first and second letters, of the English or Latin name of the element. The molecule of a substance is represented by a formula: thus, ° is the symbol of an atom of oxygen, and C of an atom of carbon, and O₂ is the formula of a molecule of oxygen, and CO₂ the formula for a molecule of carbon dioxide. The proportions of the constituent elements of a solid or liquid compound are also represented by a formula; thus, calcite is CaCO₃. It should be clearly understood that this formula merely means that calcite is composed of calcium, carbon, and oxygen in the proportions of one atom of calcium, one atom of carbon, and three atoms of oxygen; it does not stand for a “molecule” of calcite” (see previous paragraph).

Atomic and Molecular Weights

The atomic weight of an element is the weight of an atom of the element compared with the weight of an atom of oxygen taken as 16. A table of atomic weights is given below.

The molecular weight of a substance is the sum of the atomic weights of the atoms composing a molecule of the substance. In the case of a solid, the formula weight is a convenient quantity, and is the sum of the weights of the atoms making up the formula of the compound. Thus, the atomic weight of calcium is 40, of carbon is 12, and of oxygen is 16; the formula weight of calcite (CaCO₃) is therefore (40+12+3×16) = 100.

Valency

The valency of an element is measured by the number of its atoms which will combine with or replace one atom of hydrogen. For example, chlorine combines with one atom of hydrogen and is therefore univalent; calcium replaces two atoms of hydrogen and is therefore divalent, and so on. . Several of the elements have different valencies in different compounds; thus iron is divalent in the compound FeO, or trivalent in the compound Fe₂0₃. The usual valencies of the commoner elements are given below:
Univalent: H, Cl, Br, I, F, Li, Na, K, Ag, Cu, Au. Divalent: 0, S, Se, Te, Be, Mg, Ca, Sr, Ba, Pb, Hg, Cu, Zn, Co, Ni, Fe, Mn, Cr, Sn.
Trivalent: B, Au, AI, Fe, Mn, Cr, Co, Ni, N, P, As, Sb, Bi.
Quadrivalent: C, S, Si, Ti, Zr, Sr, Mn, Pb.
Quinquavalent: P, As, Sb, Bi, Ta.
Hexavalent: S, Cr, Mo, W, U.
Heptavalent: Mn.
Note that some elements show variable valency, e.g. Fe, S, Mn.

The Structure of the Atom

According to views developed early in this century, the atoms themselves may be regarded as built up of still smaller units, called electrons and protons. The electron has a unit negative electric charge, and a mass about 1/1860 of that of the lightest atom, hydrogen; the proton has a mass about equal to that of’ the hydrogen atom and carries a unit positive charge. Although other similar small units exist, it is convenient to regard the electron and the proton as the bricks from which the atoms of the elements are built. In the Rutherford-Bohr theory, the atom consists of a central nucleus surrounded by electrons moving in orbits, rather like the planets round the sun. Most of the mass of the atom is concentrated in the nucleus, which is small compared with the diameter of the whole atom as defined by the outermost electrons. The nucleus carries a positive charge equal in magnitude to the total charge of the orbital electrons, so that the whole atom is electrically neutral.

Thus the lightest atom, hydrogen, consists of a single orbital electron and a nucleus of unit mass carrying unit positive charge (i.e. one proton). The next atom in order of weight, helium, consists of two orbital electrons and a nucleus of 4 units of mass with two positive charges, and so on with successively heavier atoms. Each atom differs from its next lighter neighbor in having one more orbital electron and, on the average, two more units of mass in the nucleus.

Ions

An atom which has lost or gained one or more electrons, and is thus no longer electrically neutral, is called an ion. There is plenty of evidence to show that a compound such as common salt (NaCl) is not an aggregate of chlorine and sodium atoms, but of sodium and chlorine ions, the sodium ion having a unit positive charge and the chlorine ion a unit negative charge (written Na⁺, Cl^- ‘}. The electrostatic attraction between these oppositely charged ions constitutes the binding or valency force in the compound sodium chloride, and is known as an electrovalency. If they can be melted or dissolved in water, such compounds conduct electricity and are called “ionic” or “polar” compounds j other examples are calcium sulphate (the mineral anhydrite) and iron sulphide (pyrites). But not all valency links are of this kind: in many cases the link is in the nature of a sharing of one or more electrons between atoms, and is known as the “co-valent” or “homopolar” bond.