Mineralogy

Metals and Non-Metals

The elements may be divided roughly into two classes, metals and non-metals. There is no hard-and-fast line of division between the two classes, and the metalloids (e.g. arsenic) combine characteristics of both divisions. The physical distinction between the two classes is readily understood with reference to luster, malleability, conduction of heat and of electricity, etc., but, as will be seen later, this division is also of great chemical importance.

Metals: AI, Sb, As, Ba, Bi, Cd, Ca, Cr, Co, Cu, Au, Fe, Pb, Mg, Mn, Hg, Mo, Ni, Pt, K, Ag, Na, Sr, Sn, Ti, W, Zn, etc.

Non-Metals: B, Br, C, CI, F, H, I, N, 0, P, S, Si, etc.

The Periodic Classification

When the elements are listed in the order of their atomic weights, they may be divided into groups so that elements of similar chemical properties are brought together. This was first shown by Mendeleeff, and modern views on the structures of the atoms have given a physical basis to his Periodic Law. In the table on page 14, each element is shown by its symbol and by a number, known as the “atomic number,” which indicates its position in the list of elements arranged in the order of their atomic weights. The atomic number of an element is also equal to the number of positive charges on the nucleus of its atom (and therefore to the number of orbital electrons). The rows across the Table correspond to Mendeleeff’s original periods, and elements connected by lines running from top to bottom of the table form the groups. Elements in the same group show similar chemical properties: they have the same main valency, and tend to replace one another in varying degree in minerals. Their compounds often crystallise in similar forms and they often occur together in nature. Thus, there are marked similarities between the corresponding minerals of Li, N a, and K; and of Ca, Sr and Ba. Elements in the 4th, 5th and 6th periods which are shown surrounded by a frame are of variable valency, having special features in their electronic structure, and are known as “transitional.”

Classes of Compounds

Oxides

Compounds of oxygen with another element are called oxides, and are a very important class of minerals. As examples may be given corundum (Al₂0₃), tinstone (Sn0₂), and quartz (Si0₂). The chemical composition of complex minerals can be written as a combination of various oxides, e.g. orthoclase, KAISi₃0₈, could be written K₂0.AI₂0₃.6Si0₂. This used to be the accepted way of formulating minerals, but it is misleading, because the oxides are not present as such in the mineral. In this resource the formulae of minerals are now written as far as possible in accordance with their atomic structure as revealed by X-ray studies, a matter of special importance in connection with the silicates.

Periodic Table of the Elements

Click for enlargement

Acids and Bases

The oxides of non-metals are acidic, and most of them dissolve in water to form acids. All acids are compounds of hydrogen, which is capable of being replaced by a metal; the group of atoms combined with the hydrogen is termed the acid. radicle. Thus sulphur, carbon and nitrogen give rise respectively to sulphuric, carbonic and nitric acids. The oxide of silicon, silica (Si0₂), is acidic but is not readily soluble in water and does not give rise to silicic acid.

The oxides of metals are, in general, basic. Some combine with water to form bases, e.g. caustic soda (NaOH), calcium hydroxide (Ca(OH)₂). Many metal hydroxides, some of which occur as minerals, are insoluble and therefore not formed in this way, but, on heating, they lose their water and form the basic oxides. Examples are: Mg(OH)₂, brucite; AI₂(OH)₆, gibbsite.

Salts

By the combination of an acid and a base, the hydrogen of the acid is replaced by the metal of the base, and the result is the formation of a salt. Thus the action of hydrochloric acid (HCI) on the base caustic soda (NaOH) gives the salt sodium chloride (NaCI), together with water (H₂0), as shown in the equation below:

HCl+NaOH=NaCI+ H₂0

acid + base = salt + water

Many minerals are salts, and the names of the commoner acids and their corresponding salts are tabulated below:

Name of Acid	Name of Salt	Example of Salt
Hydrochloric (HCI)	Chloride	Rock-salt (NaCl)
Hydrobromic (HBr)	Bromide	Bromyrite (AgBr)
Hydriodic (HI)	Iodide	Iodyrite (AgI)
Hydrofluoric (HF)	Fluoride	Fluor-spar (CaF2)
Nitric (HNO3)	Nitrate	Nitre (KNO3)
Sulphuric (H2S04)	Sulphate	Barytes (BaS04)
Sulphuretted Hydrogen (H2S)	Sulphide	Galena (PbS)
Carbonic (H2CO3)	Carbonate	Calcite (CaCO3)
Pyroboric (H2B407)	Borate	Borax (Na2B407)Aq.
Phosphoric (H3P04)	Phosphate	Apatite [Ca3 (PO 4)2]

The large group of silicate minerals used to be regarded as derived from a number of hypothetical silicic acids. The structure of these minerals is dealt with later.

In the examples of mineral salts given in the table above, all the hydrogen of the acids has been replaced by metallic elements, and the resulting salts are called normal salts. When only a part of the hydrogen is replaced acid salts are produced. For example, K₂S0₄ is normal potassium sulphate, KHS0₄ is acid potassium sulphate. In basic salts, the whole of the base has not been neutralized by the acid portion; thus, the mineral malachite is a basic carbonate of copper and its composition may be written CuC0₃.Cu(OH)₂.

Water of Crystallization

When certain minerals crystallise they combine with a number of molecules of water, which are loosely attached to the compound, and do not enter into its inner chemical constitution. This water is called water of crystallisation and can be driven off from the compound at a moderate heat. Gypsum has two molecules of water of crystallisation, as CaS0₄ + 2H₂0; borax has ten, as Na₂B₄0₇ + 10H₂0. Such minerals are said to be hydrated.

Isomorphism

It is found that certain minerals of analogous composition crystallise in forms showing close relation one with another. Such minerals have their atoms arranged on similar plans. This phenomenon is called isomorphism. The members of an isomorphous series are often salts of those metals which are contained in the same group of the Periodic Classification.

The calcite group of minerals is an example of an isomorphous series, consisting of the following chief members: calcite (CaCO3), dolomite (CaCO₃.MgC0₃), ankerite [CaCO₃Mg,Fe)C0₃], magnesite (MgC0₃), mesitite (2MgCO₃.FeCO₃), siderite (FeC0₃), rhodochrosite (MnC0₃). This list shows the presence of links between the simple compounds. The important group of minerals known as the plagioclase felspars constitutes an excellent example of a series showing isomorphous mixture, there being a gradation in chemical composition, crystalline form, specific gravity and optical properties from one extreme, albite NaAlSi₃0₈, to the other, anorthite CaAl₂Si₂0₈.

In isomorphous series one element replaces another, and this finds expression in the forrnulee of the individuals of such series. The olivine group varies from pure magnesium silicate (Mg₂SiO₄) to pure iron silicate, fayalite, (Fe₂SiO₄). The formula . for a slightly ferriferous olivine would be written as (Mg,Fe)₂Si0₄, whereas that of an olivine in which iron predominates would be written (Fe,Mg)₂Si0₄.

Oxidation and Reduction

A chemical change by which oxygen is added to an element or compound is called oxidation. The term reduction is applied to a change in which the oxygen or other non-metal is taken away from a compound:

When metallic copper is heated in contact with air it is changed into a black oxide of copper, as in the following equation:

2 Cu + O₂ = 2 CuO.

Here oxidation of the copper has taken place. The reverse process may be studied by heating the copper oxide in a current of hydrogen, with the result that metallic copper and water are formed. This is a case of reduction, and the changes may be represented as:

CuO + H₂ Cu + H₂0

Another example of oxidisation is the change from ferrous to ferric oxide, thus, 2FeO + ° = Fe₂0₃. This is an important change in connection with the alteration of minerals. Oxidation and reduction are of very great importance in the blowpipe analysis of minerals.
Synthesis and Analysis

The building-up of a compound by the union of one element with others is termed synthesis; the splitting-up of such a compound into its con. stituent elements is called analysis. It is by means of synthesis and analysis that the operations of the chemist are carried on.

Analysis

The first step in analysis consists in determining the nature of the elementary substances contained in a compound, the next in determining the proportions of these constituents. The former is called qualitative, and the latter quantitative analysis.

In a qualitative analysis the recognition of the constituents hinges upon the fact that certain bases and certain acids produce well-marked phenomena in the presence of. known substances or preparations termed reagents. The characteristic effect produced by a reagent is spoken of as a reaction. Thus hydrochloric acid is a reagent, and when added to clear solutions containing salts of lead, silver or mercury, it produces a dense white precipitate consisting of the chlorides of those metala.. - a reaction denoting the presence of one or more of them in the original solution. This reaction must be supplemented by others in order to determine which of the three metals is present in the salt.

Such investigations conducted in solutions are called analyses by the wet way. There is, however, a dry ‘way which is extremely convenient for the purposes of the mineralogist, and this is now described.

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.

Chemistry of Minerals					Â
	Atomic		Weights
		Atomic			Atomic
Element	Symbol	Weight	Element		Symbol	Weight
Aluminium	Al	26·97	Neodymium		Nd	144·27
Antimony	Sb	121·76	Neon		Ne	20·183
Argon	A	39·944	Nickel		Ni	58·69
Arsenic	As	74·91	Niobium		Kb
Barium	Ba	137·36	(Columbium)		(Cb)	92·91
Beryllium	Be	9·02	Nitrogen		N	14·008
Bismuth	Bi	209·00	Osmium		Os	190·2
Boron	B	10·82	Oxygen		0	16·0000
Bromine	Br	79·916	Palladium		Pd	106·7
Cadmium	Cd	112·41	Phosphorus		P	30·98
Ceesium	Cs	132·91	Platinum		Pt	195·23
Calcium	Ca	40·08	Potassium		K	39·096
Carbon	C	12·01	Praseodymium		Pr	140·92
Cerium	Ce	140·13	Radium		Ra	226·05
Chlorine	CI	35·457	Radon		Ra	222
Chromium	Cr	52·01	Rhenium		Re	186·31
Cobalt	Co	58·94	Rhodium		Rh	102·91
Copper	Cu	63·57	Rubidium		Rb	85·48
Dysprosium	Dy	162·46	Ruthenium		Ru	101·7
Erbium	Er	167·2	Samarium		Sm	150·43
Europium	Eu	152·0	Scandium		Sc	45·10
Fluorine	F	19·00	Selenium		Se	78·96
Gadolinium	Gd	156·9	Silicon		Si	28·06
Gallium	Ga	69·72	Silver		Ag	107·880
Germanium	Ge	72·60	Sodium		Na	22·997
Gold	Au	197·2	Strontium		Sr	87·63
Hafnium	Hf	178·6	Sulphur		S	32·06
Helium	He	4·003	Tantalum		Ta	180·88
Holmium	Ho	164·94	Tellurium		Te	127·61
Hydrogen	H	1·0080	Terbium		Tb	159·2
Indium	In	114.·76	Thallium		TI	204·39
Iodine	r	126·92	Thorium		Th	232·12
Iridium	Ir	193·1	Thulium		Tm	169·4
Iron	Fe	55·85	Tin		Sn	118·70
Krypton	Kr	83·7	Titanium		Ti	47·90
Lanthanum	La	138·92	Tungsten		W	183·92
Lead	Pb	207·21	Uranium		U	238·07
Lithium	Li	6·940	Vanadium		V	50·95
Lutecium	Lu	174·99	Xenon		Xe	131·3
Magnesium	Mg	24·32	Ytterbium		Yb	173·04
Manganese	Mn	54·93	Yttrium		Y	88·92
Mercury	Hg	200·61	Zinc		Zn	65·38
Molybdenum	Mo	95·95	Zirconium		Zr	91·22

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.

The Mineral Kingdom

It has long been the custom to divide nature into three great departments, the animal, vegetable and mineral kingdoms. The mineral kingdom comprises the materials that make the crust of the earth and a part of this kingdom is dealt with in .the science of mineralogy. Whether or not any definite boundaries exist between the three kingdoms is a subject which remains to be investigated.

The different members of the animal and vegetable kingdoms are characterised by the development of special organs, or of certain peculiarities of structure, by means of which they pass through a series of changes known as life and growth. This latter phenomenon takes place by the absorption of various kinds of matter which then undergoes conversion by chemical processes into substances similar to those making the plant or animal. In this way the waste which accompanies life is replaced. The bones and shells of animals consist to a great extent of mineral matter. Plants are capable of deriving earthy substances from the soil in which they grow. But mineral matter which has thus been utilised by organisms passes, in the rigid interpretation of the term, beyond the pale of mineralogy, for it assumes a structure, governed by the nature and requirements of the animal or plant, that ‘it would not possess as an ordinary portion of the earth’s crust. For example, a pearl would be regarded as an organic substance and not a true mineral, although it consists of mineral matter. Again, coal, being a substance derived from the decomposition of vegetable matter, would not be rigidly classed with minerals.

Minerals

A most important characteristic of a mineral is the possession of a definite chemical composition. Some qualification of this statement is, however, necessary. Certain minerals form a closely related series in which there is a gradual replacement of one element by another, the two end-members of the series being connected by a number of transitional types of intermediate composition. In order to avoid the establishment of a great number of slightly differing mineral species, it is usual in such cases to consider the series as a whole, definite names being given to the end-members and possibly to certain intermediate types of historic or other interest. The variations of the chemical compositions of such series are not haphazard but are governed by certain rules.

The possession of a definite chemical composition does not suffice in all cases to fix the mineral species. It is found that two minerals with markedly different physical properties, such as colour, hardness, form, density and so on, have identical chemical compositions. In cases such as these, the two mineral species have their atoms arranged on different plans with the result that they have different physical properties. Under favourable conditions, the internal atomic structure of minerals finds expression in their external forms which are bounded by flat surfaces arranged in characteristic ways. Minerals with such external forms provide the beautiful objects known as crystals.

It follows from the requisite of a definite chemical composition and a definite atomic structure that minerals must be homogeneous, that is, each part, however small, must have the same chemical and physical properties.

Definition of a Mineral

A mineral is a substance having a definite chemical composition and atomic structure and formed by the inorganic processes of nature.

If we follow this definition rigidly, we are bound to consider the naturally occurring pure gases amongst the minerals. We should not include air, however, since it is a mixture of nitrogen and oxygen and is therefore not homogeneous. Again, water, snow and ice come within the definition since they are naturally occurring homogeneous inorganic substances of a definite chemical composition. The so-called mineral oils are mixtures of several hydrocarbons and therefore cannot be considered as mineral species.

What should be included within the rigid definition of a mineral is thus clear, but the term is often employed in a more extended sense, a usage which has been the cause of several celebrated law-suits. Thus, a miner considers a mineral to be anything of economic value that can be extracted from the earth. The national statistical summaries of mineral production include details of materials such as chalk, clay, coal, petroleum, and igneous rocks that do not come within the definition of a mineral. In this book it is proposed to discuss not only those substances which fulfill the term, but also a few materials whose origin may not always be free from organic causes or whose chemical composition may not be constant. Coal, mineral oils, limestones and some phosphate are examples of such substances.

Bodies in no way to be distinguished from actual minerals have at various times been artificially formed, either purposely in the laboratory or by accident in industrial processes; but although identical with true minerals of like chemical composition, they are the outcome of processes controlled by human agency, and consequently are not included among minerals. They have, nevertheless, a profound interest for the mineralogist inasmuch as they serve to a certain degree to elucidate the conditions under which the corresponding minerals have been formed.

Rocks

The popular usage of the term mineral includes, as we have already seen, certain substances which are more properly called rocks. A rock is a portion of the earth’s crust which has some individuality; it is the working unit of the field geologist and the distribution of the various kinds of rocks is shown upon geological maps. A rock has no distinctive shape of its own, it has no definite chemical composition and it is not homogeneous.

Examination shows that in most cases rocks consist of a mixture of various minerals. The heterogeneous rock can be taken to pieces and the several homogeneous minerals that compose it separated out. For example, consider the well-known rock granite. It can be seen by inspection of a hand-specimen of this rock that it is made up of three constituents-one white or pink and cleavable, which is the mineral orthoclase; another, clear glassy and with no cleavage, which is the mineral quartz; and a third, glistening, scaly and soft, which ‘is the mineral mica. Detailed chemical and physical investigation would show that the components, orthoclase, quartz and mica, fulfil the requisites of minerals. They are the mineral units which have been aggregated together to form the rock granite. These three constituents occur in varying proportions in different granites and even in different parts of the same granite mass. It sometimes happens that a rock, in the geological sense of an individual portion of the earth’s crust, may be composed of one mineral only. For example, a pure statuary marble consists of the single mineral calcite.