Descriptive Mineralogy PDF

# The Study of Minerals ## INSIDE THIS CHAPTER ### PART C. FORMATION AND DESCRIPTIVE STUDY OF MINERALS - 11.1. Introduction - 11.2. Minerals from Magmas - 11.3. Minerals from Gases - 11.4. Minerals due to Recrystallisation - 11.5. Minerals From Solutions - 11.6. Rock Forming Minerals - 11.7. The...

# The Study of Minerals ## INSIDE THIS CHAPTER ### PART C. FORMATION AND DESCRIPTIVE STUDY OF MINERALS - 11.1. Introduction - 11.2. Minerals from Magmas - 11.3. Minerals from Gases - 11.4. Minerals due to Recrystallisation - 11.5. Minerals From Solutions - 11.6. Rock Forming Minerals - 11.7. The Silicate Group - 11.8. The Felspar Group - 11.9. Pyroxene Group - 11.10. Amphibole Group - 11.11. Comparative Study of Pyroxenes, and Amphiboles - 11.12. Mica Group - 11.13. Oxide Minerals - 11.14. Carbonate Minerals ## PART C. FORMATION AND DESCRIPTIVE STUDY OF MINERALS ### 11.1. INTRODUCTION Minerals are natural products in solid state and may be formed in a number of ways such as: - **Solidification from an originally hot, molten material through cooling.** This process is called crystallisation and is the most common method of formation of minerals. - **Solidification from gaseous state directly,** generally because of rapid cooling. This process is called sublimation and is rather a rare process occurring mostly near active volcanoes. - **Metamorphism involving change of composition of an originally formed mineral due to a change in temperature, pressure or chemical environment acting independently or in close cooperation of one another.** This process is sometimes referred as recrystallisation and takes place in solid state. - **Precipitation and evaporation from natural solutions under favourable conditions of temperature and chemical concentration.** Precipitates and evaporates are group of minerals formed in this manner ### 11.2. MINERALS FROM MAGMAS The naturally formed hot molten material that is believed to exist at certain depths below the surface of the Earth is called magma. The same material when erupted through volcanoes is called lava. Magma is the source of the largest number of minerals in nature. Many of them are quite rare and precious such as diamonds, rubies, sapphires and aquamarines; other are slightly more common but still highly valuable such as those of gold, silver, tin, copper, lead and zinc etc. Still others are quite common and important as well such as quartz (silica), magnesite (iron), chromite (chromium), uraninite (uranium) and monazite (thorium). The most common minerals - silicates of aluminium with alkalies and other metals – which make the bulk of the crust of the Earth are also crystalline in their origin. ### The Study of Minerals The magma is a natural hot melt with great variation in its chemical composition, original temperature, viscosity and related physical and chemical properties. It continues to exist as a melt there is a change in one or more of these conditions (smelt remain unchanged. As and whet long the einemical conditions surrounding the melt properties. It continues to exist as a melt, there is a change in one or more of these conditions (smelt remain unchanged. As and whet long the chemical conditions surrounding the melt's properties, It continues to exist as a melt long the chemical concentration), one or more constituents separate out as crystals. The presence of some substances like water vapours, carbon dioxide, sulphur dioxide, chlorine, fluorine and boric acid greatly facilitates the process of formation of minerals from the magma by the process of crystallisation. They add to the mobility of the magma and assist in the transfer of molecules of the elements of minerals dispersed throughout the melt to the centres of crystallisation. These substances are thus sometimes rightly called as mineralizers. We shall discuss magma and crystallisation process in a greater detail in a subsequent chapter. It will be sufficient to remember at this stage that the formation of minerals from magma is a highly complex process which is controlled by factors like chemical composition, relative concentration of different components, original temperature of the melt, its viscosity, presence of the mineralizers, rate of cooling as also nature of physical environment in which cooling takes place and so on. ### 11.3. MINERALS FROM GASES Hot gases form a very important group of emanations from active volcanoes. In majority of cases these gases escape into the atmosphere. Sometimes, however, a gas may change directly into a solid substance under the conditions prevailing around a volcano. It is called a sublimate. In some volcanoes called Fumaroles, sublimates of quite a few compounds are of common occurrence. Among the important minerals formed in this manner, native sulphur deserves special mention. It is of common occurrence in the volcanic regions of Japan, Mexico and Italy. Similarly, Sylvite (KCl) is another mineral formed as a sublimate around volcanoes in Germany, Poland and Italy. ### 11.4. MINERALS DUE TO RECRYSTALLISATION Quite a large number of minerals are formed through metamorphism-solid to solid recrystallisation. This process takes place when an already formed mineral is subjected to a changed environment, such as when there is a rise in temperature around it or pressure acting on it is increased or it comes in contact with chemically reactive gases or vapours. The original mineral may suffer a change in its atomic constitution with or without a change in its total chemical composition. One thing is, however, certain: a new mineral is formed at the cost of an earlier mineral. Minerals formed in this manner may be grouped as metamorphic minerals. These form the bulk of rocks called the Metamorphic Rocks. Among the minerals formed as a result of recrystallisation may be mentioned the three polymorphic silicates-Kyanite, Andalusite, and Sillimanite, all having the same empirical formula: Al₂Si₂O₅, but differing in atomic structure. These minerals are produced as a result of high-temperature, high pressure effects on originally alumina-rich minerals. Similarly, minerals of chlorite group (alumino silicates of Mg, Fe, Ni and Cr etc.) provide examples of 'alteration' products from original silicates of broadly similar chemical composition but of igneous origin. The change in their case is brought about at moderate to low temperatures primarily under the influence of chemically active fluids. Penninite, Clinocore and Prochlorite are important examples. ### 11.5. MINERALS FROM SOLUTIONS Water is a great solvent. During its passage along the cracks or over surface and subsurface rocks, it may take into solution quite a number of elements and compounds. This solvent action of water takes place even at ordinary temperature and under atmospheric pressure, producing natural solutions. The waters of rivers, springs, lakes, seas and ocean all contain dissolved salts in variable concentrations. From such natural solutions also minerals may be formed in quite good abundance through the so simple and commonly known processes as evaporation and precipitation. Sodium chloride, the common salt is actually a mineral called halite and provides best example of evaporates. It crystallizes in cubic system and forms easily and abundantly when the sea-water or lake-water collected in shallow ponds starts saturating with progressive evaporation. This mineral is found naturally (rock salt) and also produced as a crop by evaporating saltish sea water in specially prepared salt fields. Anhydrite (CaSO₄) and Gypsum (CaSO₄.₂H₂O) provide examples of another group of minerals formed from solutions by the process of evaporation. Calcite (CaCO₃) that often forms features of fantastic shapes in limestone caves the world over is also a mineral precipitated from carbonated solutions due to loss of CO₂. (In most cases, however, calcite may be of a different origin: organic as in limestones and recrystallised as in marbles). ### 11.6. ROCK FORMING MINERALS As mentioned earlier, 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. ## 11.7. THE SILICATE GROUP ### 11.7.1. General 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. ### 11.7.2. 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. ### 11.7.3. Atomic Structure 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. It will be beyond the scope of this book to discuss the atomic structure of the silicates. Only, most important conclusions are mentioned below: **The Fundamental Unit.** 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⁺⁴ 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 con oxygen tetrahedron at the corner is 16 A. Different types of silicate structures: - **Independent Tetrahedra.** A unit SiO₄⁻⁴ 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 magnesíum which have four and two valences respectively, easily combine with an independent SiO₄⁻⁴ 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. - **Doubly Linked Tetrahedra.** In some cases SiO₄⁻⁴ 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 Si₂O₇⁻⁶ group. This double-tetrahedron is also capable of independent existence when its six negative charges are satisfied by equal number of positive metallic elements. - **Complex-Linked Tetrahedron.** In some cases, three, four and six tetrahedral may be linked together in such a way that they form closed ring-type structures. - **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]²⁻. 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 [SiO₁₆]¹⁶⁻. The amphibole group of minerals have 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 tetrahedra linked together in the above manner.** - **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 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 tetrahedrons at all the four corners are shared, each oxygen ion being shared by two adjacent tetrahedra. ## 11.8. THE FELSPAR GROUP ### 11.8.1. General 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. ### 11.8.2. Chemical Composition In chemical constitution, felspars are chiefly aluminosilicates (also referred as alumosilicates) of Na, K and Ca with following general formula : $WZ_{4}O_{8}$ 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: - Na Al Si₃ O₈ - K Al Si₃ O₈ - 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 as described subsequently. ### 11.8.3. 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 is 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. ### 11.8.4. 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. ### 11.8.5. 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: **Potash Felspars.** Orthoclase (K Al Si₃ Og), Sanidine (K Al Si₂ Og) and Microline (K Al Si₂ Og). **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 Og and Ca Al₂ Si₂ Og as the end members. - Albite - Labradorite - Oligoclase - Bytwonite - Andesine - Anorthite. The above series is also known as Albite-Anorthite series. Crystallographically, Felspars fall into two crystal systems. **Monoclinic Felspars** - Orthoclase (K Al Si₂ Og) - Sanidine (K Al Si₂ Og) **Triclinic Felspars** - Microcline (K Al Si₂ Og) - Albite-Anorthite series (six minerals). ### 11.8.6. 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. ### 11.8.7. Description Among the Felspar Group, the following mineral species are quite common as rock forming minerals and hence are described in some detail. ### 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 Si₂ Og - **Optical:** Optically negative (-) - **Occurrence:** A most common and essential constituent of many igneous rocks, especially granites. - **Economic Use:** As a ceramic material. - **Varieties:** - Adularia- a transparent orthoclase. - Sanidine- a high temperature variety stable above 900°C. ### 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. - **Streak:** Colourless - **Hardness:** 6-6.5 - **Sp. Gravity:** 2.54 to 2.57 - **Composition:** K Al Si₂ Og - **Optical:** The mineral is not easily distinguished in hand specimens from orthoclase except when perfectly crystallized. 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:** - As a ceramic material - As a semi-precious stone (amazonite). - **Varieties:** Anorthoclase- (meaning- not orthoclase). It is a triclinic felspar containing also sodium aluminium silicate. ### 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:** - As a ceramic material - As an ornamental stone in polished form. ### 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. - **Varleties:** Composition of other members of plagioclase felspars has already been given above. These may be broadly considered the varieties of plagioclase felspars. ## 11.9. PYROXENE GROUP ### 11.9.1. General 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. ### 11.9.2. 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. ### 11.9.3. Atomic Structure The pyroxenes show the single-chain structure of silicates. In this type of constitution, the fundamental silicon-oxygen tetrahedron 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. ### 11.9.4. 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. ### 11.9.5. Classification Pyroxenes are commonly classified on the basis of their crystallisation in two groups: - **Orthorhombic Pyroxenes** - Enstatite: MgSiO₃ - Hypersthene: (Fe, Mg) SiO₃ - **Monoclinic Pyroxenes** - Clinoenstatite: MgSiO₃ - Clinohypersthene: (Fe, Mg) SiO₃ - Diopside: CaMgSi₂O₆ or CaMg (SiO₃)₂ - Hedenberguite: CaFeSi₂O₆ or Ca (SiO₃)₂ - Augite: Complex silicate of Ca, Mg, Fe and Al. - Acmite (Aegirine): NaFe (SiO₃)₂ - Jaedite: NaAl (SiO₃)₂ - Spodumene: LiAl (SiO₃)₂ ### 11.9.6. Physical Properties The pyroxene minerals as listed above exhibit similar physical properties of their identical atomic constitution. They are generally dark in colour, their hardness varies between 5 and 6 and Sp.Gr. from 3 - 3.3. Pyroxene crystals are generally short and stout. Prismatic cleavage is prominent in most cases. ### 11.9.7. Descriptive Following members of pyroxene group are of very common occurrence in the rocks and hence deserve individual description. ### 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. ### 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. ### 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. ### 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 specially rich in augite. ### 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 specially common in Nepheline syenites. ### 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 - **Occurrence:** Occurs as boulders and in metamorphic rocks. It is used as an ornamental stone after polishing. ### 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; - **Optical:** Optically (+) - **Occurrence:** The mineral occurs in pegmatites and is used as à semi precious stone. ## 11.10. AMPHIBOLE GROUP ### 11.10.1. General 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. ### 11.10.2. 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 Cl, 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. ### 11.10.3. 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. ### 11.10.4. 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°. ### 11.10.5. Classification Amphiboles are commonly divided in two groups on the basis of their crystallisation: Orthorhombic amphiboles and monoclinic amphiboles. **Orthorhombic Amphiboles:** - Anthophyllite – (Mg, Fe)₂ (Si₄O₁₁) (OH)₂ **Monoclinic Amphiboles:** - Tremolite: Ca₂Mg₅[Si₄O₁₁]₂ [OH]₂ - Actinolite: Ca₂(Mg, Fe)₅ [Si₄O₁₁]₂ [OH]₂ - Homblende: 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 ### 11.10.6. 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. ### 11.10.7. Descriptive ### 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. ### 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. ### 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:** 5.5 - 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. **Note:** The commercial asbestos is derived from other minerals called chrysotile and Serpentine which are hydrous magnesium silicates. ### Occurrence Actinolite is confined in its occurrence to metamorphic rocks such as crystalline schists. ### HORNBLENDE Ca₂Na (Mg, Fe) (Al, Fe) [(SiAl)₄O₁₁]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 ### Glaucophane Na₂ (Mg, Fe)₃Al₂ [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 - **Composition:** In composition, glaucophane differs from other amphiboles in that a part of hydroxyl group is replaced by fluorine. It is optically (-) ### Arfvedsonite Na₃ (Mg, Fe), (Fe, Al) [Si₄O₁₁₂ [OH, F₁₂ 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. ### 11.11. COMPARATIVE STUDY OF PYROXENES AND AMPHIBOLES