CHAPTER 2: MINERALS
INTRODUCTION From the earliest time, man has found important uses of minerals. E.g. clay for bricks and pottery; quartz and jade for weapons, garnet, amethyst and other coloured stones for ornaments and also gold, silver and copper for ornaments and utensils. An in depth study of geology usually begins with an introduction to mineral, considering that earth's solid surface is composed of rocks and soils that are primarily mineral aggregates. Mineralogy is therefore a subdivision of geology since minerals constitute the rocks of the earth's crust.
In civil engineering, the study of minerals is important because: The minerals, rocks, and soils that occur at and beneath the earth's surface are the materials with which the engineer must work. In the designing of any structure, engineers must be able to evaluate and distribute natural materials present at site to base the design upon this assessment, which is impossible without the general understanding of the physical and chemical characteristics of the minerals and rocks that make up the earth's crust. Knowledge of minerals is essential for engineers who deals with earth materials since minerals are partially responsible for the physical and mechanical properties of rock and soil encountered in mines, tunnels and excavations. In industry, minerals are directly incorporated into chemicals, abrasives, and fertilizers and are processed into thousands of other useful products.
The Nature and Origin of Minerals Minerals are formed in various ways and different conditions. Most of the minerals require thousands of year to develop and others need just a few years. There are few cases that need only a few hours to develop. The mineral formations takes places either in the molten rock or magma, near the Earth surface or deep in the Earth crust as a result of transforming. What is a mineral? They occur naturally as inorganic solids. They have a specific internal structure; that is, their atoms are precisely arranged into a crystalline solid. They have a chemical composition that varies within definite limits and can be expressed by chemical formula. They have definite set of physical properties (hardness, cleavage, crystal form etc) that result from their crystalline structure and composition.
Physical Properties of Minerals The minerals can be identified by their physical properties. Which are characteristics that can be observed or determined by simple tests. The physical properties are: (a) Crystal form and shape (external appearance) (b) Colour (b) Streak (c) Cleavage and fracture (d) Luster (e) Hardness (f) Reaction with acid
Crystal form and shape (external appearance) The crystal faces reflect the internal symmetry of the crystal structure that makes the mineral unique. MINERALS are characterized by their crystal structures. Crystal structures consist of definite arrangements of the atoms of the elements that make up the minerals. In contrast to chemical molecules, which have defined sizes, crystal structures have no size limits although a crystal structure does have a basic building unit called a unit cell. In a crystal, there can be any number of unit cells repeating in any direction.
Cont d Therefore the decisive factor in determining the external appearance and the physical properties of a mineral is its internal structure which is formed by the arrangement of the smallest structure parts, the atoms, ions or molecules. By packaging in regular order then it forms as crystal lattice. Minerals with a crystal lattice are called crystalline, those without a crystal structure are amorphous.
Schematic presentation of the crystal lattice of NaCl. For each Na atom there is one Cl atom. Each Na is surrounded by Cl and each Cl is surrounded by Na. The charge on each Cl is -1 and the charge on each Na is +1 to give a charged balanced crystal. CRYSTAL LATTICE
The six crystal systems, with sketches of typical mineral forms are: (1) Isometric: Where all angles are 90 degrees and all axes are equal in length (2)Tetragonal: Where all angles are 90 degrees, two axes are equal in length and the third axis is of a different length.
Cont d (3) Hexagonal: Where there are three axes of equal length in the same plane but at 120 degrees to one another and a forth axis, perpendicular to the plane of the other three axes that differs in length. (4)Orthorhombic: Where all axes are at 90 degrees to one another, but all axes have different lengths.
Cont d (5) Monoclinic: Where all three axes are of different lengths and one inter-axial angle is not 90 degrees. (6) Triclinic: Where all three axes are of different length and all angles are different from one another.
SOME SYMMETRICAL EXAMPLES ISOMETRIC DIAMOND TETRAGONAL WULFENITE ORTHORHOMBIC TANZANITE MONOCLINIC GYPSUM
Cont d HEXAGONAL BERYL TRIGONAL QUARTZ variety AMETHYST TRICLINIC MONTEBRASITE AMORPHOUS AMBER
Colour The colour of the mineral = seen by eye. Colour results from a mineral s chemical composition, impurities that may present in the sample, flaws or damage in the internal structure, the light in the room or strong reflective surfaces. Unfortunately, even though color is the easiest physical property to determine, it is not the most useful in helping to characterize a particular mineral. The problem is some minerals display a rainbow of colors (shown by the mineral fluorite (CaF 2 ) ). Therefore, colour is a general rather than specific indicator. Quartz, for example, ranges through the spectrum from clear, colourless crystals to purple, red, white, grey and jet black.
The many colors of fluorite
Streak Streak - colour of finely powdered mineral particles produced by scraping the specimen along a roughened surface such as porcelain plate. The mark left behind can be a characteristic feature of the mineral. The streak is not necessarily the same as the colour of the mineral, haematite, for example produces a reddish brown streak, even though the sample may have a metallic grey appearance. The limitation of a streak plate is that it can only be used on minerals with a hardness less than seven. The combination of luster, color, and streak may be enough to permit identification of the mineral.
Cleavage and fracture The internal structure of the mineral is responsible for an extremely useful group of properties involving its strength. There are two ways in which a mineral can break Cleavage and fracture. Most minerals, when broken are observed to split along particular planes, these features are called cleavage planes. The cleavage direction is usually but not always parallel to one of the crystal faces. Micas are examples of minerals with excellent cleavage in one direction. Some minerals do not have a cleavage (quartz) and the surface of rupture is more irregular known as fracture.
Examples of Cleavage Fracture
Luster Luster - property that results from the manner in which light is reflected from a mineral. In another words luster is the shine of a mineral. Luster is described in terms of the degree of brightness. The terms to describe luster are : Metallic, earthy, waxy, greasy, vitreous (glassy), adamantine (or brilliant, as in a faceted diamond). Other shiny, but somewhat translucent or transparent luster (glassy, adamantine), along with dull, earthy, waxy, and resinous luster, are grouped as non-metallic. Metallic : like polished metal e.g. galena Submetallic : less brilliant Dull : e.g. chalk Vitreous : like broken glass e.g. quartz
Cont d Type of Lustre: Vitreous Luster a mineral having a glassy shine. E.g. Quartz and Calcite. Pearly Lustre a mineral having a pearly shine. E.g. Muscovite. Metallic lustre a mineral with a metallic shine. E.g. Magnetite (Iron Ore). Silky lustre a mineral with a silky shine. E.g. Asbestos. Resinious lustre a mineral with a greasy shine like resin. E.g. Talc. Admantine lustre The mineral having a diamond like shine. E.g. Diamond and Zircon
Cont d Transparency: Transparency is the degree to which a medium allows light to pass through it. The transparency may be either opaque, translucent, or transparent. Type of Transparency: Opaque A mineral which does not pass any light, and nothing can be seen through it. The light is refracted again and again at many boundary surfaces until it finally becomes reflected and absorbed. Granular, fibrous or columnar as well as aggregates always opaque. E.g. Orthoclase, Magnetite And Hornblende. Transparent - Mineral which allows the light pass through fully and objects on the other sides are seen clearly through the mineral. E.g. Colourless Quartz and calcite. Semi Transparent Mineral which allows light pass partially and objects are seen hazy through the mineral. E.g. Slightly milky white varieties Quartz and Calcite. Translucent A mineral which allows only some diffused light to pass through it. E.g. milky white varieties Quartz and Calcite.
Cont d Transparent - Quartz Semi Transparent- Sulfur
Hardness Hardness is a measure of a mineral's resistance to abrasion. This property is easily determined and is used widely for field identification of minerals In case of mineral identification, hardness is a relative scale that refers to the difficulty of scratching the mineral. The hardness is described using an arbitrary scale of ten standard minerals. The scale is called the MOH's scale of hardness. The hardness of any object is controlled by the strength of bonds between atoms and is measured by the ease or difficulty with which it can be scratched.
Reaction with acid When dilute hydrochloric acid (typically 10%) is dripped onto some minerals a reaction takes place. On calcite (CaCO 3 ), bubbles of carbon dioxide are produced; in some iron sulphide ores, hydrogen sulphide is produced
Silicate Mineral What are silicate minerals? A group of minerals contains SiO 4 4- as the dominant polyanion. In these minerals the Si 4+ cation is always surrounded by 4 oxygens in the form of a tetrahedron. Because Si and O are the most abundant elements in the Earth, this is the largest group of minerals and is divided into subgroups based on the degree of polymerization of the SiO 4 tetrahedra. Approximately 30% of all minerals are silicates and some geologists estimate that the crust has been about 95% silicate minerals, of which some 60% is feldspar and 12% quartz.
Structure and Classification of the Silicates In all silicate structures investigated, the silicon atoms are in fourfold coordination with oxygen. This arrangement appears to be universal in these compounds, and the bonds between silicon and oxygen are so strong that the four oxygen are always found at the corners of a tetrahedron of nearly constant dimensions and regular shape, whatever the rest of the structure may be like. Hence the existence of a silicon tetrahedron will make a mineral as a silicate mineral and its absence will make it as a non-silicate mineral.
The silicon-oxygen tetrahedron is the basic building block of the silicate minerals. This is the most important building block in geology because it is the basic unit for 95% of the minerals in the crust
Silicate classification is based on the following types of linkages: 1. Single chains pyroxene 2. Double chains amphiboles 3. Two dimensional sheets minerals - micas, chlorites, and clay minerals. 4. Three dimensional frameworks - feldspar and quartz
Silicon-oxygen tetrahedral groups can form single chains, double chains and sheets by sharing of oxygen ions among silica ions Single chain Double chain Sheet
Rock Forming Minerals Minerals vary greatly in their chemical composition and physical properties. Before we begin the study of rocks it is necessary to know the chief rock forming minerals. Although there are more than 2000 known minerals, only a few are abundant in the most common rock forming minerals and can be identified by its physical properties by simple tests. Minerals are classified according to chemical composition and structure. The composition of the most common rock forming minerals is limited by the abundant of elements in the crust. In fact, only eight elements constitute about 98% of the weight of the earth's crust.
Oxygen and Silicon make up approximately 75% of weight of rocks. Silicon and Oxygen occur on combination with other abundant element to form silicate minerals. This group is called the silicate group because all its members contain a specific structural combination of silicon and oxygen, even though most silicate minerals also contain other elements. Thus silicate minerals is the chief rock forming minerals.
Quartz Most common of silica group minerals. Crystallization from the magma took place below 867 C and stable practically over the whole range of geological conditions. Present in silica-rich igneous rocks both volcanic and plutonic and can be recognized by glassy grains of irregular shape without cleavage. Stable both physically and chemically, therefore difficult mineral to alter or breakdown once formed. Important constituent in most metamorphic rocks, usually colourless or white, but can occur in practically any shade, glassy luster. Can be utilized in construction industry.
Quartz Mineral
Feldspar group Most important group, abundant and constitute the most of rock forming minerals. Make up to 60% of the earth's crust Found almost on all of the igneous rocks, in some sedimentary and many metamorphic rocks. Two major types of feldspar: Potassium feldspar (Kfeldspar) and Plagioclase feldspar. Good cleavage in two directions, porcelain luster and hardness of 6.
Cont d The plagioclase feldspars: 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) The K-feldspars or alkali felspars: Microcline, (Potassium aluminum silicate) Sanidine, (Potassium sodium aluminum silicate) Orthoclase, (Potassium aluminum silicate)
Feldspar Mineral Albite Oligoclase Andesine Anorthite
Mica Micas are a group of monoclinic minerals and are characterized by perfect cleavage. Typically paper thin, shiny, elastic cleavage plates. Only two common occurring mica known as biotite (dark brown to black), usually less commercial value and muscovite (colourless or slightly tinted). Abundant in granite and in many metamorphic rocks and is also a significant component of many sandstones.
Pyroxene High temperature minerals found in many igneous and metamorphic rocks. Usually dark coloured (dark green to black) and contains silicates of iron and magnesium. Occurs in basic and ultrabasic rocks.
Olivine Occurs chiefly in basic and ultrabasic rocks with (MgFe) 2 SiO 4 present. Crystallizes at a high temperature, over 1000 o 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 beneath the Earth's crust.
Amphiboles This mineral has much in common with pyroxenes and consist of complex silicates which are magnesium, calcium and iron. Hornblende the most abundant amphibole is a common constituent of igneous and metamorphic rock. Colour ranges from green to black. Common in metamorphic rock known as amphibolite.
Calcite Composed of calcium carbonate (CaCO3) and principal mineral of limestone. Can be precipitated directly from seawater and removed from it by organisms to make shells. Dissolved in groundwater and reprecipitated as new crystals in caves and fractures in rock. Soft (3.0) and easy to scratch, effervesces (bubbles) in dilute hydrochloric acid, perfect cleavage in three directions but not at right angle. Major component of limestone and major mineral metamorphic rock, marble.
Dolomite Composed of magnesium and carbonate (CO 2 ). widespread in sedimentary rocks, forming when calcite reacts with solutions of magnesium carbonate in seawater or groundwater. It will effervesces in dilute hydrochloric acid only if it is in powdered form.
Clay minerals Constitute major part of the soil and thus encountered more frequently than other minerals. Form when air and water interact with the various silicate minerals breaking them to form clay and other products.
Halite and Gypsum Two most common minerals formed by the evaporation of seawater or saline lake water. Halite (common salt, NaCI) easily identified by its taste, very soft and scratched easily with finger nail. Gypsum composed of calcium sulphate and water (CaSO 4 2H 2 O).
Chlorite (MgFe) 5 Al(Si 3 AI)O 10 (OH) 8 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.
Serpentine Mg 6 Si 4 O 10 (OH) 8 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.
Talc Mg 3 Si 4 O 1O (OH) 3 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. Kaolin (China Clay) AI 4 Si 4 O 10 (OH) 8 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.
Non Silicate Minerals Refer Table 2.0 for common, economically important non-silicate mineral. Oxides and Hydroxides: These are minerals that are form by combination of various cations with oxygen. Some examples of this type of minerals are hematite, ilmenite, magnetite, Bauxite, Limonite and Cassiterite. Carbonates and Sulfates: Consist of framework similar to the silica tetrahedra. An important mineral in this group is gypsum, the main ingredient in building materials. The most important carbonate minerals are calcite which combines calcium with the carbonate ion, and dolomite which contains calcium and magnesium in its structure.
Table 2.0: Examples of important non-silicate minerals
Halides: Often occurring as chemical deposited sediments formed by evaporation and as vein minerals in igneous rocks. Example of halide mineral is halite or rock salt deposit from the evaporation of enclosed bodies of salt water.
Minerals which make up the three broad categories of rocks Igneous rocks Sedimentary rocks Metamorphic rocks quartz, biotite, muscovite, amphiboles (e.g. hornblende), pyroxenes (e.g. augite), orthoclase, olivine parent igneous rocks - quartz and feldspar the earth's surface minerals clay minerals, hydrous aluminum silicates, carbonates, calcite and dolomite, those deposited from saline waters - rock salt and gypsum quartz, feldspar, amphiboles, pyroxenes, micas, garnet chlorites, the carbonates metamorphosed limestone
End of the Chapter 2 Q & A