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ROCK FORMING MINERALS

CHAPTER 2.2

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INTRODUCTION TO MINERALOGY

LEARNING OUT-

COMES (LO)

LO1 : The student should understand the physical properties of minerals

LO2 : The student should understand the rock forming mineral / silicate structure

Earth crust is basically made up from the composition of eight major element ;

Oxygen, 46.6% Silicon, 27.7% Aluminum, 8.1%

Iron, 5.0% Calcium, 3.6%

Sodium, 2.8% Potassium, 2.6% Magnesium, 2.1%

Others, 1.5% (Titanium, Hydrogen, etc)

Basically the rock forming minerals are divided into 3 major groups.

ACIDIC SILICATE BASE MINERALS

BASIC SILICATE BASE MINERALS

NON SILICATE MINERALS

2.2.1 SILICATE MINERALS

Almost silicate struc-tures are in fourfold

coordination with oxy-gen. This arrangement appears to be univer-

sal in these com-pounds, and the bonds

between silicon and oxygen are so strong.

Structure and Classification of the Silicates

Silicate classification is based on the following

types of linkages:

Single chains – e.g pyroxene

Double chains – e.g amphiboles

Two dimensional sheets – e.g micas, chlorites and clay minerals

Three dimensional frameworks – e.g feldspar and quartz

Structures of The Common Silicate

Ferromagnesian Silicates / Basic Silicates• Ferromagnesian silicates are silicates that

contain iron, magnesium, or both as in Olivine. Other minerals include the Pyrox-

ene and the Amphiboles

Non-Ferromagnesian Silicates / Acid Sil-icates

• Non-Ferromagnesian silicates are silicates that lack iron and magnesium, as in

Feldspar. Other minerals include the Quartz and the Muscovite

• Silicate minerals are divided into:i) Quartz group (non-ferromagnesian)ii) Feldspar group (non-

ferromagnesian)iii) Pyroxene group (ferromagnesian)iv) Amphibole group (ferromagnesian)v) Mica group (non-ferromagnesian)vi) Miscellaneous groups of silicate

such as olivine and clay minerals

Quartz

Albite from feldspar group

Augite from pyroxene

group

Hornblende from

amphibole group

Muscovite from mica

group

Quartz group• Quartz is the most common mineral on the face of the Earth and it is frequently the primary mineral, > 98 %. • It is found in nearly every geological environment and is at least a component of almost every rock type.• Colour is as variable as a spectrum, but clear quartz is by far the most common colour followed by white or cloudy (milky quartz), purple (Amethyst), pink (Rose Quartz), grey or brown to black (S-moky Quartz).• Lustre is glassy to vitreous as crystals, while cryptocrystalline form are usually waxy to dull but can be vitreous.• Crystals are transparent to translucent and cryptocrystalline form can be translucent or opaque.• Crystal System varies and most common is hexagonal with two rhombohedrons (six sided pyramid) and some are trigonal.• Cleavage is very weak in three directions (rhombohedral) and the fracture is conchoidal.

Feldspar Group• The feldspars are a group of minerals that have similar character-

istics due to a similar structure.

• All feldspars have low symmetry, being only monoclinic, to triclinic.

• Tend to twin easily and one crystal can even be multiply twinned on the same plane, producing parallel layers of twinned crystals.

• They are slightly hard at around 6, and have an average den-sity at 2.55 to 2.76.

• They have a rather dull to rarely vitreous luster.

• They have two directions of cleavage at nearly right angles.

• Feldspars also tend to crystallize in igneous environments, but are also present in many metamorphic rocks

Some of the more common feldspar minerals are:

a) The Plagioclase Feldspar- Albite, (Sodium aluminum silicate)- Oligoclase, (Sodium calcium aluminum silicate)- Andesine, (Sodium calcium aluminum silicate)- Labradorite, (Calcium sodium aluminum silicate)- Bytownite, (Calcium sodium aluminum silicate)- Anorthite, (Calcium aluminum silicate)

b) The K-feldspar or Alkali Feldspar- Microcline, (Potassium aluminum silicate)- Sanidine, (Potassium sodium aluminum silicate)- Orthoclase, (Potassium aluminum silicate)

Albite

Oligoclase

Andesine

Anorthite

Orthoclase

Microcline

Pyroxene group• Common rock forming minerals and are represented in

most igneous and metamorphic rocks.

• Usually dark coloured (dark green to black).

• Typical structure contains chains of SiO3 tetrahedrons which has one flat edge that lies on the ‘base’ of the structure as if the entire chain were a chain of connec-ted three sided pyramids.

• The slope of the tetrahedral pyramids helps to determ-ine the cleavage angle of the pyroxenes at nearly 90o.

• Members of the pyroxene group include:a) Monoclinic

- Augite (Sodium iron silicate)- Diopside (Calcium magnesium silicate)

b) Orthorhombic- Enstatite (Magnesium silicate)- Ferrosilite (Iron magnesium silicate)

Augite

Enstatite

Amphibole group

• Minerals are mainly monoclinic and the crystals are elongated in the c-direction and usually bounded by six vertical faces with the angle intersect of 124o.

• Hornblende is the most abund-ant amphibole as a common constituent in igneous and metamorphic rock.

• Colour ranges from green to brown.

Hornblende

Mica group• A group of monoclinic miner-

als whose property of split-ting into very thin flakes is characteristic and recognis-able.

• This is due to the perfect cleavage parallel to the basal plane in mica crystal which in turn results from the layered atomic structure.

• Abundant in granite, in many metamorphic rocks and sig-nificant component of many sandstones.

Muscovite

Olivine• Occurs chiefly in basic and ultrabasic rocks with

(MgFe)2 SiO4 present

• Crystallises at high temperature, over 1000 C, one of the first minerals to form from basic magmas, and common in basalt

• The only mineral clearly visible in the hand specimen

• Probably the major constituent of the material be-neath the earth’s crust

Calcite• Composed of calcium carbonate (CaCO3) and principal

mineral of limestone

• Can be precipitated directly from seawater and re-moved from it by organism to make shells

• Dissolved in groundwater and re-precipitated as new crystals in caves and fractures in rock

• Soft and easy to scratch, bubbles in dilute hydrochloric acid, perfect cleavage in three directions but not at right angle

• Major mineral in metamorphic rock (marble)

Halite and Gypsum• Two most common minerals form by the evaporation

of seawater or saline lake water

Halite (common salt, NaCl) easily identified by its taste, very soft and scratched easily with fingernail

Gypsum composed of calcium sulphate and water (CaSO42H2O).

Chlorite• A green flaky minerals formed by hydrous silicates of

magnesium and aluminum

• Found in Igneous rocks and in metamorphic rocks such as chlorite-schist and in some clays

Talc• Soft flaky mineral, white or greenish white, easily scratched

by finger nails

• Occurs as a secondary product in basic and ultrabasic rocks and in talc-schist

Kaoline• Derived from breakdown of feldspar by action of water

and carbon dioxide

• White or grey, soft with texture of flour and clayey smell when damp

Serpentine• An alteration of olivine, pyroxene or hornblende

• Change from olivine to serpentine may be brought about by action of water and silica

• Found in basic and ultrabasic rocks

2.2.2 Non Silicate Minerals

1)Oxides and Hydroxides2)Halides

3)Carbonates and Sulphates

Oxides and Hydroxides

• Form by combination of various cations with

oxygen. Some examples are haematite, il-menite, magnetite, bauxite, limonite and

cassiterite

Halides

• Often occurring as chemical deposited sed-iments formed by evaporation and as vein

minerals in igneous rocks

Carbonates and Sulphates

• Consist of framework similar to the silica tetrahedra. An important mineral in this

group is gypsum – almost used in building materials.

• The most important carbonate minerals are calcite (calcium + carbonate ion) and

dolomite (calcium + magnesium) in its structure