Primary Minerals

The thickness of the earth's crust varies from 10 km under the ocean to 30 km under the continents.
Of the 88 naturally occuring elements on earth, only 8 make of most of the crust.

The earth's crust and soils are dominated by the silicic acid in combination with Na, Al, K, Ca , Fe and O ions.

In Table 4.1.1 the mean elemental content of soil and crustal rocks, and the soil enrichment factors are listed. Elements with high enrichment factors (EF) are C, N, S , and elements with low EF are Na, Mg , Al, P, Cl, K, Ca, Mn and Fe. The latter ones are important nutrients for plant growth.

Those elements are components of primary minerals, whereas primary minerals are components of parent rocks.

There are almost 3000 known minerals, but only 20 are common and just 10 minerals make up 90 % of the earth's crust.

Primary minerals are defined as minerals found in soil but not formed in soil. This definition is different from that of secondary minerals, which are defined as minerals fomed in soils.

Most of the primary minerals (primary silicates) have a crystalline structure, i.e., a structure in which ions are arranged in an orderly and repeated spatial pattern.

The fundamental unit in silicates is the silicon-oxygen tetrahedron, which is composed of a central silicon ion surrounded by four closely-packed and equally-spaced oxygen ions.

The four positive charges of Si4+ are balanced by four negative charges from the four oxygen ions (O2-), one from each ion, thus each discrete tetrahedron has four negative charges. The central ion may be either Al3+, Fe2+, or Mg2+.

When in six-folded coordination, oxygens form an eight-sided octahedron. If larger Ca2+, Na+, or K+ ions are present, they occur at the center of clusters of tetrahedra, with each tetradedron supplying a part of all the oxygens needed for eight-fold or twelve fold coordination.

In this arrangement, the larger cations provide a center of positive charge that attracts and holds the clusters of tetrahedra together.

The cations occuring in this position, i.e., outside or between neighboring tetrahedra are called accessory cations. Si4+ and Al3+ ions are small and have a high charge (valence).

In general, the smaller the cation and the higher its valence the stronger the bond between it and the oxygen.

Stability in minerals requires their structure to be electrically neutral, i.e., the negative charge of the O2- in the structure must be equally balanced by the positive charge of the cations. Isomorphous substitution is the replacement of an ion with higher valence by some other kind of cation. This process is supported by a high concentration of substituting ions in a mineral-forming medium so as to increase their chance of entering the mineral structure in place. The pattern of substitution is generally the following: Al3+ substitutes for Si4+, and Fe2+ and Mg2+ substitutes for Al3+. An electrical imbalance occurs because the valence of the substituting ions is lower than that of the ions in replace. Neutralization of the excess negative charge is accomplished by the inclusion of accessory cations in the structure. In the primary silicates, Ca2+, Na+, K+ are the principal accessory cations that neutralize the negative charge resulting from ion substitution. The most important primary silicates are discussed in the following:

Framework Silicates: They are composed of tetrahedra linked trough their corners into a continuous 3D-structure. Quartz is a framework silicate composed entirely of silicon-oxygen tetrahedra. The bulk density of quartz is 2.65 g/cm3 and quartz is highly resistant to mechanical abrasion and chemical weathering. Quartz is very common in most igneous, metamorphic and sedimentary rocks. In feldspars the Si4+ is partly replaced by Al3+, which results in a positive charge balanced by Na+, K+ or Ca2+ ions. In the alkali feldspars Na+ and K+, and in the plagioclase, Na+ and Ca2+ are the dominant accessory cations. Feldspars are the most abundant minerals in the earth's crust; they make up 50 - 60 % of the crustal rocks.