Mineral:

A mineral is a naturally occurring homogenous solid inorganically formed with a definite chemical composition and an orderly atomic arrangement. More than 90% of all of the minerals in the Earth’s Crust are made up of compounds containing Silicon and Oxygen, the two most abundant elements on Earth. There are over 2000 minerals on Earth, but only 100 are commonly found. 30 minerals make up the majority of the rocks on Earth.

A mineral, by definition, must satisfy five conditions:

• It must be naturally occurring.

• It must be inorganic.

• It must be a solid element or compound.

• It must have a definite composition.

• It must have a regular internal crystal structure

Formation of minerals

When the molten magma solidifies, the different elements present in it freely arrange themselves in accordance with attractive forces and geometric form.

The earth’s crust contains dominant amount of oxygen (46.60%) followed by silicon (27.72%).

In order to achieve neutrality between the negatively charged oxygen and the positively charged silicon, there would be a greater tendency for silicon and oxygen to combine to form the basic compound, called the silicon – oxygen tetrahedron (SiO4)4-.

This explains the dominance (>90%) of silicate minerals in the earth’s crust.

Geometrically, it is possible to arrange only 4 oxygen anions around a central silicon cation so that all are touching each other. This is the arrangement of a tetrahedron.

The amount of charge carried by silicon ion is 4+ and by oxygen is 2-. In order to attain neutrality, one silicon (4+) ion would combine with two oxygen ion (2 x 2-) to form SiO2 but geometrically stable structure is formed when 1 silicon combines with 4 oxygen ions to form tetrahedron (SiO4)4- which carries a net negative charge of 4-.

Classification of minerals  

i. Based on the mode of origin

Primary minerals:  The minerals formed cooling and solidification of original molten material is known as the primary minerals eg. Feldspar, Hornblende, Mica, Quartz etc;

1. Quartz: SiO2

• Most common soil forming mineral

• Make up 13% of earth's crust and from 30 to 40% of the average soil

• Commonly a translucent milky-white color

• Hard enough to scratch glass

• Resistant to weathering

• Present in granite; absent from basalt

• Present in almost all sandstone

• Does not contribute plant nutrients to the soil

2. Feldspar - alumino-silicates with bases of K, Na, and Ca

• Account for 60% of the earth's crust

A. Orthoclase Feldspar---KA1Si3O8

• Slightly harder than glass

• Commonly white, orange, or pink in color

• Fine wavy lines may occur within crystals

• Flat surfaces are common (intersecting at 88-90° angles)

• The most abundant mineral in granite

• Is an important source of potassium

B. Plagioclase feldspar--Na AlSi308↔Ca Al2Si2O8

• Slightly harder than glass

• Common gray color (from almost white to dark bluish gray)

• Commonly has striations (flat faces within crystals seen as straight lines

On surface)

• Flat surfaces are common (interesting at 87-89° angles)

• Weathers more readily than orthoclase

3. Horneblende --- NaCa2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2

• Slightly harder than glass

• Black, dark brown or dark green in color

• One of the dark-colored minerals in granite

• Weathers more rapidly than feldspar, but persist in soils as dark colored

gravel

4. Micas- alumino-silicates with K, Mg, and Fe basic components

• Easily spilt into thin flexible elastic plates

• Has shiny surface

• Present in granite, basalt, loess, and glacial till

• Muscovite (white Mica) KAl3Si3O10(OH)2 : is Al mica and is colorless

• Contains more Potassium than Biotite

• Biotite (black mica) KA1(MgFe)3Si3O10(OH)2 : is Mg, Fe mica and is black

 • Has more Iron and Magnesium

Secondary minerals:  Minerals that are formed, by the weathering of primary minerals are  known as secondary mineral. Eg: Limonite, Gibbsite etc., and clay minerals like Kaolinite, Montmorillonite etc.,

1. Gypsum - CaSO4 2H2O

• Forms from evaporating calcium sulfate-bearing waters

• Very soft and weathers fairly readily

• Accumulate in large quantities in semi-arid regions

• Can be both a Primary and Secondary mineral

2. Calcite - CaCO3(carbonates)

• Commonly found in limestone and Marble

• Much softer than glass; harder than fingernails

• White or colored by impurities

• Slightly soluble in water

• Effervesces in dilute HCI (release bubbles of CO2)

3. Dolomite - CaCO3 MgCO3

• Most common liming material in NC

• Similar to calcite

• Contains Mg

4. Iron oxides

• Formed through chemical weathering

• Geothite (FeOOH): gives yellow color in soils

• Hematite (Fe2O3): responsible for red coloration in soils

5. Clay Minerals (kaolinite)

• Highly colloidal

• Formed primarily from chemical weathering of primary minerals

• Ability to absorb or hold nutrient ions on their surfaces

ii. Based on its importance or amount

Essential mineral: Those minerals which form the chief constituents of a rock are known as essential minerals. They are present in large quantities varying from 95-98% eg. Calcite and Silicate minerals.

Accessory minerals:  Those minerals which occur only in small quantities and whose presence / absence is of no consequence as far as the character of the rock is concerned are called accessory minerals. Eg. Tourmaline, magnetite, pyrites etc

Silicate minerals

Ferro magnesian silicate minerals

Inosilicates (Pyroxenes and amphiboles)

The pyroxenes and amphiboles are two groups of ferromagnesian minerals. The structure 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 double chains (alternately 2 and 3 oxygen atoms share the successive tetrahedra). These chain silicates are sometimes referred to inosilicates

Phyllosilicates

The phyllosilicates are an important group of soil – forming minerals and are represented by micas (biotite, muscovite). They have sheet structure of tetrahedra where each silicon ion shares three oxygen ions with adjacent silicon ion to form a honey – comb like pattern. The fourth unshared oxygen ion of each tetrahedron stands above the plane of all others.  The basic structural unit of phyllosilicates is formed by the condensation of two sheets of silicon-tetrahedra with one sheet of aluminum or magnesium octahedron.

Non Ferro magnesian minerals

Tectosilicates: The most common minerals of this group are feldspars and quartz.

Feldspars:

• Feldspars are aluminosilicates of K, Na and Ca.  The feldspar structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedra.

• The tetrahedra contain mainly Si ions with sufficient Al substitution. It belongs to the group of minerals that are light in weight.

• There are two groups of feldspars: (i) Potassium feldspars (KAlSi3O8) include orthoclase and microcline. 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) plagioclase feldspars- a series consisting of solid solution of albite (NaAlSi3O8) high in sodium and anorthite (CaAl2Si3O8) high in calcium.

• Plagioclase weathers more rapidly than orthoclase.

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, and free from any substitution. It is the most abundant mineral next to feldspars.

Non-silicate minerals

Oxides:       Haematite (Fe2O3)

                     Limonite (Fe2O3, 3H2O)

                     Goethite (FeO (OH) H2O)

                     Gibbsite (Al2O3H2O)

The red, yellow or brown colours in soils are due to the presence of goethite and haematite , which occur as coatings on the surface of soil particles.

Carbonates :  Calcite (CaCO3)

                       Dolomite (CaMgCO3)

Sulphates    :  Gypsum (CaSO4.2H2O)

Phosphates :  Apatite (Rock phosphate Ca3 (PO4)2 - primary source of phosphorus

Formation of secondary mineral

         The secondary minerals are formed at the earth’s surface by weathering of the pre-existing primary minerals under variable conditions of temperature and pressure. These clay minerals are of size <0.002 mm and are considered to be the most reactive part of soil. Important soil properties like nutrient and water holding capacity are controlled by clay minerals. These minerals are layered silicates consisting of silica tetrahedron and aluminium octahedron.

1 silicon tetrahedron + 1 aluminium octahedron     1:1 clay mineral (Kaolinite)

2: 1 non-expanding clay mineral                           i. Black mica (Biotite)

                                                                                      ii.White mica (Muscovite)

                                                                                      iii.Weathered mica (illite)

2: 1 expanding clay mineral                        i.  Partially expanding (Vermiculite)

                                                                             ii. Fully expanding (Montmorillonite)

2: 2 clay mineral                                            i. Chlorite

1:1 type minerals. The most important mineral in this type, commonly found in soils, is kaolinite whose structure is shown.The chemical composition of kaolinite is Si4Al4O10(OH)8. The two sheets of each crystal unit of kaolinite are held together by oxygen atoms which are mutually shared by the silicon and aluminium atoms in their respective sheets. These units are held together very rigidly by hydrogen bonding (-H-) to the oxygen plane of the adjacent layer. So the lattice is fixed and kaolinite mineral does not allow water to penetrate between the layers and has almost no plasticity, cohesion, shrinkage and swelling properties. Besides kaolinite, there are other minerals in this type namely halloysite, anauxite and dickite.

2:1 type minerals (Expanding lattice type). There are some important minerals in this type which includes montmorillonite ( smectitic group), vermiculite and other smectitic group of minerals like beidellite, nontronite and saponite etc.Montmorillonite is best known example of this type of minerals and its structure is shown.

Montmorillonite : The flakelike crystals of this mineral are composed of 2:1 type crystal units. These crystal units are loosely held together by very weak oxygen to oxygen linkages. Water molecules as well as cations are attracted between crystal units, causing expansion of the crystal lattice. The spacing (C-Axis) of the layers ranges from 12 to 18 A (1.2-1.8 nanometers) and is variable with the exchangeable cation species and the degree of inter layer spacing. Montmorillonites are the swelling and sticky type clays. Internal surface, cohesion and plasticity of this mineral are also very high. The size of this mineral is very small (0.01-1.0µm or microns). On drying, it shows shrinkage and as a result wide cracks are formed in the soils dominated by this type of mineral. The dry aggregates or clods are very hard, making such soils difficult to till.

Vermiculite : Vermiculite clays are common in most of the soils. The layers are held more weakly together by hydrated magnesium (six water molecules in octahedral coordination with Mg2+) rather than tightly held by potassium ions (K+). Thus, Vermiculite has swelling but not as much as montmorillonite. It has a high cation exchange capacity.

2:1 non-expanding type minerals. In this group, hydrous mica or illite is the most important in soils. Like mont-morillonite, illite has a 2:1 type lattice. The structure of illite is presented diagrammatically. However, about 15 per cent of silicon in silica sheets is substituted by aluminium. The excess of negative charge is satisfied largely by potassium in the interlattice layers, thus making the lattice structure of the non-expanding type.  Hence illite is relatively non-expanding. Thus hydrous mica has slight to moderate swelling. The properties of water adsorption, cation exchange and other physical properties lie in between those of kaolinite and montmorillonite type of minerals.

2:2 type minerals: Chlorite (2:2 or 2:1:1 layer silicates) occurs extensively in soils. Chlorites are basically silicates of magnesium with some iron and aluminium present and it is composed of alternate talc (similar to a montmorillonite crystal unit) and brucite (Mg(OH)2) layers. Chlorite mineral is similar to the unit lattice of vermiculite, except the hydrated Mg in vermiculite is a firmly bonded magnesium hydroxide octahedral sheet. Thus, a layer of chlorite has 2 silica tetrahedral, an aluminium octahedral and a magnesium octahedral steet (2:2 or 2:1:1). Chlorite does not swell on wetting and has low cation exchange capacities. It is almost non-expanding type of mineral because of its very little water adsorption.

Amorphous minerals

A mineral is a solid material, made of one substance that occurs naturally on Earth. Most of the common minerals are made of crystals.

A Crystal is a solid formed by a repeating, three-dimensional pattern of atoms, ions, or molecules and having fixed distances between the different parts.

Minerals that do not grow in these regular or crystalline patterns are called Amorphous solids. Eg. Allophane.