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EPS 50 Lab 01: Properties of Minerals and Rocks Grotzinger and Jordan, Chapter 3

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EPS 50 Lab 01: Properties of Minerals and Rocks Grotzinger and Jordan, Chapter 3 Name: ______________________________ EPS 50 Lab 01: Properties of Minerals and Rocks Grotzinger and Jordan, Chapter 3 Due: Monday 1 pm Introduction: Geologists study rocks and minerals because these materials provide information about the formation and evolution of our planet, including Earth’s composition, age, resources, and dominant processes. Because rocks and minerals persist for thousands, millions, or even billions of years, they provide a record of Earth’s history. This record is difficult to interpret, but with careful observations and analyses, we can get a glimpse into deep time. The first step is being able to identify these materials. Objective: In this lab you will learn to distinguish amongst minerals based on a few qualitative diagnostic tests. Along the way you will begin to recognize some of Earth's basic minerals and their general chemistries. You will also learn to determine major rock types. Be sure to keep samples in their proper boxes! Answers: Please answer the numbered questions on the separate answer sheet handed out during lab. Only the answer sheet will be graded. Explanations should be concise - a few sentences or less. All answers should be your own, but we encourage you to discuss and check your answers with a few other students as you move through the exercises, unless specified otherwise. -------------------------------------------------------------------------------------------------------------- Basic Observations 1) Look at the large piece of granite in Tray 1 and write down at least five observations about this rock. Draw a sketch showing the details of these features. (3 pts). 2) Note the smaller hand samples in Tray 1. These samples are examples of the diversity of minerals within granites. List at least five criteria you can use to distinguish these samples from one another in terms of the minerals present and their relative abundances. (3 pts). Part 1: Minerals and Mineral Properties The different substances within the piece of granite you just studied are called minerals. Geologists define a mineral as a naturally occurring, inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and other identifying physical properties. In simpler terms, minerals are formed from the specific bonding of different combinations of atoms in a solid form. The atoms can be of the same element, or of many different ones. To keep track of the type and amount of different elements, each mineral is given a specific chemical formula (e.g. FeS2 or CaCO3). A rock is an aggregate of minerals. Thousands of different minerals have been identified and cataloged by mineralogists, but the vast majority of rock-forming minerals belong to one of these six groups (you should know these!): Feldspar Quartz Mica Pyroxene Amphibole Calcite Other common minerals include oxides (e.g. hematite: Fe2O3), salts (e.g. halite: NaCl), and sulfides (e.g. pyrite: FeS2). Each mineral has different physical and chemical properties. Physical properties you will use to identify the minerals include color, streak, hardness, luster, density, habit, cleavage, magnetism, fluorescence, and reaction to chemicals. Refer to Appendix 3 in the back of your book or any reference book for properties of common minerals. Mineral Color and Streak The color we perceive from minerals results from light reflected off the surface of the crystal; it is determined by elements or defects in the crystal structure. When reporting mineral or rock colors in this lab (and in your field work), use objective descriptive words. Avoid subjective adjectives like 'cream', 'buff', or 'rosy'. Instead, use conventional terms such as 'very light yellow', 'light brown', and 'pale orange-pink'. While color can be a useful tool for identifying some minerals, it is not generally diagnostic. Quartz is made of silica and oxygen (SiO2) often packed into a six-sided prism shape that ends in pyramids. If only two elements (Si and O) are present, the mineral quartz appears transparent. Yet, if a tiny amount of an impurity is present in the crystal, the result is colored quartz. 3) Tray 2 contains several different quartz samples. Write down the different colors of quartz you observe using objective descriptive terms. (4 pts). Another test performed in conjunction with surface color is the streak test. A streak is made by firmly drawing the mineral sample across an unglazed porcelain tile called a streak plate. The resulting mark of powdered mineral is often an excellent diagnostic tool. If a mineral is harder than the porcelain tile, it will not leave a streak (the streak is said to be 'colorless'). Some streaks will appear white and are best seen on a black or dark colored porcelain tile. 4) Using Tray 3 describe the colors of the two varieties of hematite, limonite, magnetite, and pyrite. Write in the chemical formula and describe the streak color for each. (5 pts). Mineral Hardness The hardness of a mineral is a measure of its resistance to scratching and is related to the interatomic bond strength. The stronger the bond between the atoms, the harder the mineral. Friedrich Mohs, a 19th century German mineralogist, devised a relative scale on which minerals are ranked by hardness. Talc and graphite are the softest minerals with a hardness of 1; diamond is the hardest mineral at 10. The Mohs hardness scale is shown below, along with some common items that can be used for reference. Examine a mineral hardness kit and briefly look up the descriptions of the Mohs reference minerals so you can become familiar with them. Care should be exercised in determining the relative hardness of two minerals. Sometimes when one mineral is softer than another mineral, parts of the first may rub off on the second and leave a mark that may be mistaken for a scratch. Such a mark can be easily removed with a damp finger whereas a true scratch is permanent. It is always advisable to confirm the result of a scratch test by reversing the procedure: see whether the second mineral will scratch the first. 5) Test the hardness of the minerals found in Tray 4. Also note the chemical formula and the mineral color. (4 pts). 6) While diamond and graphite are chemically identical (they both consist of pure carbon), they occur at the opposite ends of the hardness scale. Why is this? (2 pts). Mineral Luster Luster refers to the way light is reflected from the surface of a mineral. Luster is divided into two main groups: metallic and nonmetallic. The luster of pyrite is a typical metallic luster. Nonmetallic luster can be further described by such terms listed below. Luster description is often subjective, and therefore not very diagnostic. To help understand luster, examine the minerals in Tray 5. Mineral Luster Terms ~ Metallic: strong, mirror-like reflections produced by opaque substances ~ Non-metallic - Vitreous: producing a bright, hard reflection, as in glass - Resinous: characteristic of resins, as in amber - Waxy: reflecting a fuzzy, diffuse light - Pearly: iridescence similar to the appearance of a pearl - Silky: sheen of fibrous materials such as silk - Earthy: dull and powdery, as in a clump of dirt 7) Describe the luster, chemical formula and color of the minerals from Tray 5: galena, limonite, quartz, biotite, and two samples of gypsum. (6 pts). 8) Explain why you think the luster is different for the gypsum A and gypsum B samples. (2 pts). Mineral Density The specific gravity of a mineral is the ratio of its weight to the weight of an equal volume of pure water at 4˚C, which has a density of 1 g/cm3. Common rock forming minerals have specific gravities in the range of 2.6 - 2.8, but heavier minerals can have specific gravities up to 8. Specific gravity can be measured very precisely in the laboratory but may also be roughly estimated simply by holding the specimen in hand. 9) Classify the samples in Tray 6 as 'heavy', 'average' or 'light' by weighing each specimen in your hand. Record the chemical formula for each. (4 pts). 10) Make a guess as to why the heavy minerals are so heavy. Hint: see the periodic table. (2 pts). Crystal Shape or Habit The form a crystal takes is a function of its chemical bonding pattern. As a mineral grows, it builds up a 3-dimensional array of the elements in its chemical formula in a crystal form. With no barriers to its external growth, a mineral will form a perfect crystal. A perfect crystal is rare, however, and more commonly, adjacent crystals interfere with each other’s boundaries during later growth stages. Also, internally, the patterns formed may have planes of weakness. In spite of this, the fundamental form of a crystal, called the crystal habit, is generally distinguishable, and is a useful diagnostic tool. Crystals of the same mineral will always form the same angles between the same crystal faces. These interfacial angles are a good diagnostic tool, and can be precisely measured using a contact goniometer. Simple minerals, such as halite (NaCl) often form simple crystals. More complex minerals with complicated chemical bonding can form more interesting shapes. Crystal habits Massive Indiscernible masses of crystals usually too fine to see. Lazurite forms massive examples. Micaceous Flaky to platy crystals compacted together in sparkling masses. Mica group. Bladed Elongated and flattened like a blade of grass. More elongated than platy and thinner than tabular. Kyanite forms crystals that are a good example of bladed crystals. Equant Any three perpendicular axes through the crystal are more or less equal. Can be used to describe rounded as well as angular crystals. Fluorite forms crystals that are a good example of equant crystals. Prismatic One of the most common of crystal habits.
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