Subject Name Environmental Sciences Paper No: Environmental Geology Module: 16: Mineralogy- Minerals, types Development Team Principal Investigator Prof. R.K. Kohli & Prof. V.K. Garg & Prof. Ashok Dhawan Co- Principal Investigator Central University of Punjab, Bathinda Prof. R. Baskar Paper Coordinator Guru Jambheshwar University of Science and Technology, Hisar Dr. Rachna Bhateria Content Writer Maharshi Dayanand University, Rohtak Content Reviewer Dr. Meenal Mishra IGNOU, New Delhi Anchor Institute Central University of Punjab Environmental Geology Paper Name Module 16. Mineralogy- Minerals, types Name/Title Module Id Pre-requisites To study about Mineralogy and Minerals in general Objectives To discuss the physical properties of Minerals To discuss the classification of minerals Mineralogy, mineral, naturally occurring, inorganic compounds Keywords Objectives To study about Mineralogy and Minerals in general To discuss the physical properties of Minerals To discuss the classification of minerals Introduction Different geological processes are responsible for the concentration of minerals in the parts of crust. Quartz contributes about 70 % of the continental crust and is the most common mineral. Minerals which contain both silicon and oxygen are known as silicates. Oxygen and silicon can be joined together in different ways to form silicate structures like feldspar and augite, olivine, etc. (Figure 1). About 96 percent of the minerals found in Earth’s crust are silicates. There are over 100 elements known presently but over 99% of the Earth’s crust is composed of just 8 elements i.e. oxygen (O), silicon(Si), aluminium (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K). About 4000 plus minerals have been so far discovered. Out of which only a few hundred are considered to be common. An economic concentration or reserves of metallic minerals in a rock is known as ore. Noneconomic minerals like quartz, feldspar and calcite which are found in association with ore minerals are known as gangue (pronounced "gang") and are considered waste. It is insufficient to form an ore from average concentration of minerals in the crust. Figure 1: Minerals (Source: ftp://ftpdata.dnr.sc.gov/geology/Education/PDF/Minerals.pdf) 16.1 Mineralogy Mineralogy is the branch of science which deals with the physico-chemical study of naturally occurring solid and crystalline materials. We can also say that it is the scientific study of all aspects of minerals concerning with conditions of formation during their origin and natural distribution. Internal structure of crystal, physical properties and chemical composition of minerals all are included in mineralogy. A mineral is a homogeneous naturally occurring solid substance formed by inorganic processes having a definite but not fixed chemical composition with an ordered atomic arrangement. 16.2 Chemical Composition of Minerals Elements are the building blocks of minerals. Geochemical factors such as element abundance, solid solution limits, mineral stability, place a limit on the composition and stability of naturally occurring compounds, hence, there are relatively less number of minerals. Minerals and synthetic compounds may have identical structures. However, they differ in the fact that minerals are rarely pure substances and typically show wide variation in their composition. According to the composition, variation of minerals ranges from pure elements (Fe, Au, Ag) with relatively simple compounds (eg. PbS galena, KCI- sylvite) to very complex compounds (eg. Steenstrupine). Chemically simple minerals (SiO2) do not necessarily have simple structures, e.g., alpha quartz. Chemical classifications of minerals are based on the predominant anion or anionic group. The following classes are recognized: (1) native elements; (2) sulphide, telluride, arsenide and selenide minerals including sulphosalts of antimony and bismuth; (3) halides; (4) oxides; (5) hydroxides; (6) carbonates; (7) nitrates; (8) borates; (9) chromates; (10) tungstates; (11) molybdates; (12) phosphates; (13) arsenates; (14) vanadates; (15) silicates and aluminosilicates. Because of the dominance of oxygen, silicon and aluminum in the earth, silicates and aluminosilicates are quantitatively the most important class of minerals. Minerals of mixed anion composition, e.g. F in topaz or apatite, S in pyrite, OH in talc and mica, Cl in biotite, are usually classified according to the nature of the dominant anion. Hybrid minerals, such as valleriite and tochilinite, are sulphides containing layers of hydroxides, and are not common. Each major compositional class of minerals is subdivided into groups of minerals having similar crystal structures (Table 1). Table 1: Classification of Minerals Mineral Group Anionic Representative Minerals or Anionic Complex Native element - Sulfur, gold, silver, copper, diamond graphite Sulfides S2- Pyrite, galena, sphalerite, chalcopyrite Oxides O2- Hematite, magnetite, chromite Halides Cl2-, F- Halite, fluorite 2- Sulfates (SO4) Anhydrite, gypsum, barite 2- Carbonates (CO3) Calcite, dolomite 3- Phosphates (PO4) Apatite 2- Silicates (SiO4) Quart, feldspar 16.3 Structural Classification of Silicates The silicate and alumino silicate class comprise orthosilicates, sorosilicates, inosilicates, phyllosilicates and tectosilicates, etc. Each of these divisions is further subdivided into mineral groups of different structure, e.g. the cyclosilicates into, beryl, tourmaline and axinite groups. The divisions arise because atoms of similar size and bonding character adopt similar structures with a particular anion or anionic group. Hence, minerals of very different composition possess the same crystal structure. Silicates are compounds where Si and O are abundant and are major 4- mineral components of the earth’s crust and mantle’. The basic unit for all silicates is the (SiO4) 4- tetrahedron (Figure 2). Variety of silicate minerals are produced by the (SiO4) tetrahedra linking to self-similar units sharing one, two, three, or all four corner oxygens of the tetrahedron. Silica minerals are classified based on how the silica tetrahedral are linked. O2-- Si4+ = 4- Figure 2: Silicate structure (SiO4 ) Tetrahedra may be isolated or be linked in rings, single chains, double chains, sheets, or frameworks. Let us discuss: 4- (1) Nesosilicates (Independent/Isolated tetrahedral group) [(SiO4) ]: When the isolated tetrahedra are linked by the bonding of each oxygen ion of the tetrahedron to a cation, the cation in turn bond to the oxygen of other tetrahedral (Figure 3a). Thus the tetrahedra are isolated from one another by cations on all sides. The ratio of oxygen to silica is 4:1. Examples of such silicate minerals are olivine, garnet, zircon, etc. 6- (2) Sorosilicates (Double Tetrahedral group) [(Si2O7) ]: In this type two tetrahedra are linked by a single oxygen atom or in other words, two tetrahedra share one oxygen (Fig. 3b). The ratio of oxygen to silica is 2:7 or 3.5:1 (Figure 3b). Example of this type of silicate structure is epidote, melilite. 12- 6- (3) Cyclosilicates (Ring structure) [(Si6O18) ] or [(Si3O9) ]: When angular position of tetrahedra is such that it forms a ring. Closed rings of tetrahedral each sharing 2 oxygen ((Figure 3c). The ratio of oxygen to silica is 3:1. It forms following three types of closed rings: (i) each of 3 tetrahedra sharing an oxygen ion such as in mineral benitoite (ii) each of 4 tetrahedra sharing an oxygen ion such as in mineral axinite (iii)each of 6 tetrahedra sharing an oxygen ion such as in mineral beryl. (a) (b) (c) Figure 3: Crystal structure of (a) nesosilicate; (b) sorosilicates, (c) cyclosilicate minerals. (4) Inosilicates 2- (a) Single Chain (SiO3) : Single chains also form by sharing oxygen but in this case, two oxygens of each tetrahedron bond to adjacent tetrahedra but in an open-ended chain instead of a closed ring. Single chains are linked to other chains by cations (Figure 4a). The ratio of oxygen to silica is 3:1. Example of such silicates are the pyroxene group of minerals. 6- (b) Double Chain (Si4O11) : These are continuous double chains of tetrahedral alternatively sharing two and three oxygen. In this case, two single chains combine to form double chains linked to each other by shared oxygens. Adjacent double chains linked by cations form the structure of the amphibole group of minerals (Figure 4b). A common mineral, hornblende has a complex composition including calcium, sodium, magnesium, iron and aluminium. The ratio of oxygen to silica is 2.75:1. (a) (b) Figure 4: Crystal structure of (a) single chain and (b) double chain silicate minerals. 2- (5) Phyllosilicates (sheet silicates) [(Si2O5) ]: Sheets are structures in which each tetrahedron shares three of its oxygens with adjacent tetrahedra to build stacked sheets of tetrahedra. Cations may be interlayered with tetrahedra sheets (Figure 5a). The ratio of oxygen to silica is 2.5:1. The micas and clay minerals are the most abundant sheet silicates. The minerals with sheet structures can be separated into extremely thin sheets. 0 (6) Tectosilicates (3-D Framework) [(SiO2) ]: Three dimensional framework form when each tetrahedron shares all its oxygens with other tetrahedra (Figure 5b). In this silicate structure, the ratio of oxygen to silica is 2:1. Minerals of feldspars and quartz are the examples of this type of silicate structure. (a) (b) Figure 5: Crystal structure of (a) phyllosilicate minerals and (b) tectosilicate minerals. 16.4 Physical Properties of Mineral The mineral