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SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 13: Minerals 1 Lecture 13: Introduction to Minerals 2 3 4 Minerals Almost every manufactured product contains minerals. They are essential for the development of a modern society. All geological processes on earth such as volcanic eruption, earthquake, landslide, and erosion involve earth materials (and minerals are the building blocks of earth materials). Basic knowledge of earth materials is essential to the understanding of all geological phenomena. Geological defination of mineral: any naturally occurring inorganic solid that possess an orderly crystalline structure and definite chemical composition. Naturally ocurring – formed by natural geologic processes. Exclude all synthetic materials produced in the lab. Solid within the temperature ranges normally experienced at Earth’s surface. Ice is a mineral but water is not. Orderly crystalline structure – atoms are arranged in an orderly, repetitive manner. Forming regularly shaped objects : crystals. Volcanic glass is not considered as mineral. Definate chemical composition. Most minerals are chemical compounds made up of 2 or more elements. Quartz = SiO2; Calcite = CaCO3; Gold = Au; Sulphur = S Generally inorganic. Sugar = crystalline organic compound is not a mineral. Organic compounds such as those in shells and coral reefs are generally considered minerals. 5 Example of a mineral: salt or NaCl Sodium and chloride ions are arranged in an orderly and repetitive manner into a building block having a cubic shape. 6 Rock Defination: any solid mass of mineral, or mineral-like matter that occur naturally as part of the Earth. Most rocks occur as aggregate of several types of minerals (eg. granite quartz, K-feldspar, plagioclase, etc.) Some rocks are composed of only 1 mineral (eg. limestone calcite) Few rocks are composed of non-mineral matter (eg. pumice volcanic glass; coal organic matter) 7 Properties of Minerals Each mineral has a definite chemical composition and crystalline structure, which give it a unique set of physical properties. These properties are used in the identification of the minerals. Primary properties Crystal form Lustre Colour Streak Hardness Cleavage Fracture Specific gravity 8 Properties of Minerals Crystal form The internal orderly arrangement of the atoms of a mineral is reflected in its crystal form When a mineral forms without space restriction it will develop individual crystals with well-formed crystal faces Crystal growth is often severly constrained, stunted due to competition for space, forming small intergrowth of crystals or aggregates of crystals 9 10 Properties of Minerals Lustre: The appearance or quality of light reflected from the surface of a mineral Metallic appearance of metals – metallic lustre, opaque Submetallic lustre – imperfect metallic lustre Adamantine lustre – diamond Vitreous lustre – appearance of a broken glass (most common in minerals) Resinous lustre – resin-like Greasy lustre – perfectly smooth appearance Pearly lustre – pearl-like due to reflection of light Silky lustre – due to reflection off small parallel fibers Earthy lustre – dull, lack of lustre 11 Properties of Minerals Colour – Not a reliable diagnostic property 12 Properties of Minerals Streak – colour of a mineral in its powdered form. It is much more reliable indication of colour. It is obtained by rubbing the mineral across a piece of hard, unglazed porceline called a streak plate Even if a mineral occurs in more than one colour, its streak usually shows the same colour Minerals with metallic lustre have a dense dark streak 13 Properties of Minerals Hardness: a measure of the resistance of a mineral to abrasion or scratching. One of the most useful diagnostic properties. Determined by rubbing the mineral to be identified against another mineral of know hardness. Standard hardness scale: Mohs scale (10:hardest – 1:softest) 14 Properties of Minerals Cleavage - is breakage along planar surfaces, which are parallel to possible external faces on the crystal. This is because bonds between layers of atoms aligned in certain directions are weaker than bonds between different layers. In some minerals, a single direction of weakness exists, but in others, two, three, four, or as many as six may be present. Where more than one direction of cleavage is present, it is important to determine the angular relation between the resulting cleavage surfaces: are they perpendicular to each other (right angle), or do they meet at an acute or obtuse angle? Mica is a good example – breaking along very closely spaced flat planes that yield thin "sheets." 15 Properties of Minerals Fracture: When a mineral without cleavage breaks, the fracture surfaces can be used for their identification Conchoidal fracture is smooth curved surface like broken glass Subconchoidal fracture is similar to conchoidal, just not as curved, but still smooth Splintery fracture is a fracture type that occurs in fibrous or finely acicular minerals Earthy fracture is a fracture that produces a texture similar to broken clay Jagged or hackly fracture has sharp points or edges that catch on a finger that's rubbed across the surface. 16 Properties of Minerals Specific gravity (G) is defined as the ratio between the weight of a substance and the weight of an equal volume of water at 4° C. Thus a mineral with a specific gravity of 2 weighs twice as much as the same volume of water. Since it is a ratio, specific gravity has no units. The specific gravity of a mineral depends on the atomic weights of all its constituent elements and the manner the atoms are arranged. Most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5 - 3.5. Metallic minerals have higher specific gravity. Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids. Mineral G Quartz (SiO2) 2.65 Calcite (CaCO3) 2.7 Pyrite (FeS2) 5 Copper (Cu) 8.9 Gold (Au) 19.3 17 Mineral Groups There about 4,000 minerals described. <100 are abundant, and they make up most of the rocks in the Earth’s crust. They are classified as rock-forming minerals. The bulk of these minerals are made up of 8 elements. The most common rock-forming minerals are silicates (90%): formed by the combination of oxygen, silicon and one or more metals. Other minerals are grouped under nonsiliates. Many are important economically. Elemental Composition of the Earth’s crust Element Wt % Vol % Oxygen, O 46.6 93.77 Silicon, Si 27.7 0.86 Aluminium, Al 8.13 0.47 Iron, Fe 5.00 0.43 Magnesium, Mg 2.09 0.29 Calcium, Ca 3.63 Sodium, Na Abundance of minerals on Earth’s surface Mineral Area % Feldspars 30 Quartz 28 Clay minerals & mica 18 1.03 Calcite 9 2.83 1.32 Iron oxides 4 Potassium, K 2.59 1.82 Others (trace) 1.43 0.01 Others 11 18 Mineral Group native elements Anion or Anionic Complex Representative Minerals - sulfur, gold, silver, copper, diamond, graphite sulfides S-2 pyrite, galena, sphalerite, chalcopyrite oxides O-2 hematite, magnetite, chromite halides Cl -1, F-1 halite, flourite sulfates (SO4)-2 anhydrite, gypsum, barite carbonates (CO3)-2 calcite, dolomite phosphates (PO4)-3 apatite silicates (SiO4)-2 quartz, feldspar 19 Silicate Minerals Silicate minerals are the most common rock-forming minerals. Their atomic structure is based on silica tetrahedron (SiO4-2), in which four oxygen atoms are bond to each silicon (Si) atom. The mineral structures are constructed by sharing of oxygen between Si atoms producing linkages of tetrahedra. This sharing of oxygen between Si tetrahedra produces chains and other 3D Si tetrahedra structures, which are themselves linked together through bonds between O and other atoms (e.g., Al, Mg and Fe). Chemically, silicate minerals can be separated into two major types: ferromagnesian (iron/magnesian) - olivine, pyroxenes, amphiboles, biotite felsic (silica/aluminum) - quartz, plagioclase, potassium feldspars Silicate minerals, however, are typically classified on the basis of their silica tetrahedra polymerization. The simplest silicate mineral structures have isolated Si tetrahedra linked together through bonds between oxygen and cations other than silicon. More complicated structures involve tetrahedra linked together to form rings (beryl), single chains (pyroxenes), double chains (amphiboles), sheets (micas and clay minerals), and 3D frameworks or networks (quartz and feldspars). 20 21