Minerals: Definition, Occurrence and Classification

After reading this article you will learn about:- 1. Definition of Minerals 2. Occurrence of Soil Forming Minerals 3. Formation 4. Classification.

Definition of Minerals:

Minerals are naturally occurring inorganic solid homogeneous substances composed of atoms having an orderly and regular arrangement with definite chemical composition and a characteristic geometric form, such as quartz (SiO₂), Orthoclase (KAlSi₃O₈), Calcite (CaCO₃), Olivine [(Mg, Fe)2SiO₄] and gypsum (CaSO₄.2H₂O).

Occurrence of Soil Forming Minerals:

The inorganic constituents of soils occur mainly in a limited number of minerals or inorganic compounds of defined crystal out of more than 2000 known minerals. Compositions of given soil forming minerals can vary greatly because of isomorphous substitution of different ions in a given crystal structural position. Therefore, each mineral structure tends to form an isomorphous series of minerals.

Many of the essential trace elements occur mainly as isomorphous substitutions in host minerals, and this prevalence of a variety of ions in minerals of various rocks enabled plants to evolve with a dependence on a wide range of micronutrient elements in addition to major nutrient elements.

Alumino silicates are the most common and principal soil forming minerals. A majority of the soil-forming minerals are light (based on weight) minerals having specific gravity less than 2.9. The relative occurrence of minerals are given in Table 2.3.

Formation of Minerals:

Minerals can be formed as a result of solidification of molten magma in such a fashion that the different elements present therein arrange themselves in accordance with attractive forces and geometric form. The silicate tetrahedron is the principal building block of all the silicate minerals of the earth’s crust. Geometrically one siliconcation (radius 0.42 Ã…, Angstrom is surrounded by four oxygen anions (radius 1.32 Ã…) touching with each other.

The amount of charge carried by silicon is 4⁺ and by oxygen is 2^–. For bringing neutrality, one silicion (4⁺) requires two oxygen ions (2 × 2^–) to form SiO₂, but for the formation of geometrically stable structure one silicon cation needs to be combined with four oxygen ions which results a net negative charge of 4^–.

Classification of Minerals:

Minerals can be classified as below:

1. Feldspars:

Feldspars are anhydrous aluminosilicates of K, Na and Ca and occasionally of other large cations such as Ba. The feldspar structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedra. The terahedra contain mainly Si ions with sufficient Al substitution. In belongs to the group of minerals that are light in weight.

There are two groups of feldspars:

(i) Potassium feldspars (KAlSi₃O₈) include orthoclase, microcline, adularia and sanidine. Orthoclase and microcline are more common in the plutonic and metamorphic rocks. The potassium feldspars occur commonly in the silts and sands of soils and also abundant in clay-size fractions of soil.

(ii) Plagioclage feldspars—a series consisting of a solid solution of albite (NaAlSi₃O₈) high in sodium and anorthite (CaAl₂Si₂O₈) high in calcium. Plagioclase weathers more rapidly than orthoclase.

2. Pyroxenes and Amphiboles:

The pyroxenes and amphiboles are two groups of ferromagnesian minerals (heavy group) the structure of which consists of long chains of linked silica tetrahedra. The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of a double chain (alternately 2 and 3 oxygen atoms shared successive tetrahedra).

These chain silicates are sometimes referred to inosilicates. The pyroxene group of minerals comprised of different minerals namely enstatite, hypersthene, diopside and augite, of which augite is the most important minerals in soils and it is found in basic rocks.

The amphibole groups of minerals are common in acidic rocks and it can be represented by the isomorphous series between tremoliteactinolite olivine and hornblende.

Hornblende weathers fairly rapidly. Olivine (olive-green) minerals from an isomorphous series between forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). Pyroxenes are more basic in character and therefore it weathers more rapidly than amphiboles.

3. Micas:

Micas occur extensively in soils. They are primarily originated from the parent rock from which the soil is derived. Generally soils are inherited from well-ordered and imperfectly ordered micas. Well-ordered micas are derived from igneous and metamorphic rocks whereas imperfectly ordered micas are derived from sedimentary rocks.

The most common well-ordered micas are muscovite, paragonite, biotite and phlogopite (trioctahedral). The imperfectly ordered micas contain less potassium and more water as compared to well- ordered micas and this type of micas are most abundant in the clay fraction of soils. Among the ordered micas, biotite weathers more rapidly than muscovite.

In imperfectly ordered micas, many of the illite-type specimens as well as the disordered micas of soils exhibits some mixed-layering with phases of vermiculite, smectitic group of minerals, chlorite and intergrades of several of these species.

4. Quartz:

It is very densely packed and occurs in a high degree of purity. It is strongly resistant to weathering as the structure is densely packed, electrically neutral and free from any substitution. It is the most abundant mineral next to feldspars. Serpentine, a hydrous magnesium silicate occurs more commonly as a secondary product. Garnets are characteristic of metamorphic rocks and are very hard and most resistant to weathering.

5. Clay Minerals:

Of the naturally occurring inorganic crystalline minerals found in the clay fraction (< 2µ) of soils, the most common are layer silicates (illite, montmorillonite, chlorite, vermiculite, kaolinite). Most of the physico-chemical and morphological properties of soil are influenced by these clay minerals.

They are variable in colour like white, grey, light yellow depending upon their chemical composition. In soils, the clays and oxy-hydrates of iron form coatings on mineral grains and impart shades of yellow, brown or red colour to soils.

Clay minerals are colloidal in nature and carry a negative electrical charge on their surfaces. Due to presence of negative charges, the cations are attracted to its surfaces. These cations do not get bounded permanently and take part in exchange reactions and ultimately clay minerals act as a source of cation adsorption and cation release which are very important in soil fertility.

The kind and proportion of clay minerals found in a soil depend on the kind of parent material and the intensity of weathering from which the soil has been derived.