Mineral Identification Questions Packet
Lab #8 Mineral Identification Name I. Introduction to Minerals Mineral -a naturally occurring, solid crystalline substance with a specific chemical composition. Geologists must develop the ability to identify, on sight, the most important and abundant minerals. The only way to obtain this skill is by experience and a concerted effort to learn the diagnostic characteristics of each mineral. Your textbook provides a summary of the most important physical properties used in hand specimen mineral identification. In today’s lab, we will use physical properties to identify minerals. 1. Hardness: Hardness refers to a mineral’s resistance to scratching and is generally quite useful in identifying minerals. A mineral’s hardness is measured by its relative “scratchability,” or by one mineral’s ability to scratch another. A very soft mineral can be scratched with a fingernail. A hard mineral cannot be scratched by a knife and will scratch glass. The Mohs scale of hardness is widely used among geologists to identify minerals. (12 pts) Determine the relative hardness of each of the following minerals by scratching them against each other: amphibole, quartz, graphite, mica, olivine, talc. Rank the minerals in order from softest (#1) to hardest (#6). 1. SOFTEST 2. 3. 4. 5. 6. HARDEST 2A. Cleavage: Cleavage describes a mineral’s tendency to break along surfaces parallel to planes of weakness within the crystal structure. The number of cleavage planes and the angles between them can be very useful in distinguishing one mineral from another. Most minerals have 0, 1, 2, or 3 cleavage planes. If bonding is equally strong in all directions, the mineral tends to break along irregular surfaces instead of cleavage planes. These minerals are said to have fracture. The most common type of fracture that we will see in this lab is conchoidal fracture, which is characterized by curved surfaces. Quartz, olivine, and broken glass all exhibit conchoidal fracture. (30 pts) Determine whether the following minerals exhibit distinct cleavage. If they do, record the number of cleavage planes and the angles between the planes (if possible). Mineral Exhibits cleavage? # of planes Angles b/w planes Mica Quartz Amphibole Feldspar Calcite 2B. Crystal Habit: The crystal habit is the growth shape of a single mineral crystal. The crystal habit is determined by the orientations of the atoms and molecules that form the crystal. It will probably be difficult for you to tell the difference between crystal habit and cleavage planes. (3 pts) Look at a crystal of quartz. Describe and sketch its crystal habit. How does quartz’s crystal habit differ from its cleavage or fracture? (1 pt) Look at the garnet crystals with a magnifying glass. Garnet has 12 faces in a wellformed crystal, but they are too difficult to see in this sample. Describe what you can observe. 3A. Luster and Color: Luster refers to the way in which light is reflected from the surface of a mineral. Luster is controlled by the types of elements present and their bonding, both of which affect the way light passes through or is reflected by the mineral. The first-order (and most obvious) subdivision of luster is metallic versus non-metallic. Other common descriptions of non-metallic luster include vitreous (glassy), resinous, greasy, pearly, earthy, and dull. For the purposes of this lab, let’s stick with dull, vitreous, and metallic. (3 pts) Describe the luster of the following minerals. Mineral Luster Galena Quartz Hematite Color is probably the first property you notice about a mineral, but unfortunately it is not one of the most diagnostic for mineral identification. In many cases, a mineral may exhibit a variety of colors. Quartz, for example, can be clear, pink, white, yellow, or purple. (3 pts) Why might a mineral have more than one color? 3B. Streak: The streak of a mineral is simply the color of its powder. Streak is most useful in identifying opaque minerals. The color of the hand specimen of an opaque mineral may be affected by tarnish on the surface or by the luster of the mineral, but the mineral’s streak is not. (6 pts) Test the streak of the following minerals by dragging the mineral across a ceramic plate. Mineral Color of Hand Specimen Color of Streak Hematite K Feldspar Sulfur 4A. Specific Gravity (S.G.): This is a measure of density, which is mass per volume. Specific Gravity is specified as the ratio between the weight of a volume of the mineral and the weight of an equal volume of water. Thus, the specific gravity of water is 1. A mineral weighing three times as much as an equal volume of water would have a S.G. of 3. Most minerals have a specific gravity between 2 and 8, with non-metallic minerals clustering below 4 and metallic minerals generally being higher. Very few, such as gold and platinum, may be as high as 19. (3 pts) Arrange the following minerals in order of increasing specific gravity as best you can based on their “heft” in your hand: hematite, galena, sulfur. 1. 2. 3. 4B. Other properties: There are several other properties that are useful in the identification of specific minerals or mineral groups. These include calcite’s reaction to hydrochloric acid (HCl), and magnetite’s attraction to a magnet. Be sure you are familiar with these properties and how to test for them. 1. (3 pt) Put a drop of dilute hydrochloric acid on calcite. Do not get the HCl on your clothes or in your eyes. Describe the reaction. Why do you think this occurs? 2. (2 pt) How does magnetite react to a magnet? (32 pts) Identifying Unknowns – A Now you are familiar with the most important physical properties used to identify minerals. Use the mineral chart below to identify the eight unknowns, then enter the corresponding letter into the Bowen’s Reaction Series on the next page. Sample Luster Streak Color Hardness Cleavage Crystal habit Name A B C D E F G H Possible Unknowns: Mineral Plagioclase Feldspar Orthoclase (K) Feldspar Mica (Muscovite) Mica (Biotite) Luster Streak Color Grey to white Pink to red Light tan to clear Brown to black Hardness Cleavage Crystal Habit Vitreous/Dull White 6 2 planes at 90° tabular Vitreous/Dull White 6 2 planes at 90° tabular Vitreous White 2 1 plane tabular Vitreous Brown 2 1 plane tabular Garnet Vitreous None Dark red 7.5 None Dodecahedron (12 sides) Calcite Vitreous white White to clear 3 3 planes at 75° various Olivine Vitreous None Green 7 None n/a Quartz Vitreous None/white Various 7 None Pyramid, 6 sides Pyroxene Vitreous Black Black 6 2 planes at 90° prismatic Pyrite metallic Black Gold 6.5 cube Amphibole Vitreous Black Black 6 None 2 planes at 55° and 125° prismatic Identify Unknowns – B. Bowen’s Reaction Series Please enter the letter of the sample that corresponds to the mineral (pts included in above exercise). Olivine Pyroxene Plagioclase Amphibole Biotite Orthoclase Muscovite Quartz
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