The abundance of an element is different from its mode of occurrence, or availability. Some elements present in the crust in considerable amounts never occur in any concentrated form; they are dispersed in common minerals and are called dispersed elements.
Thus, copper, much less abundant than rubidium, forms rich ore deposits. On the other hand, rubidium is found dispersed in potassium minerals. Similarly, gallium is dispersed in aluminium minerals; but chromium and lead, both comparatively less abundant metals, are known to form large scale ores. Indium, hafnium, and rhenium are similarly dispersed elements.
The availability of an element depends largely on its ability to form concentrated mineral sources. Even then, elements with rich “ore deposits” also have a larger fraction unavailable as dispersed amount. Extraction of the element from such dispersed states is impractical for technical and economic reasons. Thus magnesium in large quantities (∼30%) is present in vast deposits of olivine.
But it is extracted from sea-water, where it is present only to the extent of 0.03%. Sometimes, a relatively rare element gains importance due to its usefulness, e.g., Be. Sometimes, a rare element may be obtained as an industrial by-product. Electrolytic refining of copper can provide large quantities of tellurium; about 1000 tons of gallium and germanium may be obtained annually from the ashes of certain coals. It is time that we should explore possibilities of utilizing these elements on a commercial scale.
The main forms of existence of the common elements on the earth are shown in Table 1.3. It appears that oxygen compounds, mainly in the form of silicates, predominate on the earth’s crust. Carbonates, sulfides and sulfates occur in gradually decreasing quantities.
Geochemical Classification of the Elements:
According to Goldschmidt, distribution of the elements in the crust, mantle and the core of the earth was initiated by formation of different layers of varying densities during the solidification of the earth. Different elements were then distributed among these layers according to their affinity. On this basis he classified the elements into four groups according to their relative affinities for metallic, sulfidic, silicate and gaseous phases.
(a) Siderophilic or Iron-Loving (Greek Sideros ≡ Iron):
These elements are believed to occur free in the core of the earth together with iron. However, many of them occur in the crust in other combined forms. Except Ru, Rh, Pd, Os, Ir, Pt, Au and C, the remaining elements occur in combination with sulfur in the crust of the earth (chalcophilic). Ru, Rh, Pd and Os also show some affinity for sulfur. Carbon occurs as carbonates in rocks (lithophilic). Some elements, viz., Fe, Co, Ni, Cu, Ga, Ge and Sn are also found as their oxides in rocks (lithophilic).
This group includes elements which have incompletely filled outermost shells of electrons. They are relatively inert chemically—the triads of group VIII (Mendeleef classification) are most important in this category (Table 1.3).
The siderophilic elements have high negative electrochemical potentials and they occur near the minima in the atomic volume (vs. atomic no.) curve. The heats of formation of oxides of these elements are lower than that of FeO.
(b) Chalcophilic or Copper Loving (Greek Chalcos ≡ Copper):
This group generally contains elements whose ions contain 18 electrons in the outer shell and whose ionization energies are greater than about 600 kJ mol-1 (6-7 eV) and which form covalently bonded sulfides. Notable among them are Mo, Fe, Cu, Ag, Zn, Cd, Hg, Ga, In, Tl, Pb, As, Sb, Bi, S, Se, Te. Since copper is chiefly found as its sulfide mineral, these elements are also found principally as sulfides (Table 1.3). These elements have intermediate potentials in the electrochemical series. In the atomic volume curve, they occupy regions in which atomic volume increases with atomic number.
(c) Lithophilic or Stone-Loving (Greek Lithos, Stone):
These elements readily form ions with an octet in the outermost shell. They have low ionization energies (< ~ 600 kJ mol–1) and high positive potentials (1-3 volts). They combine strongly and ionically to oxygen—the heats of formation of these oxides are greater than that of FeO. The difference between the two heats of formation is a measure of the intensity of the lithophile character. In the atomic volume curve, they appear near the maxima and on the declining parts.
When sufficiently basic, the oxides of the lithophilic elements combine with silica to form silicates. Hydrogen and the halogens, however, occur in different forms – hydrogen as oxide and the halogens as their halides.
(d) Atmophilic or Vapour Loving:
These elements are gaseous at ordinary temperatures. Hydrogen and oxygen are included in this group as members showing secondary affinity.
The distribution of any element depends on the temperature, pressure and the chemical environment of the system as a whole. As a result, some elements may show affinity for more than one group. Thus chromium is a strongly lithophile element in the crust of the earth; but it becomes chalcophilic in an oxygen-deficient atmosphere, as is found in some meteorites. Carbon and phosphorus become siderophilic under strongly reducing conditions.
c → chalcophilic in the crust of the earth. l → lithophilic in the crust of the earth.
In the last three groups, the affinity is much less pronounced for the elements in normal (not bold) types.
The above geochemical classification of an element refers to its behaviour in liquid phase equilibria in melts. The actual mineralogy of an element is somewhat different because there are many secondary factors responsible for mineral formation. Thus, thallium, a chalcophile element, forms sulphide minerals. But the greater part of thallium in the earth’s crust is found in potassium minerals where Tl+ ions replace K+ ions. In the following section, we shall briefly survey the principal factors responsible for mineral formation.
The Formation of Minerals:
Minerals are naturally occurring substances, generally solid and inorganic, whose atoms and ions are arranged in regular, three-dimensional patterns that give each mineral a characteristic set of physical properties. A mineral may be composed of either a single element (graphite) or of several elements. Minerals may have widely different chemical compositions and structures. They present a great diversity of forms, structures and physical and chemical properties. We identify a mineral by its form, colour, transparency, hardness, lustre, crystal structure etc.
About 3300 different kinds of minerals have been known to be present in nature—of which only a few hundred are common.
A mineral rarely exists alone in the crust. Usually two or three are combined in regular pattern and definite proportion to form different rocks. According to their origin of formation we group various rocks into igneous, sedimentary and metamorphic rocks. The mother of these rocks may be identified as the magma, or molten rock material, mainly silicate.
Temperature in the crust increases at the rate of about 1°C for each 33 m depth and it does not become steady until the depth is very great. From 60 km onward, the temperature ranges from 500-1200°C, or even 2000°C. This temperature is sufficient to melt any rock. But at the very high pressure of the upper crust (~500,000 atmosphere), rocks do not melt entirely but turn into a thick fluid, the magma.
(1) Igneous Rocks:
Igneous rocks result from freezing of magma. They may be of two types— intrusive rocks and extrusive rocks. The magma intrudes upward through cracks into surrounding rocks—with a sharp fall in temperature and pressure it differentiates and crystallizes to form minerals. The “frozen” magma now becomes a body of intrusive rocks. The lava poured out from a volcano solidifies into extrusive rocks.
Almost all igneous rocks are crystalline. Intrusive rocks are formed by slow cooling and are usually coarse-grained. But extrusive or volcanic rocks are formed by rapid cooling, and so large crystals cannot be formed; they are either fine-grained or glassy.
Granites and basalts are common examples of igneous rocks. Chemically, silicates are the main constituent of these rocks. Pegmatites are another kind of igneous rock which crystallize during the last stages of cooling and solidification of magma. They consist of coarser grains than granites and contain many rare elements. Igneous rocks are the major kinds of rocks in the crust, making up about 65 per cent of the total by weight.
(2) Sedimentary Rocks:
Sedimentary rocks are formed by the erosion of igneous rocks by exogenic forces. Rocks exposed on the surface are acted upon by sun rays, ice freezing, wind, rains, plant roots, etc. With this sort of continuous weathering they get transformed into debris, sand and soil. These are then carried out by water current, wind, glacier, etc. and get deposited in rivers, lakes, sea-beds and the like. The sediments grow in thickness over years.
As the pressure and temperature increase, water is squeezed out of the pores in the sediments and the pores are filled by silica, iron oxide or carbonates from solution. Their cementing action converts the loose grainy sediment to sedimentary rocks. Mudstone, sandstone, limestone, dolomite, etc. are examples of sedimentary rocks. Such rocks can be readily identified by their layer structure or stratification.
Sedimentary rocks are nearly 8% of the total weight of the crust. But they are widely distributed over continental surfaces and sea-beds. Almost 75% of the continental surfaces are covered by them.
(3) Metamorphic Rocks:
These rocks are formed by the transformation of igneous and sedimentary rocks. The latter, buried at great depths in the crust, get heated by intruding hot magma. At the high temperature and pressure, the rocks undergo a change of structure, texture and ingredients, too; ultimately they are converted to metamorphic rocks. Slate and marble are common examples of such rocks. They make up about 27% of the whole crust.
Factors Controlling the Formation of Minerals:
The actual processes involved in the formation of rocks are a series of complicated chemical reactions taking place over a very long period of time under varying external conditions. The elements may be broadly classified according to their geochemical affinity into lithophilic, siderophilic, etc. groups. This classification may be used as a rough starting point for the formation of minerals.
As the earth gradually attained its present form, a phase differentiation was brought about by gravity. The compositions of these phases were determined by the most abundant elements—Fe, Mg, Si, O, S and Al. The amount of oxygen was evidently more than that of sulfur; again the total oxygen plus sulfur was insufficient to combine with the entire amount of electropositive elements. Among them, iron was more abundant than magnesium and silicon, and possessed the greatest affinity for sulfur.
At the same time, it was most easily reduced to the free metal. As a consequence, three distinct phases were formed—free iron (left uncombined), iron sulfide and iron-magnesium silicate. Elements more electropositive than iron displace iron from the silicates.
The less electropositive elements were displaced by iron from the silicate phase. Elements which form essentially homopolar bonds with sulfur—the sulfide group metals in analytical chemistry and the metalloids—were accommodated in the sulfide phase.
This interpretation is also in keeping with the classification of elements as class-a and class- b. The class-a metals are usually found associated with oxygen and oxoanions such as phosphates, silicates and carbonates. The class-b metals are usually found as their sulfides, e.g., zinc, cadmium and mercury.
It is likely that in the reducing atmosphere prevailing when the earth’s crust solidified, these and other chalcophilic metals separated out in the sulfide phase. Subsequently, as rocks were weathered, some of the sulfides were leached out and eventually precipitated as carbonate, silicate or phosphate. This idea appears to be very much applicable to the minerals of zinc.
Further insight into the process of mineral formation involves detailed consideration of the various factors determining the formation and stability of compounds in general. The atoms and ions are striving toward stability through stages of redistribution and recombination under different prevailing conditions. The principal controlling factors for such changes may be broadly generalized as—(i) structural control (ii) thermodynamic control and (iii) kinetic control.
Ionic size, ease of packing and electrical charge balance is the most important structural factors in the formation of minerals. Thus ions of similar radii but different chemical character often occur together in silicate minerals. Thallium exists in potassium minerals which may be attributed to similar ionic radii (K+ 0.14 nm; Tl+ 0.15 nm). Some other examples are shown in Table 1.5.
Examples of Structural Control in Mineral Formation:
The ease of packing of cations and anions in a particular mineral is largely determined by radius-ratio effects. When a cation has a counter-ion with radius-ratio lying near the boundary between two types of coordination, it may occur in both coordination numbers. The temperature and pressure at which crystallization took place gains importance in such cases.
High temperature and low pressure favour the lower coordination number. Thus aluminium has a radius-ratio value 0.38 with oxide ion. It assumes C.N. 4 in typically high-temperature minerals and substitutes silicon (e.g., feldspars). In minerals formed at lower temperatures, it tends to attain C.N. 6.
The importance of electrical stability in crystals is nicely summarized in the following rule of Pauling – “In a stable structure the total strength of the valency bonds which reach an anion from all the neighbouring cations is equal to the charge on the anion”. Thus feldspars contain 4-coordinated Si or Al with oxide ions. Only one-fourth of the valency of an oxygen atom is left unsatisfied by the Si or Al. Mg or Fe in six-fold coordination would contribute at least 2/6 or one-third unit of charge.
This would be greater than the “charge-requirement” of the oxygen atom and hence Mg or Fe cannot enter such structures. But Na, K, etc. univalent ions and large bivalent cations like Ca2+ have radius-ratios (with the oxide ion) greater than 0.71, nearly that required for C.N. 8. They can meet the residual charge requirement of the oxide ion. So we find feldspars with Ca, Na and K but not with Mg or Fe in their structures.
Formation of isomorphous crystals or solid solutions and atomic substitution are other examples of structural control on the formation of minerals. Examples of isomorphism are already well-known. Solid solution formation is independent of isomorphism, e.g., between FeS and ZnS. In atomic substitution, atoms or ions replace others of similar size and charge. Sometimes the charge may be balanced by the inclusion of an additional ion, or by way of coupled substitution – Mg2+ – Si4+ ion pairs in diopside (CaMgSi2O6) are replaced in part by Al3+ – Al3+.
Substitution does not usually occur when the ionic charges differ by more than unity – Y3+ not by Na+; nor Zr4+ by Mn2+. The presence of iron and manganese in dolomites (calcium magnesium carbonate) is now interpreted in terms of atomic substitution of magnesium. This is often referred to as diadochy, the ability of different elements to occupy the same lattice position.
Crystal field stabilization energy provides another interesting illustration of various stabilizing factors involved in mineral formation. Pending our introduction to the nature of this energy chemistry, we simply accept that certain ions are preferentially stabilized in octahedral sites, e.g., the divalent ions of Ti, V, Cr, Fe, Co, Ni and Cu. The non-transition elements and Mn(II), Zn(II) ions lack this stabilization. The order of decreasing octahedral site preference energy for some of the +2 and +3 transition metals is –
+2: Ni > Co > Mn; +3: Cr > V > Sc
This order suggests that Ni2+ would be more readily depleted from the magma during crystallization of olivine and pyroxene than would Co2+. The structures of inverse spinels also point out the importance of crystal field stabilization.