Agriculture for Engineers 4 (3+1)

Lesson 2. Rocks and Minerals

2.2 CLASSIFICATION

Rocks are divided into three natural groups based on mode of origin or formation. These are:

1. Igneous rocks: formed from molten material on cooling

2. Sedimentary rocks: formed from sediments under pressure

3. Metamorphic rocks: formed from pre-existing rocks through action of heat and pressure

2.2.1 Igneous rocks (Latin ignis, means fire)

 The igneous rocks are formed by solidification of molten material (magma). All rocks formed directly from molten material are called igneous. These rocks are first to form and hence they are called as oldest rocks. The igneous rock make up about 95 % of the earth’s crust and about 88% of the mass of these rocks consist of feldspars, quartz and amphiboles.

Igneous rocks, on the basis of the depth of formation, are classified into:

(1) Plutonic - When the magma solidifies at great depth, about 2 to 3 miles deep under the surface of the earth, the igneous rock formed is called plutonic e.g. granite, synite, gabbro, norite etc. These are crystalline rocks as the sizes of crystals are big.

(2) Intrusive or dyke - When the magma solidifies at moderate depth is called Intrusive or dyke e.g. pegmatite, dolerite. The crystals are of smaller size in the rock. The rocks consolidated in vertical cracks and formed wall like masses are known dykes, whereas those consolidated in horizontal cracks are known as sills. In some cases the molten material is consolidated in irregular and narrow cracks is called a vein (Fig. 2.1)

Fig.2.1: A schematic diagram of volcanic eruption showing the occurrence of plutonic (intrusive) and volcanic (extrusive) igneous rocks.

 (3) Extrusive or effusive - When the solidification takes place on the surface of the earth as a result of volcanic activities, the igneous rock formed is called Extrusive or effusive e.g. rhyolite, pumis, chalcidian, basalt, trap. On the basis of percentage silica content, igneous rocks are divided into:

(a) Acidic: SiO₂  content is more than 65% e.g. granite, pegmatite, rhyolite

(b) Intermediate: SiO₂ content is between 55 and 65% (sub-acidic SiO₂ 60% to 65%, e.g. syenite; sub-basic, SiO₂  55 to 60 % e.g., diorite)

(c) Basic: SiO₂  content is between 44 and 55 % e.g. basalt and

(d) Ultra basic: SiO₂ content is less than 44% e.g. picrite.

 2.2.2 Sedimentary rocks (Latin sedimentas, means settling)

As soon as igneous rocks are exposed to weathering processes, they start breaking down physically and chemically into soluble and insoluble products when transported by water or glaciers form new deposits which in time become cemented and solidified into new forms of rocks called the sedimentary rocks. Stratification is the most common feature of these rocks, so they are also termed as stratified rocks. e.g. lime stone, sand stone, silt stone, shale and conglomerate.

Water plays important role information of these types of rocks so they are also known as aqueous rocks. The different layers are formed by sediment deposition, which are cemented together with silica, lime, iron oxide etc., although the sedimentary rocks form only 5% of the earth crust. They are of great importance because they occur extensively at or near the surface and are source of important product used in agriculture or industry.

Depending upon grain size sedimentary rocks are grouped as:

(1) Rudaceous: If the individual grains forming rocks are of the size of boulders and pebbles, called rudaceous. Grit - is composed of sharp and angular pebbles, Shingle- is composed of large rounded pebbles, Conglqmemte- consist of rounded or sub angular pebbles, and  Breccja – angular fragments.

(2) Arenaceous: These rocks have individual grains of sand size e.g. sand stone.

(3) Argillaceous: These rocks have individual grain size of clay. In their formation, these may either have loose or consolidated rocks with various clays or loose sediments. China clay (Kaoline) - formed , from decomposition of feldspar, Pipe clay - Iron free clay, Fire clay - Free from lime and alkalies and Laterite - Reddish clay formed by decomposition of basalt and granite.

 2.2.3. Metamorphic rocks

Metamorphic rocks are formed from igneous and sedimentary rocks through action of heat, pressure and chemically, active liquids and gases. Metamorphism may result in changes mainly physical, chemical or both. Heat, pressure and water are called agents of metamorphism. Thus, when igneous and sedimentary rocks are subjected to tremendous pressure and high temperature, metamorphism takes place and metamorphic rocks are formed. e.g.

2.3 Occurrence of soil forming rocks

The composition of the upper 5 km of the Earth crust is as follows:

Sedimentary rocks       -  Shales 52 %

                                        - Sandstones 15 %

                                        - Limestones and dolomite 7 %

Igneous rocks              -  Granite 15 %

                                         -  Basalt  3%

Other rocks                  -   8%

So it is found that five kinds of rocks mentioned above occupy more than 9O% of the total continental area.

2.4 MINERALS

Minerals are naturally occurring Solid homogeneous substances composed of atoms having an orderly and regular arrangement with definite chemical composition and a characteristic geometric form as Quartz (SiO₂), orthoclase (KAlSi₃O₈), calcite (CaCO₃) , olivine [(Mg, Fe)₂ SiO₄] and gypsum (CaSO₄ .2H₂O)

2.4.1 Classification

Minerals can be classified on the basis of their amounts, mode of origin, composition and specific gravity are given below:

(A) On the basis of origin and mode of formation

       1. Primary minerals

When a mineral arises from the cooling and solidification of a molten mass is called primary minerals. e.g. (i) orthoclase feldspar –KAlSi₃O₈ (ii) plagioclase feldspar- NaAlSi₃O ₈, (iii) anorthite feldspar - Ca(Al₂Si₂O₈), (iv) quartz - SiO₂ , (v) hornblende - Ca₂Al₂Mg₂Fe₃ Si₆O₂₂(OH)₂ (vi) muscovite – KAl₃Si₃O_1O(OH)₂, (vii) biotite - KAl(Mg.Fe)₃ Si₃O_1O(OH)₂ and (viii) augite - Ca₂(Al₂.Fe₄(Mg.Fe)₄ Si₆ O₂₄

       2. Secondary minerals

When it arises through the metamorphism or weathering of Primary or other pre-existing minerals, it is called secondary minerals e.g. (i) calcite - CaCO_3, (ii) magnesite - MgCO₃, (iii) dolomite - CaMg(CO₃)₂, (iv) silirite - FeCO₃, (v) gypsum - CaSO₄.2H₂O, (vi) apatite – Ca₅(F,Cl)(PO₄)₃, (vii) limonite - Fe₂O3.3H₂O, (viii) hematite- Fe₂O₃ and (ix) gibbsite – Al₂O₃. 3H₂O

       3. Silicate clay minerals: kaolinite - Al₄ [Si₄O₁₀] (OH)₈, montmorillonite – Al₂ [Si₄O_1O](OH)₂.4H₂O and hydrous mica - K₂Al₄ (AlSi₇)O_2O (OH)₄

 (B) On the basis of chemical composition

According to their chemical composition, minerals are divided into eight groups.

      1. Silicate: These are the salts of silicic acid _(H4SiO4). e.g. (i) muscovite – KAl₃Si₃O₁₀(OH)₂, (ii) biotite - KAl(Mg.Fe)₃ Si₃O_1O(OH)₂, (iii) epidote - Ca₂(Al,Fe)Al₂O(OH)(SiO₄)(Si₂O₇), (iv) orthoclase feldspar – KalSi₃O₈, (v) zeolite – CaAlSi₃O_1O.7H₂O, (vi) olivine - (Mg,Fe)₂SiO₄ and (vi) amphibole - (Na,Ca)₂.(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂

      2. Native elements: The minerals of this group are made up of only one element. (i) graphite - C, (ii) diamond - C and (iii) sulphur - S.

      3. Oxides: e.g. (i) quartz - SiO₂, (ii) hematite - Fe₂O3, (iii) geothite - Fe₂O3. H₂O, (iv) Limonite - Fe₂O3 3H₂O, (v) magnetite - Fe₃O₄, (vi) pyrolusite - MnO₂, (vii) rutile - TiO₂ and (viii)  gibbsite – Al₂O₃.3H₂O

     4. Sulphate: e.g. (i) gypsum - CaSO₄. 2H₂O, (ii) barite - BaSO₄ and (iii) gypsum anhydrite - CaSO₄,

     5. Carbonate:  e.g. (i) calcite - CaCO3. (ii) magnite - MgCO₃ and (iii) dolomite -CaMg(CO₃)_2.

     6. Sulphide: e.g. (i) copper pyrite - CuS₂ and (ii) iron pyrite - FeS₂

     7. Phosphate: e.g. (i) apatite – Ca₅ (F,Cl,OH,O)(PO₄)₃ Apatite is the source of P in soils.

     8. Halides: e.g. (i) flurorite (Flurospar) - CaF₂ , (ii) rock salt (Halite)- NaCl and KCl

(C) Based on specific gravity

Depending on chemical properties minerals have different specific gravity. Based on Specific gravity (g/cm³) mineral can be grouped into (i) Light minerals: The minerals having sp. gravity < 2.85 are called light minerals: e.g . quartz (2.6), feldspar(2.65), muscovite (2.5 to 2.75) and (ii) Heavy minerals: The minerals having sp. gravity> 2.85 are called heavy minerals e.g. hematite (5.3) pyrite (5.0), limonite (3.8), augite (2.9-3.8), olivine (3.5).

2.4.2 Clay minerals

Naturally occurring inorganic materials (usually crystalline) found in soils. There are mostly newly formed crystals reformed from the soluble 'products of the primary minerals and considered secondary minerals having a significant surface area.

The clay fraction of the soil particles has a diameter less than 0.002 mm. The mineral present in clay fraction of the soil is called clay minerals. Clay minerals use the most important secondary minerals. They are colloidal and crystalline in nature. They carry a negative electrical charge on their surface. Most of the physical, chemical and morphological properties of soils are influenced by these clay minerals.

Clay minerals are hydrous aluminium silicate, frequently with some replacement of aluminium by iron and magnesium. The three most important groups of silicate clay minerals are kaolinite (1:1 lattice non-expanding type), montmorillonite (2:1 lattice expanding type) and illite (2:1 lattice non-expanding type). The relative occurrences of minerals in soil are given in Table 2.1.

Table 2.1: Relative occurrences of minerals in soil

S. No.	Name of minerals	Distribution (%)
1	Feldspars	60
2	Quartz	12
3	Amphiboles and pyroxenes	17
4	Micas	4
5	Silicates	6
6	Rest of the minerals	1