MINERAL CHEMISTRY

Chemistry of Industrial Raw Materials (CHM 405) Mineral Chemistry Minerals are the building blocks of rocks. A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement. The majority of minerals observed are derived from the Earth's crust. Eight elements account for most of the key components of minerals, due to their abundance in the crust. These eight elements, summing to over 98% of the crust by weight, are, in order of decreasing abundance: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium and potassium. Oxygen and silicon are by far the two most important — oxygen composes 46.6% of the crust by weight, and silicon accounts for 27.7%. Minerals can be formed through the following processes: 1. Precipitation from a fluid like H20: This can take place within the Earth through hydrothermal process, diagenesis and metamorphism and or near the Earth’s surface as a result of evaporation weathering or biological activity. 2. Sublimation from a vapour: This process is rare, but can take place at a volcanic vent, or deep in space where the pressure is near vacuum. 3. Crystallization from a liquid: It takes place during crystallization of molten rock (magma) either below or at the Earth's surface. 4. Solid - Solid reactions: This process involves minerals reacting with other minerals in the solid state to produce one or more new minerals. Such processes take place during metamorphism and diagenesis due to changing temperature and pressure conditions. Properties of Minerals a) Colour and streaks: some minerals can be identified by their colour this property can be misleading, because mineral colour is often affected by traces of impurities. Streak, however, is a very reliable identifying feature. Streak refers to the colour of the powder produced when one mineral is scratched by another harder mineral. Fluorite, for example, comes in a great range of colours, yet its streak is always white. b) Hardness: a mineral’s hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. c) Luster: it refers to a mineral's appearance when light reflects off its surface. There are various kinds of luster, all having descriptive names. Thus, metals have a metallic luster, quartz has a vitreous or glassy luster, and chalk has a dull or earthy luster. d) Cleavage and fracture: some minerals, when struck with force, will cleanly break parallel to planes of weakness in their atomic structure. This breakage is called cleavage. Muscovite cleaves in one direction only, producing thin flat sheets. Halite cleaves in three directions, all perpendicular to each other, forming cubes. A mineral's cleavage directions may reveal the crystal system to which it belongs. e) Specific gravity: two minerals can look alike, yet a piece of one may be much heavier than an identical-sized piece of the other. The heavier the mineral, the higher the specific gravity of the mineral. When pure, each mineral has a predictable specific gravity. Therefore, this property is a very reliable clue to a mineral's identity. f) Other identifying properties: minerals have unusual properties that further aid identification. Fluorescent minerals viewed under ultraviolet light glow with various colours. Phosphorescent minerals glow in the dark after exposure to ordinary light. Triboluminescent minerals give off light when crushed or hit. Several minerals containing iron, nickel, or cobalt are magnetic. Over 100 minerals contain uranium, thorium, or other radioactive elements and are therefore radioactive. These are only a few of the unique properties that can be used to identify minerals. Finally, an experienced mineralogist will take into account the location in which an unknown mineral is found. The nature of the surrounding rocks and the presence of other minerals and elements all provide clues to help in identification. Chemical Classification of Minerals Minerals can be chemically classified according to the nature of anionic group present. The chemical classification is as outlined below: 1. Native elements 2. Sulfides (including sulfosalts) 3. Oxides and hydroxides 4. Halides 5. Carbonates, nitrates, borates, iodates. 6. Sulfates, chromates, molybdates, tungstates 7. Phosphates, arsenates, vanadates 8. Silicates. 1. Native elements: These elements include all mineral species which are composed entirely of atoms in an uncombined state. Such minerals either contain the atoms of only one element or else are metal alloys. The native elements are divided into metallic, semimetallic, and nonmetallic subgroups. Metals tend to be a dense and malleable substance which possess a characteristic metallic luster and conduct electricity well. Semimetals and nonmetals are brittle and are poor conductors of electricity when compared to metals. a. Metals Metallic elements which are found in the native state include gold, silver, copper, lead, iron, nickel, platinum, and the rarer elements palladium, iridium, and osmium. Mercury, tantalum, tin, zinc e.t.c. The uncombined atoms of the metals act as perfect spheres and are relatively inert; they tend to form lattices of face-centered cubic, body-centered cubic or hexagonal close-packed structure. The lattice structures of these native metals are composed of metallic bonds, which are relatively weak and produce soft, malleable, ductile, and sectile substances with rather low melting points. The native metals are very conductive, and because light cannot propagate inside a good conductor, the metals possess the characteristic of highly reflective 'metallic' luster. The most common native metals are members of the gold group. These include the elements gold (Au), silver (Ag), copper (Cu), and lead (Pb). The iron group contains iron (Fe) and nickel (Ni). These metals possess a body-centered cubic structure. Nickel and iron are mutually soluble because their atomic radii are both equal to 1.24 angstrom. This solid solution is frequently found in meteorites and probably constitutes much of the earth's core. P The platinum group consist of uncombined common metals like Platinum (Pt), palladium (Pd), iridium (Ir), and osmium (Os). The mineral species which form from the uncombined atoms of these elements are platinum, palladium, platiniridium and iridosmine. Platiniridium is a rare alloy of iridium and platinum, while iridosmine is an equally rare alloy of iridium and osmium. Both alloys possess hexagonal close-packed structure, while platinum and palladium are cubic close packed. The metals of the platinum group are harder and possess higher melting points than those of the gold group. b. Semimetals The native semi-metals include arsenic (As), antimony (Sb), and bismuth (Bi), as well as the less common elements selenium (Se) and tellurium (Te). Arsenic, antimony and bismuth crystallize in the hexagonal-scalenohedral class while selenium and tellurium crystallize in the trigonal-trapezoidal class. Natural crystals of all three species are rare. The semimetals are brittle, and conduct heat and electricity poorly compared to the metals. However, like the metals they display a metallic luster. The lattices of the semimetals are composed of bonds intermediate in type between metallic and covalent. Such bonds are stronger than metallic bonds but are also more directional. The structure of the hexagonal semimetals is therefore based on a distorted form of cubic closest packing in which sheets of atoms parallel to the base of the crystal separate into pairs. This sheetlike structure results in perfect basal cleavage, or perfect cleavage along the paired planes. c. Nonmetals The native nonmetals diamond, fullerene, graphite, and sulfur are structurally distinct from the metals and semimetals. Polymorphism in Diamond and Graphite The mineral species diamond and graphite offer a spectacular example of the trait of polymorphism. Polymorphism occurs when two or more mineral species contain exactly the same elements in exactly the same proportions, and therefore share a chemical formula, yet possess dissimilar lattice structures. Two polymorphic minerals possess identical chemical formulae but different crystal structures; the minerals may therefore exhibit very different physical traits. Chemically the two species diamond and graphite are identical. Both are native elements composed wholely of elemental carbon; both may be burned to carbon dioxide at high temperatures; and both contain closely similar carbon-carbon bonds. However, the two substances are structurally very different. Diamond possesses an exceptionally strong lattice in which each carbon atom is bonded by four covalent bonds to four neighboring carbon atoms, which occupy the apices of a regular tetrahedron. All four valence electrons are taken up by covalent bonds, so that none are free to conduct electricity; hence diamond forms a highly successful insulator. Graphite, on the other hand, is composed of sheets of six-atom rings in which each carbon has three neighboring atoms positioned at the corners of an equilateral triangle. Three valence electrons are occupied by covalent bonds; the fourth is free to act as a conductor of electricity. Graphite thus conducts relatively well. The sheets are stacked a distance much greater than one angstrom apart, and the van der Waals forces which bind the stacked sheets together are very weak. The wide separation and weak binding forces between parallel sheets result in perfect basal cleavage. 2. Sulfides Minerals of the sulfide class are compounds which contain the nonmetallic element of sulfur in combination with atoms of a metal or a semimetals. Compounds in which anions of antimony (Sb), arsenic (As), selenium (Se), or tellurium (Te) replace the sulfur anion and bond with metallic or semimetallic cations are classed respectively as antimonides, arsenides, selenides, and tellurides. This important class includes most of the ore minerals. Sulfides are generally opaque and exhibit distinguishing colours and streaks. The nonopaque varieties (e.g., cinnabar, realgar, and orpiment) possess high refractive indices, transmitting light only on the thin edges of a specimen. If the sulfur anion, a metallic element, and a semimetal are all present then the mineral is categorized as one of the rare sulfosalts. There are approximately 100 species constituting the rather large and very diverse sulfosalt class of minerals. Most sulfides and sulfosalts are soft, dark, heavy, and brittle, possessing a distinct metallic luster and high conductivity. the minerals of the sulfide class exhibit ionic, covalent, and metallic bonding. Examples of sulphides include chalcocite (Cu2S), bomite (Cu5FeS4), galena (PbS), sphalerite (ZnS), cinnabar (HgS), relgas (AsS), pyrite (FeS2), arsenopyrite (FeAsS), molybdenite (MoS2) while the examples of sulfosalts include pyrargyrite (Ag3SbS3), proustite (Ag3AsS3), enargite (Cu3AsS4) e.t.c 3. Oxides and hydroxides These classes consist of oxygen-bearing minerals; the oxides combine oxygen with one or more metals, while the hydroxides are characterized by hydroxyl (OH) groups. The oxides are further divided into two main types: simple and multiple. Simple oxides contain a single metal combined with oxygen in one of several possible metal, examples are cuprite (Cu2O), corundum (Al2O3), cassiterite (SnO2), pyrolusite ((MnO2). Two nonequivalent metal sites (X and Y) characterize multiple oxides, which have the form XY2O4, examples include spinel (Mg Al2O4), gahnite (ZnAl2O4), chromite (FeCr2O4) e.t.c. Unlike the minerals of the sulfide class, which exhibit ionic, covalent, and metallic bonding, oxide minerals generally display strong ionic bonding. They are relatively hard, dense, and refractory. 4. Halides These class of minerals consist of elements of the halogen group such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) bonded to a metal or semimetal cation such as sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), aluminum (Al), copper (Cu), or silver (Ag). Halides are constructed entirely of ionic bonds. The halide minerals tend to be soft (have relatively low hardness), brittle, easily soluble in water, and possess medium to high melting points. They are poor thermal and electric conductors when in solid state but when in molten state they conduct electricity well. The halides consist of about 80 chemically related minerals with diverse structures and widely varied origins. The most common are halite (NaCl), sylvite (KCl), chlorargyrite (AgCl), cryolite (Na3AlF6), fluorite (CaF2), and atacamite. 5. Carbonates, Nitrates, Borates, Iodates Mineral species which are members of the carbonate class are compounds of a metal or semimetal with the carbonate anion (CO3)2-. The plane triangular carbonate anion groups thus form the basic unit from which carbonate minerals are constructed. The nitrates are structurally very closely akin to the carbonates. Nitrogen bonds to three oxygen atoms to form the nitrate radical, (NO3)-, which forms the basic building block of the minerals of this species. The nitrates tend to be softer and possess lower melting points than the carbonates. Atoms of the element boron (B) join to three oxygen atoms in order to form the borate radical, (BO3)3-. This anion group closely resembles the carbonate and nitrate radicals in structure. However, the oxygen atoms of the borate radical may, unlike those of the carbonate or nitrate radicals, be shared between anion groups. Borate radicals may therefore be linked into polymerized chains, sheets, or multiple groups. These are the chemical structures which compose the minerals of the borate class. 6. Sulfates, Chromate, , Molybdates, Tungstates The sulfur cation may form very strong bonds with four oxygen atoms, producing the anion group (SO4)2-. This sulfate radical forms the basic structural unit of the minerals of the sulfate class. The sulfate radical does not share oxygen atoms and cannot polymerize. Minerals of the chromate class are compounds of metallic cations with the chromate anion group (CrO4)2-. Just as sulfur and chromium form the anion groups (SO4)2- and (CrO4)2-, the ions of molybdenum (Mo) and tungsten (W) bond with oxygen atoms to create the anion groups (MoO4)2- and (WO4)2-. These anion groups then bond with metal cations to form the minerals of the molybdate and tungstate classes. Molybdenum and tungsten may freely substitute for one another within the ionic groups (MoO4)2- and (WO4)2-, allowing the formation of series of solid solution. They may not, however, substitute for sulfur within the sulfate radical (SO4)2- or form solid solution with minerals of the sulfate or chromate classes. Species of the molybdate and tungstate classes are typically heavy, soft, and brittle. They tend to be dark or vividly coloured. 7. Phosphates, Arsenates, Vanadates The elements of phosphorous (P), arsenic (As), and vanadium (V) form tetrahedral anion groups combines with oxygen. The resulting phosphate radical, (PO4)3-, provides the basic structural unit of the minerals of the phosphate class; the arsenate and vanadate radicals (AsO4)3- and (VaO4)3- form the basic structural units of the arsenate and vanadate classes. The mineral species of these three classes are thus composed of the respective phosphate, arsenate, and vanadate radicals linked by various metal and semimetal cations. Phosphate, arsenic and vanadium ions may substitute for one another within the three anion groups, forming series of solid solution. 8. Silicates Oxygen and silicon together make up almost three fourths of the mass of Earth's crust. The silicate minerals, a group containing silicon and oxygen atoms, are the most abundant minerals and are the major component of nearly every kind of rock. Silicate compounds make up over 90% of the weight of Earth's crust. Most silicate minerals contain other elements in their formulas; therefore, there is a great variety of silicate minerals. In some rocks such as granite, the different silicate minerals can be seen as the small interlocking crystals of various colours. In other rocks, the mineral grains may be too small to distinguish, but they are usually silicates. Regardless of composition, all silicates have the same basic building unit, the silica tetrahedron. This consists of a silicon atom bound covalently to four oxygen atoms. The oxygen atoms occupy the corners of a geometrical shape called a tetrahedron. The silicon atom is at its center. The entire unit bears a negative electrical charge, enabling it to form compounds with cations. Non-silicate minerals The so-called non-silicate minerals consist of a variety of different mineral groups each named for a particular anion. Only a few of these minerals contribute much volume to Earth's crust, but many of them are very important minerals for manufacturing and other industrial uses. Most mineralogists recognize ten or so major non-silicate groups and a variable number of lesser groups. Uses of Minerals Everything that human kind consumes, uses, or produces has its origin in minerals. The use of minerals depends upon its deposits. The greatest use of minerals depends on its properties. Minerals are the building materials of our technological civilization, and a single mineral may have many unrelated uses. For instance, Aluminium is light, strong and durable in nature , it is used for aircraft, shipping and car industries. Minerals are used in almost all industries. Gold silver and platinum are used in the jewellery industry. Copper is used in coin industry and for making pipes and wire. Silicon obtained from quartz is used in the computer industry.