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NORTH GEOL& 101 - Geology 101 Lab 2: The rock cycle, minerals and igneous rocks

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Geology 101 Name(s): Lab 2: The rock cycle, minerals and igneous rocks Rocks are divided into three major categories on the basis of their origin: Igneous rocks (from the Latin word, ignis = fire) are composed of minerals which crystallized from molten rock, called magma. This category includes rocks that formed as a result of volcanic activity and those that crystallized from magmas cooling under the Earth's surface. Metamorphic rocks (from the Greek word, metamorphose = to transform) are derived from existing rocks which have been exposed to increased pressure and/or temperature to the point where they begin to alter, both texturally and mineralogically. Sedimentary rocks (from the Latin word, sedimentum = settling) form by the accumulation and consolidation of unconsolidated material from weathered and eroded rocks. Rocks do not remain the same throughout geologic time. They are constantly being changed by external forces. Given time, the effect of these forces is to change any one rock type into any rock type. These relationships constitute the rock cycle, as shown in the following figure: The texture and the mineralogical composition of a rock frequently reflect its geologic history and help us determine whether it has an igneous, metamorphic or sedimentary origin. Texture refers to the size, shape and relationship of Igneous Rock Sedimentary Rock Metamorphic Rockminerals within a rock. In general, igneous rocks have a crystalline texture, in which different mineral crystals have grown together and are interlocking. Metamorphic rocks often have a foliated texture, where rippled layers of single minerals align roughly parallel to each other (some single mineral metamorphic rocks have a crystalline texture). Sedimentary rocks usually have a clastic texture because they are made up of fragments (clasts) of other rocks, and often are layered. Sedimentary rocks may also contain fossils. A mineral is a naturally-occurring, solid, usually inorganic element or compound with a definite crystal structure and chemical composition which varies only within specific limits. Rocks are merely aggregates of minerals. The mineralogical composition of a rock depends on the conditions under which that rock formed. Igneous rocks tend to have minerals that form at high temperatures; sedimentary rocks contain minerals that are stable at Earth-surface conditions. Metamorphic rocks consist of minerals that form under a range of pressure and temperature conditions within the Earth. Common rock-forming minerals: Minerals in igneous rocks Minerals in metamorphic rocks Minerals in sedimentary rocks Quartz Orthoclase Plagioclase Biotite Muscovite Amphibole Pyroxene Olivine Quartz Biotite Muscovite Amphibole Garnet Talc Chlorite Staurolite Kyanite Orthoclase Plagioclase Quartz Clay minerals Iron oxide (rust) Orthoclase Biotite Muscovite Calcite Dolomite Halite Gypsum The first part of this lab is to identify mineral specimens, using the flow charts provided (Tables A-1, A-2 and A-3). Note that most rock samples will not have minerals as large as the ones you will see in this part of the lab, so take notice of diagnostic characteristics that do not depend on mineral size. Needed: Mineral testing kit and mineral samples M-1 through M-18. Please label the minerals with their M-numbers (use the lab tape and a pen) so that they can be returned to their rightful box. Using the flowcharts:To begin, determine if the mineral's luster is metallic or non-metallic. Luster refers to how the mineral reflects light; a metallic luster is how a piece of steel or bronze or copper would reflect light. Compare a piece of metal's luster to the luster of a piece of glass; the glass' luster (vitreous) is not a metallic luster. Of course, if the mineral has a dull or pearly luster, it is a non-metallic luster. 1. a. Look at minerals M-3, M-5 and M-6. Only one of these samples has a metallic luster. Which one? b. Now examine minerals M-2 and M-12; again, only one of these has a metallic luster. Which one? Hint: you may need to look at different specimens of the same mineral. By the way, since you’ll be eventually filling out the table on the next to last page, you might as well fill in the appropriate cells in the table with the information above. If the mineral has a metallic luster, determine the mineral's streak color. Streak refers to the color of the powderized mineral, most easily accomplished by rubbing a corner of the mineral sample against the porcelain streak plate provided. 2. a. Use the porcelain streak plate on sample M-4; what color does the streak turn out to be? Is it the same as the color of the mineral? b. Try streaking a few of the nonmetallic luster minerals. What seems to be the problem with the streak test and nonmetallic minerals? If necessary, use hardness (explained below) or magnetism or cleavage (explained below) or specific gravity to identify the mineral. 3. Use the magnet to determine if M-2, M-3 or M-4 is magnetic. “Weakly magnetic”, “strongly magnetic” and “not magnetic” are acceptable answers. In addition to writing your answer here, enter the information under “other properties” in the table on the appropriate row. If the mineral has a non-metallic luster, determine if the mineral has a light (closer to white than black) or a dark (closer to black than white) color. Look at the appropriate table.4. Look at samples M-15 and M-17. Which one is dark-colored? Which is light? To narrow down the possible candidate minerals, determine the relative hardness of the mineral by scratching a corner of the mineral on a piece of glass (or scratching a corner of the glass plate on the mineral). Hardness is the mineral's ability to resist scratching or abrasion. A mineral will scratch all softer minerals and will be scratched by all harder minerals. The hardness of the mineral is determined by its crystal structure and the strength of bonds which hold the crystal together. Certain index minerals define the Mohs Hardness Scale, so you can get a numerical value for hardness. 5. Use the corner of a glass plate and scratch minerals M-1, M-7 and M-13 and record the results below. Then scratch a corner of each mineral on the flat surface and record the results. Be sure to brush off any flakes of mineral to make sure that you’ve actually left a scratch! Then combine the information to draw a conclusion. Hint: there’s


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NORTH GEOL& 101 - Geology 101 Lab 2: The rock cycle, minerals and igneous rocks

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