Classification of the Elements
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.