Mineralogy

Gases and liquids

Oxygen, nitrogen and carbon dioxide are examples of natural gases; and water, mercury and petroleum are examples of natural liquids.

Solids

With the exception of mercury and the natural mineral oils, all the minerals with which we have to deal are found in the solid state, and the properties dependent on their state of aggregation are now considered.

Form

Under favorable circumstances minerals assume certain definite geometrical forms called crystals, the recognition of which is a valuable aid in the identification of minerals.

1. Crystallography or the study of crystals is dealt with in the next chapter. The following general descriptive terms are associated with the crystal characters of minerals:
• Crystallized - a term denoting that the mineral occurs as well-developed crystals. Most of the beautiful specimens in museums are of crystallized minerals.
• Crystalline - a term denoting that no definite crystals are developed, but a confused aggregate of imperfectly formed crystal grains that have interfered with one another during their growth.
2. Cryptocrystalline - a general term to denote the possession of mere traces of crystalline structure. Amorphous is used to describe the complete absence of crystalline structure, a condition found in the natural glasses but rare in minerals.
3. Minerals assume various indeterminate forms that are not necessarily dependent on crystal character. These forms are described by the following terms, which have their customary meanings:
• Acicular - in fine needle-like crystals, as in natrolite. Amygdaloidal-almond-shaped, as with the minerals known as zeolites which occupy the almond-shaped steam cavities of lavas.
• Bladed - in forms shaped like a knife-blade or a lath, a form commonly exhibited by many museum specimens of kyanite.
• Botryoidal - eonsisting of spheroidal aggregations, somewhat reo sembling a bunch of grapes, as with chalcedony.
• Capillary - exhibiting a fine hair-like form as in millerite, nickel sulphide, whence the name capillary pyrites or hair pyrites for such varieties of this mineral.
• Columnar - showing a form resembling slender columns, as in horn. blende.
• Concretionary and nodular - terms applied to minerals which are found in detached masses, the forms being spherical, ellipsoidal or irregular, as in the flint nodules found in the Chalk of the south of England.
• Dendritic and arborescent – tree-like or moss-like forms, usually produced by the deposition of the mineral in very narrow planes or crevices, as with the dendrites of manganese oxide.
• Fibrous - consisting of fine thread-like strands, as exhibited by the variety of gypsum called satin-spar, and by asbestos.
• Foliated or, better, foliaceous - consisting of thin and separable lamellae or leaves, as with mica and other micaceous minerals.
• Granular - in grains, either coarse or fine. Evenly granular aggre. gates of minerals, such as in marble, are often termed saccharoidal from their resemblance to lump sugar.
• Lamellar - consisting of separable plates or leaves as with wollastonite.
• Lenticular - with the form of flattened balls or pellets, shown by many concretionary and nodular minerals.
• Mammillated - displaying large mutually interfering spheroidal surfaces, as in malachite.
• Radiating or divergent-showing crystals or fibres arranged around a central point, as in stibnite and in many cases of concretionary forms.
• Reniform - kidney.shaped, the rounded surfaces of the mineral reo sembling those of kidneys and shown in perfection by the variety of hematite called kidney iron-ore,
• Reticulated - in the form of cross• meshes like a net, as with the rutile needles found in some micas.
• Scaly - in small plates as with tridymite.
• Stellate - showing fibres radiating from a centre to produce star-like forms, as with wavellite.
• Tabular - showing broad flat surfaces, as with wollastonite or tabular spar.
• Tuberose - showing very irregular rounded surfaces often giving rise to gnarled, rootlike shapes as in the variety of aragonite called fios-ferri.
• Wiry or filiform - in thin wires often twisted like the strands of a rope, and shown well by native silver and copper.

Pseudomorphism

Pseudomorphism is the assumption by a m’ineral of a form other than that which really belongs to it. Pseudomorphs may be formed in several ways:

1. A pseudomorph by investment or incrustation is produced by the deposition of a coating of one mineral on the crystals of another, for example, quartz on fluor-spar.
2. A pseudomorph by infiltration is formed when the cavity previously occupied by a certain crystal is refilled by the deposition in it of different mineral matter by the infiltration of a solution.
3. A pseudomorph by replacement arises by the slow and gradual substitution of particles of new and different mineral matter for the original particles which are successively removed by water or other solvents. This kind of pseudomorphism differs from the preceding in the circumstance that the new tenant enters before the old tenant has entirely evacuated its quarters.
4. A pseudomorph by alteration is due to gradual chemical change which crystals sometimes undergo, their composition becoming so altered that they are no longer the same minerals, although they still retain the old forms. As an example may be instanced the common alteration of olivine to serpentine.

Pseudomorphs may often be recognised by a want of sharpness in the edges of the crystals, whilst their surfaces usually present a dull and somewhat granular or earthy aspect.

Polymorphism

It has already been mentioned that two minerals of markedly different physical properties, such as colour, hardness, crystal form, specific gravity, etc., may have ‘identical chemical compositions. Such substances are said to be dimorphous and illustrate the general property of polymorphism. The minerals making up a polymorphous series are composed of the same atoms but have them arranged on different plans so that their physical properties differ.

As an example of dimorphism we may take the two forms of calcium carbonate occurring as the minerals calcite and aragonite. These two minerals form crystals of quite different types, their optical properties are different, and aragonite is harder and has a higher specific gravity than calcite. Again, the physically very dissimilar diamond and graphite are dimorphous forms of carbon. (See Fig. 69.) In nature titanium dioxide, Ti02, occurs in three forms or is trimorphous. The mineral anatase has a specific gravity of 3′9, brookite of 4′15, and rutile of 4′25, and their other physical characters are dissimilar, but in chemical composition they are all titanium dioxide. It is probable that the temperature, pressure, concentration, etc., operative at the time of formation of the mineral control what variety shall be produced.

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.

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.