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Lab 2: Mineralogy

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Lab 2: Mineralogy Lab 2: Mineralogy Reading Understanding Earth, Ch. 3, and Appendix 4 Objectives • To learn how minerals are classified • To learn the common properties of mineral groups • To identify common minerals found in the Earth’s crust Introduction The crust of the Earth is made up of a variety of different rocks. Each type of rock is formed from an assemblage of minerals. Minerals are the basic "building blocks" of rocks, and the recognition of common minerals and rocks is an essential part of the study of geology. A mineral is defined as a chemical element or compound that is a naturally-occurring crystalline solid and is formed as a result of inorganic processes. The terms "rock" and "mineral" are often erroneously used interchangeably. It is important, therefore, to emphasize the key differences between these materials. A mineral is a substance formed by nucleation and growth in an “orderly, three dimensional array.” It is relatively continuous and homogeneous, with a set chemical composition or range of compositions. A rock is an aggregate of minerals with possible wide variations in chemical compositions and crystal structures. A mineral can be represented by a chemical formula, while a rock cannot. Figure 2.1 The difference between a mineral and a rock. Left: The mineral garnet. Right: The rock eclogite, made up of garnet and clinopyroxene. Although there are over 3000 minerals that have been discovered, fewer than 30 make up the bulk of the earth's crust. These are called the rock-forming minerals, and are the ones you are most likely to encounter in the rocks around you. Classification of Minerals Groups of related minerals are called classes. Minerals are classified on the basis of the anion (negatively-charged atom or atom group) in their crystal structures. In nature about one-third of the known mineral types are silicates, and they make up about 95% of all the minerals in the Earth's crust. The remaining 5% include all the other groups of minerals. Table 2.1 The Major Chemical Classes of Minerals Mineral Class Anion Examples 4- silicon-oxygen tetrahedral (SiO4 ) in olivine, pyroxene, quartz, mica, Silicates a number of different linkages (pg. clays, feldspars, amphibole, 56-57, mineral book) talc, garnet, chlorite 2- Carbonates carbonate (CO3 ) dolomite, calcite, siderite Native elements no anions; minerals consist of one sulphur, graphite, diamond, native element only copper, gold pyrite, galena, chalcopyrite, Sulphides sulphur (S2-) sphalerite, pyrrhotite oxygen (O2-) and/or limonite, hematite, corundum, Oxides/hydroxides hydroxyl (OH-) magnetite, bauxite Halides fluorine (F-) or chlorine (Cl-) fluorite, halite, sylvite 2- Sulphates sulphate (SO4 ) gypsum, anhydrite, barite 3- Phosphates phosphate (PO4 ) apatite Many of the characteristics of minerals within a class have similar physical properties, because of similarities in their crystal structure. Physical Properties of Minerals Each mineral has characteristic physical properties that are easily determined and are useful in identification. A mineral's diagnostic properties are those few that are most distinct and useful for accurate identification. For example a diagnostic property of calcite is the vigorous effervescence you see when hydrochloric acid (HCl) is applied. The most important properties are hardness, habit, cleavage, and colour. There are also certain unique properties that apply to certain minerals or mineral classes, such as streak, magnetism, acid reaction, taste, and odour. 1. Lustre The lustre of a mineral is the amount and quality of light reflected from the mineral surface. The most common kind of luster is vitreous lustre. Vitreous lustre, as the name implies, means the lustre is glassy. It stands to reason that vitreous lustre is the most common lustre, because silicates are the most abundant mineral class, and the distinguishing chemical characteristic of silicates is the presence of SiO2. The amorphous state of SiO2 on its own is glass of the kind used in windows. Metallic lustre is much more rare, as it is found primarily in metal-bearing sulfides and oxides. A few minerals have a metallic look to them, but not quite as much as those with a true metallic lustre. These minerals can be described as submetallic. Table 2.2 Classification of Lustre LUSTRE APPEARANCE MINERAL EXAMPLES (proper terminology) (improper proper terminology) • transmits light as well as reflects it (like Vitreous • quartz, fluorite, olivine, garnet clear or coloured glass) • has a soft, lustrous shine to it, like • aragonite, talc (in laminae), Pearly mother-of-pearl gypsum (on cleavage faces) • kaolinite, limonite (vitreous to Earthy • dull, granular, like soil earthy) • talc (when massive), graphite, Greasy • appears to be coated by an oily substance sulphur (resinous to greasy) Adamantine • brilliant lustre • diamond • bright grey, highly reflective, (almost • galena, molybdenite Metallic like a mirror) • magnetite • dull grey/black, but "sparkly" • brassy/gold, reflective • pyrite, chalcopyrite Submetallic • borderline glassy/metallic • graphite, some hematite 2. Hardness The hardness of a mineral is one of the most useful properties we can use to identify a mineral specimen. Hardness is a measure of the specimen’s resistance to abrasion or scratching. It is usually determined by the nature of the chemical bonds within the mineral. For example, both graphite and diamond are composed of carbon atoms; however, the carbon atoms bond differently within each specimen. Diamond has strong chemical bonds between the carbon atoms and is the hardest mineral known; graphite has weakly bonded carbon atoms and can be scratched with a human fingernail. Hardness can be tested and observed by scratching the surface of a mineral sample with another reference mineral of known hardness. These reference minerals are arranged in a standard hardness scale known as “Moh’s Scale of Hardness” (see Table 1.3). The standards are numbered from 1 to 10 in order of increasing hardness. Each mineral will scratch the one below it in the scale, but not the one above it. For example, fluorite will scratch calcite, but not apatite. Also, because a significant amount of geological work is done in the field, relationships of hardness to common objects such as fingernails, knives, glass plates, and streak plates, is often used. Table 2.3 Moh’s Hardness Scale and common items that may be used for comparison Moh’s Number Mineral Common Item 1 talc 2 gypsum fingernail (2-2.5) 3 calcite copper penny (3.5) 4 fluorite 5 apatite glass plate (5-5.5) 6 feldspar steel knife (5-6) 7 quartz streak plate (6.5-7) 8 topaz 9 corundum 10 diamond TESTING HARDNESS 1. Try to find the cleanest, "freshest" looking part of the sample specimen and find a smooth surface on which to test. Press a sharp corner of the reference material (e.g., fingernail, penny, quartz) against the specimen, and make a firm upward or downward movement of a couple of millimeters. 2. Check if you have made a scratch by wiping away any powder. A fine scratch may be evident in the specimen. Use your hand lens to have a closer look. 3. If the sample specimen is scratched by the reference, then the reference is harder and has a higher number on Moh’s Scale. However, if the reference is scratched by the sample, then the sample has a higher number on Moh’s Scale. By bracketing the hardness of a mineral versus the references, you can provide a small range in which the hardness of the mineral fits. For example, if a mineral scratches a penny (with a hardness rating of 3.5) but will not scratch a glass plate (with a hardness of 5.5), then we say that the hardness of the mineral is 3.5-5.5. 3. Cleavage and Fracture Many mineral specimens are found as broken samples rather than well-defined crystals. The shape and pattern of the broken mineral is useful for identifying mineral samples. Cleavage is the tendency of a mineral to break along well-defined planes which represent planes of weakness in the crystal lattice. For example, mica can be endlessly split along its single cleavage plane, producing sheet after sheet of flat, transparent flakes. Minerals that have cleavage can have up to six distinct sets of cleavage planes. Some of these cleavage planes are illustrated in Figure 2.2 below. Figure 2.2 Types of Cleavage One direction (also called basal cleavage) cleavage plane crystal faces Two directions at 90 o Two directions not at 90 o cleavage planes fractu re s urfaces Three directions at right angles Three directons not at right angles (cubic cleavage) (rhombic cleavage) Four directions. The four cleavage planes are the surfaces represented by the cut off corners of a cube. 3 4 1 2 2 3 4 Table 2.4 below summarizes cleavages for a number of common minerals. Table 2.4 Cleavage planes of some common minerals Type of Cleavage Example No cleavage à Fracture quartz, garnet, hematite, pyrite, olivine One plane (basal cleavage) graphite, muscovite, biotite, talc Two planes at 90° pyroxene, feldspars Two planes not at 90° hornblende Three planes at 90° (cubic) galena, halite Three planes not at 90° (rhombic) calcite, dolomite Four planes fluorite More than four planes sphalerite Fracture is how a mineral breaks along a non-cleavage plane. Many minerals will break with no observable pattern and are said to have uneven fracture. Figure 2.3 below compares how light reflects off of a cleavage surface an uneven fracture surface. Some minerals break with no observable pattern, but have a more distinct fracture such as a conchoidal fracture, where the fracture surface is curved and smooth, similar to a clamshell. Conchoidal fractures may be observed on the surfaces on broken glass and obsidian. Most of the glassy minerals (quartz, garnet, olivine, etc.) have conchoidal fracture.
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