ROCK-FORMING MINERALS

Minerals that are the main constituents of rocks are called rock-forming minerals.

Despite the fact that there are a huge number of minerals known occurring on earth the

majority of rocks on earth are composed of 5 or 6 minerals.

The variation in the proportion amongst these rock-forming minerals formed the various

rocks.

SILICATE MINERALS

The vast majority of the common igneous rock-forming minerals belong to the silicate

group, which means that they are based on silicon and oxygen for their basic

elemental components. The silicates can be further divided into other elements which

occur along with the ever-present SiO2.

1. Olivine

2. Pyroxene

3. Amphibole

4. Feldspar

5. Mica, And

6. Quartz

FELDSPARS

The mineral name feldspar is comprised of two German words: feld (German for field)

and spar (German for mineral), thus the word feldspar refers to minerals that occurred in

the field.

Feldspars are the most abundant silicate mineral in the earth’s crust, making up about

58% of the total bulk of the crust.

Feldspars are formed from aluminium silicate in combination with elements of K, Na and

Ca.

Orthoclase Feldspar Albite Feldspar

Anorthite Feldspar

In crystal structures they belong in the monoclinic and triclinic crystalline systems.

Feldspars possess two sets of perfect cleavage oriented normal to each other.

Feldspar occurs most abundantly in igneous rocks, followed by metamorphic rocks and

not quite so common in certain sandstones.

MICAS

Minerals in the mica group are all foliated in shape; all of the very thin flakes are joined

through very weak bonds, and are prong to cleave apart. And hence perfect basal

cleavage is well developed.

There are two major kinds of micas: Muscovite is colourless mica constructed of silica

tetrahedrons linked by Al.

Muscovite was used as glass in the ancient days, and is excellent insulation material in

modern time.

Biotite is the dark-coloured variety of the mica group minerals. The main cations are Fe

and Mg, with a basic crystalline structure similar to that of muscovite.

QUARTZ

Quartz is a rock-forming mineral of very common occurrence, and is present in all three

major types of rocks.

Quartz is the last mineral to crystallize in the magma, and consequently occurs as filling

available interstices among minerals that crystallized earlier.

The chemical composition is the simplest SiO2 it shows vitreous luster, and does not

develop any cleavage.

Pure quartz is colourless, but may display vivid colour due to inclusion of transition

elements. Quartz is very stable, and is not readily weathered or transform to other

mineral.

Quartz with a microcrystalline structure is called chalcedony, agate, or jasper.

PYROXENES

The pyroxenes are a group of important rock-forming silicate minerals found in many

igneous and metamorphic rocks.

The pyroxene group of minerals include quite a number of minerals, all formed of silica

tetrahedrons linked together by Fe, Mg, Ca, Na and Al in single chains.

Pyroxenes have the general formula XY(Si,Al)2O6 (where X represents calcium, sodium,

iron+2 and magnesium and more rarely zinc, manganese and lithium and Y represents

ions of smaller size, such as chromium, aluminium, iron+3, magnesium, manganese,

scandium, titanium, vanadium and even iron+2).

Augite is the most common pyroxene, and jadeit is the most appreciated pyroxene.

Most of the minerals are dark green to black in colour with colourless streak.

Pyroxenes have two sets of fair cleavage intersecting at close to right angle.

Pyroxenes are high-temperature minerals, mostly occurred in igneous and metamorphic

rocks.

Augite

Jadeite, a pyroxene mineral with composition Na Al Si2O6

AMPHIBOLES

Amphibole is the name of an important group of generally dark-colored rock-

forming silicate minerals.

Rock-forming minerals of the amphibole group resemble the pyroxenes in chemical

composition and in the general properties.

However, amphibole minerals contain hydroxyls (OH)- and thus become the silicate

mineral with the most complicated chemical composition.

There are many mineral species in the amphibole group; most are long prismatic in

crystal habits. Most are black in colour.

The more common amphiboles are hornblende, actinolite and tremolite.

actinolite tremolite

OLIVINE

The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is a

common mineral in the Earth's subsurface but weathers quickly on the surface.

The element bond is very strong and so olivine is a very hard mineral with no cleavage.

Olivine is mainly green in colour with vitreous luster, and often occurs as fine crystals in

rocks.

Olivine is a high-temperature mineral, mainly forms basic igneous rocks, and is the

major constituent of the oceanic crust.

CLAY MINERALS

Clay minerals are found almost anywhere on the ground surface. They are hydrous

minerals formed when silicate minerals react with water and air during weathering

process.

Clay minerals are the main mineral component in soil.

Clay minerals occur as extremely tiny flakes; these flakes are so tiny that they are only

seen under SEM.

Clay minerals are differentiated into over ten different species basing on their crystal

structure and composition, among these kaolinite is the most common mineral.

Clay minerals include the following groups:

Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite.

Smectite group which includes dioctahedral smectites such as montmorillonite and

nontronite and trioctahedral smectites for example saponite.

Illite group which includes the clay-micas. Illite is the only common mineral.

Chlorite group includes a wide variety of similar minerals with considerable chemical

variation.

Kaolinite

Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical

composition Al2Si2O5(OH)4.

Rocks that are rich in kaolinite are known as kaolin or china clay.

The name is derived from Kao-ling a town near Jingdezhen, Jiangxi province, China.

The name entered English in 1727 from the French version of the word: "kaolin"

Occurrence

Kaolinite is one of the most common minerals; it is mined, as kaolin, in Brazil, Bulgaria,

France, United Kingdom, Iran, Germany, India, Australia, Korea, the People's

Republic of China, the Czech Republic and the United States.

Uses

1. Kaolin is used in ceramics, medicine, coated paper, as a food additive in toothpaste,

as a light diffusing material in white incandescent light bulbs, and in cosmetics. It is

generally the main component in porcelain.

2. Kaolinite can contain very small traces of uranium and thorium, and is therefore

useful in radiological dating.

Montmorillonite

Montmorillonite is a very soft minerals that typically form in microscopic crystals,

forming a clay.

It is named after Montmorillon in France.

Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2

tetrahedral sheets sandwiching a central octahedral sheet.

The particles are plate-shaped with an average diameter of approximately one

micrometre. Members of this group include saponite.

Montmorillonite is the main constituent of the volcanic ash weathering product,

bentonite.

The water content of montmorillonite is variable and it increases greatly in volume when

it absorbs water. It often occurs intermixed with chlorite, muscovite, illite, cookeite, and

kaolinite

Illite

1. Illite is similar in many respects to white mica, but has less potassium and more water in its composition.

2. Illite was first described for occurrences in

the Maquoketa shale in Calhoun County,

Illinois, USA, in 1937. The name was derived

from its type location in Illinois. Illite is also

called hydro mica or hydro muscovite.

3. Illite built up of units comprising two

tetrahedral layers separated by an

octahedral layer and forms minute flaky

crystals in a similar way to montmorillonite.

Illite

White Mica or Muscovite

Tetrahedral

Octahedron