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Only a few (not more than one hundred) minerals form the great bulk of the rocks of the crust of the Earth. These very common minerals have been grouped together as rock forming minerals. Even among these minerals, only about 25 or so make up almost 99.5 percent of the rocks one commonly comes across.
As such, practical identification of most common rocks of the Earth demands study of about 25 or so minerals in an absolutely thorough manner. This makes the job of a practising engineer quite simple. Exhaustive study of all the minerals occurring in nature falls in the domain of a specialist called mineralogist.
Among the rock forming minerals, we shall confine our study to the following three groups: Silicates, oxides and carbonates. This is because these three groups include most of the common rock forming minerals.
1. The Silicate Group:
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About eight percent of the crust of the earth is made up of silicates and the free silica. Among the silicate group, the total number of minerals known to occur in nature may easily approach to about one thousand species. A great majority of them are quite rare in occurrence. Since it is one of the biggest groups of minerals, a little knowledge about important aspects of this group will be quite useful. We shall discuss the important aspects of the group in the following order – Chemical composition, Atomic structure, classification and descriptive study of some important minerals.
Chemical Composition:
Most common silicate minerals are made up chiefly of a few of the following nine elements- Na, K, Al, Ca, Mg, Fe, Li, Si and O. Other elements are present only rarely and in traces.
Notwithstanding the fewer elements that go to make up the silicates, the variety and complexity of chemical composition of silicates still remains most challenging assignment for a chemical mineralogist. In some cases it may be impracticable to express the chemical composition of a silicate by a simple formula.
Atomic Structure:
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As a result of studies using latest techniques like X-rays, a vast amount of information has been collected about the general constitution of silicates.
Only, most important conclusions are mentioned below:
i. The Fundamental Unit:
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All silicates are simple or complex repetition of a fundamental Silicon-oxygen Tetrahedron, represented by the formula [SiO4]4-. In this tetrahedron, the very small Si ion is situated in the centre and is surrounded on the four sides by relatively big (five times in size) oxygen ions.
The dimensions of this unit cell of silicon-oxygen tetrahedron are constant. Further, the distance between the silicon ion at the centre and the oxygen atom at the corner is 106 Å (Fig. 11.1). This fundamental unit is repeated, linked and joined in different ways giving rise to different types of silicate structures.
(a) Independent Tetrahedra:
A unit SiO4 tetrahedron has four negative charges. Hence it has the capacity to exist as an isolated or independent tetrahedron provided these four negative charges are balanced by four positive ions of other metals. This actually happens in nature in orthosilicates.
Thus zircon and magnesium which have four and two valences respectively, easily combine with an independent SiO4– giving rise to a zirconium silicate ZrSiO4. Sometimes more than one (two, three or four) elements may combine with an independent tetrahedron to satisfy the four negative valences giving rise to different types of minerals.
(b) Doubly Linked Tetrahedra:
In some cases SiO4– may first get linked together in such a way that one oxygen atom is held common between the two cells. The net negative charge left in the two joint tetrahedral is 6(O-Si-O-Si-O) and the formula for such a coupled tetrahedral is (Si2O7)-6. In fact it is known as Si2O7 group. This double-tetrahedron is also capable of independent existence when its six negative charges are satisfied by equal number of positive metallic elements.
(c) Complex-Linked Tetrahedron:
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In some cases, three, four and six tetrahedral may be linked together in such a way that they form closed ring-type structures.
(d) Repetition in Space:
The single tetrahedron and the double-linked tetrahedron as described above may in themselves be repeated in space in a variety of ways giving rise to different structural forms in the silicate minerals. Among these structural forms, the following are of common occurrence.
The Chain Structure:
It results from single-dimension continuation in which each tetrahedron is linked to an adjacent tetrahedron (one on the right and one on the left) by sharing the two corners. In other words each silicon ion holds three oxygen ions in accordance with a general formula R [SiO3]. This is the characteristic structure of Pyroxene group of silicates and is commonly referred as single-chain structure.
A double-chain dimensional continuation is also possible according to formula [Si4O11]6-. The amphibole group of minerals has been found to exhibit this type of double-chain extension.
These will have six, eight and twelve free negative charges to be satisfied.
Their formulae are expressed respectively as:
[Si3O9]6-, [Si4O12]8- and, [Si6O18]12-
A large number of complex silicates have more than two fundamental tetrehedra linked together in the above manner.
The Sheet Structure:
A two-dimensional continuation of silicon tetrahedron commonly results in a layered or sheet structure. It is characterised by linking of the tetrahedrons in such a way that all the three apexes of one tetrahedron are linked with an adjoining tetrahedron resulting ultimately into a hexagonal pattern repeated lengthwise and breadthwise.
Such sheets may be linked with other identical sheets resting above or below through metallic ions resulting in a considerably weaker bond.
This is the most characteristic atomic structure (the sheet-structure) of flaky, platy and lamellar minerals like micas and chlorites.
The Network Structure:
In this type of structure, the silicon-oxygen tetrahedron are so arranged that they form a three dimensional network. In such a network, oxygen ions at all the four corners are shared, each oxygen ion being shared by two adjacent tetrahedra.
2. The Felspar Group:
The felspars (The feldspars in American terminology) are the most prominent group of minerals making more than fifty percent, by weight, crust of the Earth up to a depth of 30km. These occur chiefly in the Igneous Rocks (more than 60 percent) but also form a good proportion of their metamorphic derivatives. Felspars are also found in some sedimentary rocks like arkose and greywacks.
The group comprises about a dozen or so minerals of which 3-4 may be easily described as the most common minerals in rocks.
Chemical Composition:
In chemical constitution, felspars are chiefly aluminosilicates (also referred as alumosilicates) of Na, K and Ca with following general formula:
WZ4O8
in which W = Na, K, Ca and Ba and Z = Si and Al
The Si: Al shows a variation from 3:1 to 1:1.
Some examples of chemical composition of felspar minerals are:
(i) Na Al Si3 O8
(ii) K Al Si3 O8
(iii) Ca Al2 Si2 O8
Other metals which may be present in felspars in appreciable quantities or in traces are Barium, Lithium, Rubidium and Caesium. A very important character of chemical constitution of felspars is to occur in isomorphous series.
Atomic Structure:
At atomic level, the felspars show a continuous three-dimensional network type of structure in which the SiO4 tetrahedra are linked at all the corners, each oxygen ion being shared by two adjacent tetrahedra. The SiO4 tetrahedra are accompanied in this network by AlO4 tetrahedra so that felspars are complex three-dimensional framework of the above two types of tetrahedra. The resulting network is negatively charged and these negative charges are satisfied by the presence of positively charged K, Na, Ca and also Ba.
Crystallization:
The felspar group of minerals crystallise only in two crystallographic systems: Monoclinic and Triclinic. Infact, the plagioclase division of felspars crystallizes only in Triclinic System.
Classification:
Felspars are classified both on the basis of their chemical composition and also on their mode of crystallisation.
Chemically, felspars fall into two main groups – the potash felspars and the soda lime felspars.
Common members of the two groups are as follows:
a. Potash Felspars:
Orthoclase (K AL Si3 O8), Sanidine (K Al Si3 O8) and Microline (K Al Si3 O8).
b. Soda-Lime Felspars:
These are also called the plagioclase felspars and consist of an isomorphous series of six felspars with two components- Na Al Si3 O8 and Ca Al2 Si2 O8 as the end members.
(i) Albite
(ii) Labradorite
(iii) Oligoclase
(iv) Bytwonite
(v) Andesine
(vi) Anorthite.
The above series is also known as Albite-Anorthite series.
Crystallographically, Felspars fall into two crystal systems.
Physical Properties:
In addition to their close relationship in chemical composition, crystallography and atomic constitution, felspar group of minerals exhibit a broad similarity and closeness in their physical characters as well so that differentiation of one variety from the other requires very thorough, sometimes microscopic examination. They are generally light in colour, (because of absence of Fe and Mg), have lower specific gravity (generally around 2.6), have a double cleavage and a hardness varying between 6 – 6.5.
Description:
Among the Felspar Group, the following mineral species are quite common as rock forming minerals and hence are described in some detail:
a. Orthoclase:
Crystal System:
Monoclinic; β = 63°.57′. Crystals commonly occur in prismatic shapes.
Cleavage:
Shows cleavage in two directions. The one parallel to basal pinacoid (001) is perfect. The cleavage angle is 90°.
Colour:
Various shades of pink and red, such as flesh red, reddish white, light pink. The transparent variety is called Adularia.
Lustre:
Vitreous to semivitreous.
Hardness:
6 – 6.5
Sp. Gravity:
2.56 to 2.58
Composition:
K Al Si3 O8
Optical:
Optically negative (-)
Occurrence:
A most common and essential constituent of many igneous rocks, especially granites.
Economic Use:
As a ceramic material.
Varieties:
(i) Adularia- a transparent orthoclase.
(ii) Sanidine- a high temperature variety stable above 900°C.
b. Microcline:
Crystal System:
Triclinic; resembles closely with orthoclase in crystal habits.
Cleavage:
In two directions; the one parallel to basal pinacoid (001) is perfect.
Colour:
Similar to orthoclase. In addition, may occur as a greenish felspar, when it is called amazonite.
Crystal System:
Colourless
Cleavage:
6 – 6.5
Colour:
2.54 to 2.57
Composition:
K Al Si3 O8
The mineral is not easily distinguished in hand specimens from orthoclase except when perfectly crystallized.
Optical:
Optically negative (-)
Occurrence:
It occurs alongwith orthoclase in granites and other igneous rocks. In coarse-grained igneous rock called pegmatites, microcline is the prominent variety of felspars. Also occurs as an intergrowth with albite.
Economic Use:
(i) As a ceramic material
(ii) As a semi-precious stone (amazonite).
Varieties:
Anorthoclase – (meaning – not orthoclase). It is a triclinic felspar containing also sodium aluminium silicate.
c. Albite:
Crystal System:
Triclinic. It is the first member of the isomorphous plagioclase series of felspars the Albite- Anorthite series.
Cleavage:
Present in two directions; the one parallel to basal pinacoid (001) is perfect.
Colour:
Commonly whitish or pinkish white but shows shades of grey, green and blue.
Streak:
Colourless.
Lustre:
Vitreous to pearly. Some varieties show play of colours on the cleavage surface.
Hardness:
6 – 6.5
Sp. Gravity:
2.60 – 2.62
Composition:
Sodium aluminium silicate with NaAlSi3O8 – 100 – 90 percent and CaAl2Si2O8, 0 – 10 percent.
Optical:
Optically (+)
Occurrence:
It is an essential constituent of many igneous rocks, such as granites, syenites, rhyolites and dacites
Economic Use:
(i) As a ceramic material
(ii) As an ornamental stone in polished form.
d. Anorthite:
Crystal System:
Triclinic. It is the last member of the isomorphous plagioclase series of felspars. Crystals are commonly prismatic.
Cleavage:
Present in two directions; the one parallel to basal pinacoid (001) is perfect.
Colour:
Generally white; may also occur in reddish and light grey shades.
Streak:
Colourless.
Lustre:
Semi-vitreous.
Composition:
CaAl2Si2O8 – 100 to 90%.
Optical:
Optically (-)
Occurrence:
An important constituent of many basic types of igneous rocks.
Varieties:
Composition of other members of plagioclase felspars may be broadly considered the varieties of plagioclase felspars.
3. Pyroxene Group:
The pyroxene group of minerals forms another set of important rock-forming minerals. They occur in good abundance in the dark coloured igneous and metamorphic rocks. In fact among the ferro-magnesion minerals, pyroxenes occupy first place as rock forming group. All of them are closely related in their atomic constitution, crystallisation and general physical properties.
Chemical Composition:
In chemical composition, Pyroxenes are essentially ferro-magnesion silicates, with other bases as calcium, sodium, aluminium and lithium being also present in varying amounts in different varieties.
In its simplest form, the chemical composition of pyroxenes may be represented by the formula:
RSiO3 with R representing Ca, Na, Al and Li etc.
The most important chemical character of the pyroxenes is the Si: O ratio which is 1:3 and is explained by their atomic constitution.
Atomic Structure:
The pyroxenes show the single-chain structure of silicates. In this type of constitution, the fundamental silicon-oxygen tetrahedrons are linked together at one of the oxygen atoms. In other words, one oxygen atom is shared between two adjacent SiO4 giving rise to the typical prismatic cleavage of the group. The lateral bonding of the tetrahedra so disposed is achieved by Ca, Mg and other ions.
Crystallization:
Pyroxenes crystallize in two systems- Orthorhormbic and Monoclinic. The prism angles in pyroxenes are 87° and 93° and form a distinct feature of pyroxenes.
Classification:
Pyroxenes are commonly classified on the basis of their crystallisation in two groups:
(a) Orthorhombic Pyroxenes:
i. Enstatite – MgSiO3
ii. Hypersthene – (Fe, Mg) SiO3
(b) Monoclinic Pyroxenes:
i. Clinoenstatite – MgSiO3
ii. Clinohypersthene – (Fe, Mg) SiO3
iii. Diopside – CaMgSi2O6 or CaMg (SiO3)2
iv. Hedenberguite – CaFeSi2O6 or Ca (SiO3)2
v. Augite – Complex silicate of Ca, Mg, Fe and Al.
vi. Acmite (Aegirine) – NaFe (SiO3)2
vii. Jaedite NaAl – (SiO3)2
viii. Spodumene – LiAl (SiO3)2
Physical Properties:
The Pyroxene minerals exhibit similar physical properties of their identical atomic constitution. They are generally dark in colour, their hardness varies between 5 and 6 and sp. gravity from 3 – 3.3. Pyroxene crystals are generally short and stout. Prismatic cleavage is prominent in most cases.
Descriptive:
Following members of pyroxene group are of very common occurrence in the rocks and hence deserve individual description:
a. Enstatite (Mg SiO3):
Crystal System:
Orthorhombic. But, the mineral mostly occurs in massive and sometimes fibrous form.
Cleavage:
Prismatic (110)
Colour:
Variable between grayish white to greenish white.
Hardness:
5.5
Sp. Gravity:
3.1 – 3.3
Lustre:
Vitreous to pearly. Some varieties are translucent.
Composition:
MgSiO3
Optical:
Optically (+)
Occurrence:
A common constituent of many igneous rocks and some metamorphic rocks.
b. Hypersthene (Fe, Mg) SiO3:
Crystal System:
Orthorhombic. It is an isomorphic variety of Enstatite. Occurs commonly in massive form.
Cleavage:
Prismatic
Colour:
Commonly green, olive green to greenish black.
Hardness:
5-6
Sp. Gravity:
3.4 – 3.5
Lustre:
Pearly to Vitreous. Streak – grey.
Composition:
As given above. It is primarily a silicate of magnesium with more than 14% of FeO. Alumina is also present in some varieties.
Optical:
Optically (-)
Occurrence:
It is a more common constituent of volcanic igneous rocks like andesites and trachytes. Also found in plutonic rocks like gabbros and norites.
c. Diospide Ca Mg (Si2O6):
Crystal System:
Monoclinic; occurs in short columnar crystals.
Cleavage:
Prismatic and distinct.
Colour:
Light green, grey, colourless
Hardness:
5-6
Sp. Gravity:
3.27 – 3.38
Composition:
Calcium magnesium silicate with some Fe and Mg
Occurrence:
It is a common constituent of basic and ultra-basic igneous rocks.
d. Augite Ca (Mg, Fe, Al) (Al, Si)2O6:
Crystal System:
Monoclinic; occurs usually in short prismatic crystals and as a granular mass.
Cleavage:
Prismatic [110] and good. Commonly shows parting parallel to base (001).
Colour:
Variable, depending on chemical composition; occurs in shades of grayish, green and black.
Hardness:
5-6
Sp. Gravity:
3.25 – 3.55 (Depending primarily on iron content).
Lustre:
Commonly vitreous.
Composition:
A complex Fe-Mg silicate.
Optical:
Optically (+); Strongly pleochroic when rich in iron and titanium.
Occurrence:
A very common ferro-magnesian mineral of igneous rocks. The basic and ultra-basic rocks are especially rich in augite.
e. Aegirine – Na Fe Si2 O6 (also called Acmite):
It is also a monoclinic pyroxene with good prismatic cleavage, green to black in colour; H = 6, Sp. Gr. = 3.5 to 3.6. Crystals long and slender; optically (-). It is especially common in Nepheline syenites.
f. Jaedite (NaAlSi2O6):
It is rather a rare variety of pyroxene. It crystallizes in monoclinic system and has a good prismatic cleavage.
Colour- green; Streak- Colourless; H = 6 – 7; Sp. Gr. = 3.25 – 3.35.
Occurs as boulders and in metamorphic rocks. It is used as an ornamental stone after polishing.
g. Spodumene (LiAlSi2O6):
A rare type of pyroxene important for its lithium content.
The mineral crystallizes in long prismatic crystals of monoclinic system. Crystals as long 10- 12 m and a meter in width have been found.
Colour- White, violet, greenish; H = 6.5; Sp. Gr. = 3.1 – 3.2; Optically (+). The mineral occurs in pegmatites and is used as a semi-precious stone.
4. Amphibole Group:
This group of minerals is regarded as a parallel to the pyroxene group because most minerals of this group show a striking resemblance to the pyroxene minerals in many of their properties. They are also characterized with a double cleavage, a hardness between 5-6 and specific gravity from 3 to 3.5. Like pyroxene, they are generally dark in colour.
Chemical Composition:
Amphibole minerals are also meta-silicates with a Si: O ratio of 4: 11. The metallic ions present in amphiboles are- Ca, Mg, Fe and sometimes Mn, Na, K and H. Presence of (OH) ion, which may be replaced by F and CI, is another peculiarity of chemical composition.
The general chemical formula: [Si4O11]2 [OH]2 forms the basis for combination with the metallic ions. There is possibility of a good degree of substitution between various ions such as Al, Mg, Fe, Ca, Na and K, H and F and so on, giving rise to a variety of amphibole minerals.
Atomic Structure:
There is a basic difference in the atomic constitution of pyroxenes and amphiboles. In amphiboles, the SiO4 tetrahedra are linked in double chain; it is for this reason that the amphiboles are more complex in their chemical constitution.
Crystallization:
Most important members of amphibole group crystallize in two crystal systems.
Orthorhombic and Monoclinic – The amphibole crystals are generally long, slender and prismatic; these are sometimes fibrous in habit. The prism angle in amphiboles is 124°.
Classification:
Amphiboles are commonly divided in two groups on the basis of their crystallisation:
1. Orthorhombic amphiboles and
2. Monoclinic amphiboles.
1. Orthorhombic Amphiboles:
Anthophyllite – (Mg, Fe)7 (Si4O11) (OH)2
2. Monoclinic Amphiboles:
Tremolite – Ca2Mg5[Si4O11]2 [OH]2
Actinolite – Ca2(Mg, Fe)5 [Si4O11]2 [OH]2
Hornblende – Ca2Na(Mg, Fe)4 (AlFe) [(Si, Al)4O11]2 [OH]2
Glaucophane – Na2(Mg, Fe)3, Al2[(SiAl)4O11]2 [OH,F]2
Arfvedsonite – Na3(Fe, Mg)4, (Fe, Al) [(Si4O11]2 [OH]2
Physical Properties:
Despite wide variation in their chemical composition, amphiboles show quite a few common physical characters due to their atomic structure. Thus, all of them crystallize in only two crystal systems. Most of them are dark in colour; have a hardness ranging between 5-6 and Sp.Gr. between 2.8 to 3.6. Their crystals are elongated, slender and often fibrous in nature.
Descriptive:
a. Anthophyllite (Mg, Fe)3 [Si4O11]2 [OH]2:
Crystal System:
Orthorhombic; commonly occurs in thin, slender fibres.
Cleavage:
Perfect and prismatic.
Colour:
Grey, brownish or greenish.
Hardness:
5.5 – 6
Sp. Gravity:
2.85 – 3.20
Lustre:
Vitreous.
Optical:
Optically (+)
Occurrence:
Found only in metamorphic rocks described as crystalline schist.
b. Tremolite Ca2Mg5 [(Si4O11]2 [OH]2:
Crystal System:
Monoclinic; crystals are long, bladed.
Cleavage:
Prismatic and perfect.
Colour:
Commonly white to light grey.
Hardness:
5.5 – 6.0
Sp. Gravity:
2.9 – 3.0
Lustre:
Vitreous
Optical:
Optically (-)
Occurrence:
Igneous and metamorphic rocks, especially in metamorphosed limestones and dolomites.
c. Actinolite Ca2 (Mg, Fe)5 [(Si4O11]2 [OH]2:
Crystal System:
Monoclinic
Cleavage:
Perfect, Prismatic.
Colour:
Mostly a green amphibole. The green colour is due to ferrour iron.
Hardness:
55.5 – 6.0 (in crystals only)
Sp. Gravity:
3.1 to 3.3
Variety:
Asbestos: Actinolite and Tremolite and other minerals of amphibole group often occur in fibrous form when they are grouped as asbestos. They form long and flexible fibres.
Occurrence:
Actinolite is confined in its occurrence to metamorphic rocks such as crystalline schists.
d. Hornblende Ca2Na (Mg, Fe) (Al, Fe) [(SiAl)4 O11]2 [OH]2:
Crystal System:
Monoclinic, crystals long, slender and prismatic.
Cleavage:
Perfect, prismatic, parallel to [110]; Prismatic.
Colour:
Dark green, dark brown, black.
Hardness:
5.5 to 6
Sp. Gravity:
3.0 to 3.47 (variable, depending on composition).
Lustre:
Vitreous.
Streak:
White, with greenish tint.
Composition:
Highly variable and complex; broadly an aluminous amphibole.
Optical:
Under microscope hornblende crystals generally appear in six-sized outline. The mineral section shows strong pleochroism, an oblique extinction and is commonly optically (-)
Occurrence:
Hornblende is a common rock-forming mineral in igneous and metamorphic rocks. Amphibolite, a metamorphic group of rock may be made up chiefly of hornblende. Because of their widespread occurrence, hornblende and augite are taken as representative minerals from the amphibole and pyroxene groups respectively.
Varieties:
About half a dozen varieties of hornblende have been differentiated on the basis of variation in its chemical composition.
e. Glaucophane Na2 (Mg, Fe)3 Al2 [Si4O11]2 [OH, F]2:
This monoclinic amphibole is confined in its occurrence to crystalline metamorphic rocks called schists. It commonly occurs in grains or fibrous aggregates.
Colour:
Various shades of blue, e.g. bright blue, blue-black, grayish-blue etc.
Hardness:
6 – 6.5;
Sp. Gravity:
3.1 – 3.2
In composition, glaucophane differs from other amphiboles in that a part of hydroxyl group is replaced by fluorine. It is optically (-).
f. Arfvedsonite Na3 (Mg, Fe)4 (Fe, Al) [Si4O11]2 [OH, F]2:
It is also a monoclinic amphibole, characterized with a prismatic cleavage. It is rich in iron and magnesium and hence is black in colour as compared with glaucophane. Streak-deep blue. H = 6, Sp.Gr. = 3.4. Optically (-).
The mineral occurs chiefly in igneous rocks (compare: glaucophane) such as syenites and pegmatites. In the latter rocks, it may occur in the form of crystals of appreciable size: 10 to 20 cm in length.
Comparative Study of Pyroxenes and Amphiboles:
Among the most common rock forming minerals, the pyroxenes and amphiboles occupy a special position. They are essential constituents of many igneous and metamorphic rocks. Among themselves, the prominent minerals of these two groups show many similarities, so much so that often their identification on mere physical characteristics may become quite difficult. It is, therefore, important that we understand the qualities in which these minerals resemble closely and also the characters that make them distinct from each other. This has been attempted below in tabular form.
5. Mica Group:
Minerals of Mica Group are characterized with the presence of a micaceous structure (cleavage) by virtue of which these can be split into very thin sheets along one direction. This micaceous cleavage is explained by their atomic constitution- they consist of SiO4 tetrahedra linked at three of their corners and extending in two dimensions.
This is called sheet structure. These sheets are held together in pairs by metallic ions (e.g. K ion in muscovite, Mg or Fe ion in biotite etc.). But the bond so resulting due to the metallic ions is the weakest and hence there is an eminent cleavage present in the micas.
Micas are, besides felspars, pyroxenes and amphiboles, very common rock forming minerals forming approximately 4 percent of the crust of the Earth. Despite great variation in their chemical composition, mica minerals are easily grouped together because of their similar atomic structure.
Chemical Composition:
Mica group of minerals show a great variation in their chemical composition.
Broadly speaking, they are mainly silicates of aluminium and potassium containing one or more of:
(i) Hydroxyl group
(ii) Magnesium
(iii) Iron
(iv) Sodium
(v) Lithium and,
(vi) Fluorine.
Because of almost invariable presence of hydroxyl group, all the micas yield water when heated in a closed test tube. In view of very complex chemical composition the formulae given for different mica minerals should be considered only approximate.
Atomic Structure:
Micas are characterized with a sheet structure in atomic constitution. In this type of structure, the basic unit of silicates, SiO4 tetrahedra are linked at all their three corners (oxygen ion) resulting in Si: O ratio of 2: 5. Such a linkage when extended in two directions results in sheets of SiO4 – tetrahedra.
Two such sheets are so placed one above another that their vertices point inwards – towards each other. It is here that they are mutually cross-linked with a metallic ion, commonly Al or Mg. Other groups, especially the hydroxyl group, are also incorporated in-between these cross links.
Crystallization:
Most important members of the Mica Group crystallise in one system only- Monoclinic system. Some less important members crystallize in triclinic system.
It is typical of mica crystals that apparently they show a higher symmetry (as if they were belonging to orthorhombic or hexagonal systems). The crystals show prism angles of 60° and 120°. Because of atomic constitution, micas show excellent basal cleavage.
A six rayed percussion figure is sometimes obtained when a cleavage plate of mica is struck at with a blunt pointed tool.
The line which is most distinct in this figure is parallel to (010), face.
Classification:
Micas are generally divided into two groups based on their chemical composition (and hence colour).
(a) Light Micas:
i. Muscovite – Kal2 (AlSi2O10) (OH)2 – Potash mica
ii. Paragonite – Na Al2 (AlSi3O10) (OH)2 – Sodium mica
iii. Lepidolite – KLiAl (Si4O10) (OH)2 – Lithium mica
(b) Dark Micas:
i. Biotite – K (Mg, Fe)3, (AlSi3O10) (OH)2 – Fe, Mg, Mica
ii. Phlogopite – K, Mg3 (Al3Si3O10) (OH)2 – Mg, Mica
iii. Zinwaldite – Complex Li-Fe mica
Physical Properties:
Among the properties that are common to all the minerals of the mica group are:
(i) Perfect basal cleavage
(ii) Low hardness, between 2- 3
(iii) Vitreous luster, and
(iv) Platy habit of the crystals.
Descriptive:
From among the mica group, only muscovite and biotite are of common occurrence as rock forming minerals.
a. Muscovite – KAl2 (AlSi2O10) (OH)2; Potash Mica:
Crystal System:
Monoclinic; commonly occurs in platy form with pseudo symmetry of hexagonal or orthorhombic type.
Cleavage:
Eminent, basal (001)
Colour:
Colourless in thin sheets; as a mass may appear pale yellow.
Hardness:
2.5-3.0
Sp. Gravity:
2.7 to 3.1
Lustre:
Pearly on cleavage faces; Vitreous.
Streak:
Colourless
Optical:
Optically (-)
Occurrence:
It is the most common variety of all the micas and occurs in abundance in acidic igneous rocks such as granites and pegmatities and also in metamorphic rocks (mica schists). It is a common accessory mineral of sedimentary rocks.
Varieties:
Muscovite is a good electrical insulator and finds extensive use in electrical industry and for making fire proof material.
b. Biotite – K (Mg, Fe)3, (AlSi3O10) (OH)2 Black Mica:
Crystal System:
Monoclinic; commonly occurs in tabular sheets or short prismatic plates.
Cleavage:
Highly perfect and basal
Colour:
Black, deep green variety is also found.
Hardness:
2.5 – 3.
Sp. Gravity:
2.7 to 3.1, increases with iron content.
Streak:
Colourless
Optical:
Optically (-), strongly pleochroic in thin sections under microscope.
Occurrence:
Commonly found in igneous rocks and metamorphic rocks like gneisses. It is rare in sedimentary rocks compared with muscovite.
c. Phlogopite – KMg3 (Al3Si3O10) (OH)2:
It is a monoclinic type of mica, rather limited in occurrence and confined to crystalline limestones and dolomites.
Colour- Yellow, brown or red. H = 2.5 – 3, Sp. Gr. = 2.7 to 2.85.
d. Lepidolite – KLiAl (Si4O10) (OH)2:
This mica commonly occurs in the form of granular masses and shows the typical properties of micas- perfect cleavage, H = 2.5 to 4, Sp. Gr. = 2.8 to 3.3 and pearly luster.
In its occurrence, lepidolite is confined mostly to igneous rocks, especially to pegmatites. It is important as a source of lithium.
6. Oxide Minerals:
Next to silicate minerals, oxides occupy an important position in the list of rock forming minerals. Some of them are important as non-metallic refractory minerals (e.g. quartz, corundum, spinel and rutile). Many others are very important as source (ore) minerals of metals such as hematite, magnetite (iron), cuprite (copper), zincite (zinc), cassiterite (tin) and bauxite (aluminium).
A few more common oxide minerals are described below:
i. Quartz (SiP2):
Crystal System:
Hexagonal, (Rhombohedral). Crystals common; some crystals weighing many tones have been reported. Twinned, right-handed and left handed crystals are common.
Cleavage:
Generally absent.
Fracture:
Conchoidal.
Colour:
Colourless when pure; quartz also occurs in coloured varieties- red, green, blue and mixture.
Hardness:
7
Sp. Gravity:
2.65 – 2.66.
Streak:
White in coloured varieties.
Varieties:
It is a very common rock forming mineral and occurs in numerous varieties. A few common varieties are mentioned below:
(a) Polymorphous Transformation:
Quartz, when heated, transforms into high temperature modifications as follow:
The variety named as QUARTZ itself has two polymorphs:
α quartz, βquartz
Identification of exact type of quartz (into α and β) requires thorough investigations of the mode of formation of mineral as observed by its place of occurrence and also type of symmetry.
(b) Right Handed and Left Handed Quartz:
When occurring in distinct crystals, quartz may be distinguished into right handed and left handed types. This is done on the basis of recognition of some typical faces such as trigonal trapezohedron and dipyramid. These two faces normally occur at the edges of prism faces, one above another.
In the left handed quartz, these faces are located on the left side of the upper edge of the prism whereas in the right-handed quartz, these occur on the right upper edge of the crystals. Such a location of these faces is manifestation of an internal atomic arrangement in the crystal.
(c) Coloured Varieties:
Common pure quartz is a colourless transparent mineral. Presence of even a trace of an impurity may give it a characteristic colour and hence a variety.
A few common types of quartz distinguished on this basis are:
(i) Amethyst – Purple or violet.
(ii) Smoky – Dark to light brown, even black.
(iii) Milky – Pure white and opaque,
(iv) Rose Red – Colour is attributed to presence of titanium.
(d) Cryptocrystalline Types:
In many cases, crystallisation of pure silica to quartz remains incomplete due to interruption in the process for one reason or another. Silica occurring in these cryptocrystalline varieties, although close in composition and physical properties to quartz is named differently.
A few common varieties of cryptocrystalline silica are as follows:
1. Chalcedony:
Lustre waxy, commonly translucent, generally massive.
2. Agate:
Often banded, opaque and massive.
3. Onyx:
A regularly banded agate having alternating and evenly placed layers of different colours.
4. Flint:
A dull opaque variety of chalcedony breaking with a characteristic conchoidal fracture.
5. Jasper:
A dull red, yellow, almost amorphous variety of silica.
Occurrence:
Quartz and its varieties occur in all types of rocks; Igneous, sedimentary and metamorphic. In Igneous rocks, quartz makes up bulk of acidic varieties. In sedimentary rocks quartz makes up sandstones and ortho quartzites. Loose sands consist mostly of quartz grains. The metamorphic rocks like gneisses and schists contain good proportion of quartz in some cases. A metamorphic rock named as (Para) Quartzite is entirely made up of quartz.
ii. Corundum (Al2O3):
Crystal System:
Hexagonal – Trigonal. Columnar crystals of appreciable size (8-10 cm) are not uncommon.
Cleavage:
The mineral shows parting parallel to (0001) rather than cleavage.
Colour:
Greyish brown, occurs in many other colours also mentioned below.
Hardness:
9
Sp. Gravity:
3.9 – 4.1
Optical:
Optically (-)
Occurrence:
The common dull grayish brown variety of corundum is found distributed widely in metamorphic rocks like gneisses, schists and crystalline limestone. Corundum also occurs in some igneous rocks. The gem varieties are very rare.
Varieties:
The mineral occurs in quite a few coloured varieties some of which are highly valued as gem stones.
a. Ruby:
The transparent, red variety of corundum
b. Sapphire:
The transparent, blue variety of corundum.
c. Oriental Emerald:
Green variety of corundum.
d. Oriental Topaz:
Yellow variety of corundum.
Economic Use:
The common corundum is used as an abrasive because of its high hardness. The coloured varieties are valued as gems.
iii. Spinel (MgAl2O4):
Crystal System:
Isometric; generally occurs in loose crystals.
Colour:
Occurs in variety of colours; the transparent varieties are valued as gems. Ruby spinel- red deep; Almandine- Violet and Sapphirine- blue.
Cleavage:
Imperfect; octahedral.
Hardness:
7.5 – 8
Sp. Gravity:
3.5 – 4.5
Lustre:
Vitreous to dull in common spinel; pearly to adamantine in gem varieties.
Composition:
Spinel group of minerals though principally oxides of aluminium show some variation in their composition. This is due to partial replacement of magnesium by other metallic ions such as iron, zinc and chromium. Thus, gahnite is a zinc spinel, hercynite is an iron spinel and picolite is a chrome spinel.
Occurrence:
In metamorphic rocks and a few basic igneous rocks.
Economic Use:
As a refractory mineral.
7. Carbonate Minerals:
A few carbonate minerals are very important as rock forming minerals in sedimentary and metamorphic groups.
These include:
i. Calcite,
ii. Dolomite and
iii. Magnesite.
i. Calcite CaCO3:
Crystal System:
Hexagonal – Rhombohedral. The mineral occurs in great variety of crystals- tabular, rhombohedral, prismatic, thin and elongated.
Cleavage:
Highly perfect, rhombohedral. Parting is also common.
Colour:
Pure calcite is white and transparent. Milky-white, opaque varieties are also common. Small proportions of impurities give various tints to calcite- pink, red, violet, blue, green and black.
Hardness:
3
Sp. Gravity:
2.71.
Lustre:
Vitreous. Earthy in massive varieties.
Optical:
Under microscope calcite appears as shapeless patches showing one or more sets of cleavages and interference colours of high order.
Occurrence:
Calcite is one of the most common rock forming minerals in sedimentary rocks. Limestones are almost entirely made up of calcite and the dolomites contain this mineral to a good proportion. The recrystallized variety of calcite makes the well-known metamorphic rocks – marbles. Calcite is principally a secondary mineral formed from the carbonate rich water of sea and oceans.
Varieties:
Calcite occurs in numerous varieties including Aragonite, Iceland spar, Satin spar and chalk. The Iceland spar is a transparent crystalline variety valued as a source material for optical instruments.
ii. Dolomite Ca, Mg (CO3)2:
Crystal System:
Hexagonal, Rhombohedral division.
Cleavage:
Rhombohrdral, perfect.
Colour:
White when pure; also occurs in shades of brown, red, grey, green and black.
Hardness:
3.3 – 4
Sp. Gravity:
2.8 – 2.9.
Lustre:
Vitreous.
Optical:
Optically (-)
Occurrence:
Dolomite commonly occurs in massive forms making layers extending several kilometers across. As a mineral, it is found in veins when its igneous origin is in no doubt. As a rock constituent, however, it is believed to have been formed by action of magnesian rich sea water on original limestone deposit. This process is called in petrology dolomitization.
iii. Magnesite MgCO3:
Crystal System:
Hexagonal – Rhombohedral.
Cleavage:
Hexagonal – Rhombohedral
Colour:
White when pure; shades of grey and brown are also common.
Hardness:
3.5 – 4.5
Sp. Gravity:
3.0 – 3.1
Lustre:
Vitreous to silky in fibrous varieties.
Optical:
Optically (-)
Occurrence:
Magnesite is formed from magnesian bearing waters of sea on their coming in contact with other carbonate rocks. Large deposits of this mineral take the form of rock bodies and become the source of commercial rock.
Economic Use:
As refractory material and for chemical compounds of magnesium.