Mineralogy Engr. Heber John De Vera CHAPTER OBJECTIVES: At the end of this lecture, the student will be able to: • Describe what a mineral is, and its defining properties. • Identify minerals based on their physical characteristics or properties. • Sort minerals into the correct mineral class. • Describe minerals. how melted rock produces • Describe how rock produces different minerals. • Explain how minerals form from solutions. THE BASICS OF MINERALOGY • In general terms, a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological process (E.H. Nickel, 1995). • A mineral is a naturally occurring combination of specific elements arranged in a particular repeating three-dimensional structure or lattice. • Minerals are naturally occurring, inorganic solids with a definite composition and a crystal lattice structure. Although thousands of minerals in the earth have been identified. Just ten minerals make up most of the volume of the earth’s crust – plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay. PHYSICAL PROPERTIES OF MINERALS To be considered a mineral, an earth material must meet these requirements: • It’s solid: Minerals are solid at the temperature of earth’s surface. • Usually, it’s inorganic: Most minerals are inorganic, meaning they are not alive or the remains of a live organism. However, some animals create mineral shells. In this case, when the animal dies and these shells remain, the shell materials can be considered minerals. • It has an orderly structure: When atoms combine to form minerals, they do so in an organized way that forms a geometric pattern called a crystal. Earth materials that form without this orderly crystalline structure are described as amorphous or glassy and are not minerals. MINERALS To be considered a mineral, an earth material must meet these requirements: • It is naturally occurring: True minerals are found in nature. Humans have the technology to combine atoms into crystals in a lab, but the manmade crystals are not considered minerals. • It has specific chemical composition: Each mineral has a defined chemical composition: a combination of elements that creates its particular crystalline structure. PHYSICAL PROPERTIES OF MINERALS Minerals can be identified by their physical characteristics. The physical properties of materials are related to their chemical composition and bonding. • Color – Color is often useful but not relied upon. Different minerals may be the same color. Some minerals come in many different color. PHYSICAL PROPERTIES OF MINERALS • Luster – It describes the reflection of light off a mineral’s surface. It is how the surface of a mineral reflects light. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Standard names for luster include metallic, glassy, pearly, silky, greasy, and dull. • Metallic luster means opaque, translucent and have the appearance of polished metal. • Glassy luster is the most common of the nonmetallic lusters and means the surface of the mineral reflects light like glass. • Pearly luster refers to a subtle iridescence or color play in reflected light. • Silky luster means reflecting light with a silk-like sheen. PHYSICAL PROPERTIES OF MINERALS Six Different Types of Non-Metallic Luster Luster Appearance Adamantine Sparkly Earthy Dull, clay like Pearly Pearl-like Resinous Like resins, such as tree saps Silky Soft-looking with long fibers Vitreous Glassy PHYSICAL PROPERTIES OF MINERALS • Streak – is the color of a mineral’s powder. Streak is a more reliable property than color because it does not vary. Minerals that are the same color may have a different colored streak. Some minerals, such as the quarts do not have streak. • Specific Gravity – Density describes how much matter is in a certain amount of space. The specific gravity of a substance compares its density to that of water. Substances that are denser have higher specific gravity. • Hardness – is the strength with which a mineral resists its surface being scraped or punctured. In working with hand samples without specialized tools, mineral hardness is specified by the Mohs hardness scale. PHYSICAL PROPERTIES OF MINERALS The Mohs hardness scale is based on 10 reference minerals, from talc the softest, to diamond the hardest. Hardness Index Minerals Hardness Index Minerals 1 Talc 6 apatite 2 gypsum 7 quartz 3 calcite 8 topaz 4 fluorite 9 corundum 5 feldspar 10 diamond PHYSICAL PROPERTIES OF MINERALS • Cleavage – is the tendency of mineral to break along a certain plains to make smooth surfaces. A mineral that naturally breaks into perfectly flat surfaces is exhibiting cleavage. A cleavage represents a direction of weakness in the crystal lattice. • Fracture – is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral. • Crystal Shape – The shape of a crystal follows the symmetry of its crystal lattice. There are two complicating factors to remember: minerals do not always form nice crystals when they grow and a crystal face is different from a cleavage surface. PHYSICAL PROPERTIES OF MINERALS • Other identifying characteristics of minerals Property Description Fluorescence Mineral glows under ultraviolet light Magnetism Mineral is attracted to a magnet Radioactivity Mineral gives off radiation that can be measured with Geiger counter Reactivity Bubbles form when minerals is exposed to weak acid Smell Some minerals have a distinctive smell Taste Some minerals taste salty CLASSES OF MINERALS Minerals are classified according to their chemical properties. Except for the native element class, the chemical basis for classifying minerals is the anion, the negatively charged ion that usually shows up at the end of the chemical formula of the mineral. The major classes of minerals are: • Silicates • Sulfides • Carbonates • Oxides • Halides • Sulfates • Phosphates • Native Elements CLASSES OF MINERALS • Silicates – Based on the polyatomic anion, (SIO4)4-, which has a tetrahedral shape. Most minerals in the earth’s crust and mantle are silicate minerals. All silicate minerals are built of silicon-oxygen tetrahedra (SIO4)4- in different bonding arrangements which create different crystal lattices. • In nesosilicates, also called island silicates, the silicate tetrahedra are separate from each other and bonded completely to non silicate atoms. • In sorosilicates or paired silicates, the silicate tetrahedra are bonded in pairs. • In cyclosilicates, also called ring silicates, silicate tetrahedra are joined in rings. • In phyllosilicates or sheet silicates, the tetrahedra are bonded at three corners to form flat sheets. CLASSES OF MINERALS • In single-chain inosilicates the silicate tetrahedra are bonded in single chains. • In double-chain inosilicates the silicate tetrahedra are bonded in double chains. • In tectosilicates, also known as framework silicates, all corners of the silicate tetrahedra are bonded to corners of other silicate tetrahedra, forming a complete framework of silicate tetrahedra in all directions. • Sulfides - These are based on the sulfide ion, S2–. Some sulfides are mined as sources of such metals as zinc, lead, copper, and tin. • Carbonates - These are based on the carbonate ion, (CO3)2–. Calcite, CaCO3, and dolomite, CaMg(CO3)2, are carbonate minerals. Carbonate minerals tend to dissolve relatively easily in water, especially acid water, and natural rainwater is slightly acid. • Oxides - These are based on the oxygen anion, O2–. Examples include iron oxides such as hematite, Fe2O3 and magnetite, Fe3O4, and pyrolusite, MgO. CLASSES OF MINERALS • Sulfates - These have the polyatomic sulfate ion, (SO4)2–, as the anion. Anhydrite, CaSO4, is a sulfate. • Phosphates - These have the polyatomic phosphate ion, (PO4)3–, as the anion. Fluorapatite, Ca5(PO4)3F, which makes your teeth hard, is a phosphate mineral. • Native Elements - These are made of nothing but a single element. Gold (Au), native copper (Cu), and diamond and graphite, which are made of carbon, are all native element minerals. Recall that a mineral is defined as naturally occurring. Therefore, elements purified and crystallized in a laboratory do not qualify as minerals, unless they have also been found in nature. HOW TO IDENTIFY MINERALS First, you need good light and a hand lens or magnifying glass. A hand lens is a small, double-lens magnifying glass that has a magnification power of at least 8x and can be purchased at some bookstores and nature stores. Minerals are identified on the basis of their physical properties. When identifying a mineral, you must: 1.Look at it closely on all visible sides to see how it reflects light 2.Test its hardness 3.Identify its cleavage or fracture 4.Name its luster 5.Evaluate any other physical properties necessary to determine the mineral’s identity FORMATION OF MINERALS Minerals form under an enormous range of geologic conditions. Minerals can form from volcanic gases, sediment formation, oxidation, crystallization from magma, or deposition from a saline fluid. Formation from Hot Materials A rock is a collection of minerals. Many minerals start out in liquids that are hot enough to melt rocks. Magma is melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000°C. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly. When magma erupts onto Earth’s surface, it is called lava. Lava cools much more rapidly than magma when it is below the surface. In a cooling lava, mineral crystals do not have time to form and are very small. The chemical composition will be the same as if the magma cooled slowly. Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools. This occurs during metamorphism. FORMATION OF MINERALS Formation from Solutions 1. Minerals from Salt Water Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite easily precipitates out of water, as does calcite. 2. Minerals from Hot Underground Water Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins.
