mineral

 Lecture: 4 Minerals and their classification Minerals

Minerals are naturally occurring solids with a definite chemical composition and crystal structure. “Solid substances composed of atoms having an orderly and regular arrangement”. When molten magma solidifies, different elements present in them freely arrange in

accordance with the attractive forces and geometric form. Silica tetrahedron is the fundamental building blocks for the formation of different minerals. (SiO2). Different silicate minerals are ortho silicates, ino-silicates, phyllosilicates and tectosilicates. There are nonsilicate minerals also. These are different oxides, carbonates, sulphates, phosphates etc. Minerals that are original components of rocks are called primary minerals. (feldspar, mica, etc.).

Minerals that are formed from changes in primary minerals and rocks are called

secondary minerals (clay minerals). Those minerals that are chief constituents of rocks are called as essential minerals (Feldspars, pyroxenes micas etc) and those which are present in small quantities, whose presence or absence will not alter the properties of rocks are called accessory minerals (tourmaline, magnetite etc).

Silicate minerals occupy 90% of the mineral composition of rocks of the earth’s crust.

The silica tetrahedron is the fundamental building block of all the silicate minerals of the earth’s crust.

CLASSIFICATION OF MINERALS: 1. Based on mode of origin

 Primary Minerals  Secondary Minerals

3 Based on the Quantity

 Essential Minerals  Accessory Minerals

4 Based on Specific Gravity

 Light Minerals  Heavy Minerals

2. Based on Chemical composition * Native elements *Oxides and hydroxides * Sulphates

* Sulphides * Carbonates

* Halides * Silicates

a) Primary Minerals: The primary minerals are those which are formed owing to the crystallization of the molten magma. Depending up on the tetrahedral linkage, the silicate minerals are divided in to four groups.

1 Orthosilicates : Olivine 2 Inosilicates :

 Single chained : Pyroxenes  Double chained : Ampliboles

3 Phyllosilicates : Biotite, Muscovite 4 Tectosilicates : Quartz, Feldspars Ferro - Magnesian Minerals  Olivines, Pyroxenes, Amphiboles, Biotite

Non Ferro Magnesian Minerals  Muscovite, Orthoclase, Albite, Anorthite, Quartzb) Secondary minerals: Minerals formed due to weathering action of primary minerals. Ex.

Clay minerals

PRIMARY SILICATE MINERALS

Quartz: The frame works of quartz is very densely packed and occurs in a high degree of

purity. It is resistant to physical and chemical weathering as the structure is densely packed

electrically neutral and prevents any form of substitution. It is ubiquitous in soils but its

abundance is next only to feldspars.

Feldspars: Its frame work is less dense that quartz. There are most abundant among rock

forming minerals in the earth’s crust. These are non ferro - magenesian minerals and act as

store house of sodium, calcium, potassium minerals and many trace elements in soils.

Micas: Occur most extensively in soils. Muscovite (white mica) a non- ferromagnesian

mineral is resistant for weathering. Biotite (black mica) a ferro- magnesian is highly

susceptible for weathering. Muscovite is present only in acid igneous rocks.

Pyroxenes and Amphiboles: These two minerals are two groups of ferro- magenesian

minerals and their structure consists of long chains of linked silica tetrahedra (Inosilicates).

Due to variety of substitutions these minerals are excellent host minerals for trace cations in

soils and also for main constituent cations like Ca, Mg and Fe. Their weatherability is quite

favourable to provide sufficient amounts of these ions in available form for plant nutrition.

Pyroxenes - Single chain eg. Augite (Ca, Na)(Mg,Fe,Al)(Si,Al)2O6

Olivines: Olive green colored minerals. Olivines are relatively easily weathered. It is called

island silicate. Ex: Forsterite – Mg2SiO4 ; Fayalite – Fe2SiO4

c) Essential Minerals: The minerals which form the chief constituents of rock and which are

regarded as the characteristic components of that rock are known as “Essential Minerals” eg.

Quartz, Feldspars and Micas

d) Accessory Minerals : These minerals occur only in small quantities and whose presence

or absence is of no consequence as far as the character of the rock is concerned, are called as

accessory minerals eg. Tourmaline, Pyrite, Magnetite.

e) Light Minerals: Are the minerals which have spe cific gravity below 2.85 eg. Quartz

(2.60), Feldspar (2.65), Muscovite (2.50-2.75)f) Heavy Minerals: Having specific gravity above 2.85 g/cc e.g. Haematite (5.30), Pyrite

(5.0), Limonite (3.8), Augite (pyroxene) (3.1 – 3.6), Hornblende (amphiboles) (2.9 – 3.8),

Olivine (3.5)

SECONDARY MINERALS: The secondary minerals are formed at the earth’s surface by

the weathering of the pre-existing primary minerals under variable conditions of temperature

and pressure. Due to the action of weathering processes primary minerals are altered or

decomposed. They are two types.

a. Silicate minerals b. Non-silicate minerals

Silicate minerals: These are the minerals that contain silica along with other elements.

Basic structural units of Secondary minerals:

Silica Tetrahedron: Geometrically it is possible to arrange only four oxygen atoms (with

radius of 1.32 Ao) around a central silicon cation (with radius of 0.42Ao), so that all are

touching each other. To attain neutrality silicon would have to combine with two oxygen

atoms only. But to attain geometrically stable structure silicon ion combines with four oxygen

ions and the resultant silica tetrahedron (SiO4) 4- carries a net negative charge of four. This is

called a silica Tetrahedron.

In nature, the geometry and valence constraints are reconciled; first by linking

together tetrahedra so that oxygen ions are shared between neighbouring silicon thus reducing

the negative charge deficit; second by making use of the positive charges of other metal

cations, to balance the negative charge. Both these occur to produce a neutral mineral. Like

wise, the basic tetrahedra arrange themselves in an orderly manner, according to a fixed plan.

IMPORTANT SECONDARY MINERALS

Silicates

-Clay minerals: Illite, Montmorillonite, Kaolinite etc

Non Silicates

- Oxides, Hydroxides of Al and Fe: Hematite, Goethite, Gibbsite

- Carbonates: Calcite, Dolomite

- Sulphates: Gypsum- Phosphates: Apatite

Formulae of Different Rock Forming Minerals ; Phosphorus containing minerals Fluorapatite Ca10 (PO4) 6 F2 ; Apatite: Ca10 (PO4) 6 2+ Variscite AlPO4.2H 2O: Vivianite Fe 3(PO4)2 Strengite FePO4.2H2O

Boron containing minerals Tourmaline -Na (Mg,Fe)3 Al6 (BO3)3 Si6 O18 Iron containing minerals Limonite – Fe2O3. 3H2O Viviamite Fe3 (PO4) 2 Ilmenite - FeTiO3 ; Siderite - FeCO 3 Pyrite - FeS2 ; Hematite: Fe2O3 Chalcopyrite - CuFeS2 ; Magnetite - Fe3O4 Zinc containing minerals : Sphalerite - ZnS; Smithsonite - :ZnCO3 Mg containing minerals : Epsom - MgSO4.7H2O ; Mangnesite - MgCO3 Dolomite - CaCO3. Mg CO3; Epsomite - MgSO4.7H20 Cu containing minerals: Chalcorite - Cu2S Covellite - CuS Chalcopyrite - CuFeS2 Mn containing minerals: Pyrolusite - MnO2 ; Manganite - MnOOH Rhodocrosite - MnCO3 Mo containing minerals : Molybdenite : MoS2 Wulfenite : PbMoO4; Powellite CaMoO4