Lab2_FA22_minerals

7 GEOSCIENCE 001 SPRING 2022 Name LAB 2: PROPERTIES OF MINERALS Section Date Atomic Structure and Mineralogy The structure and chemical composition of a mineral control its physical properties. By definition, each mineral has a unique structure and chemical composition. Therefore, we expect that minerals can be distinguished by their physical properties. We would also expect that different samples of the same mineral would display similar physical properties. As rocks are aggregates of minerals, as well as fragments of other rocks, geologists must have a thorough understanding of mineral properties and how they relate to atomic structure in order to accurately analyze everything from earthquakes to volcanoes to soils – in short, everything you will be learning in the following labs and throughout this course. This lab has two main goals: 1. Gain an understanding of how mineral properties relate to atomic structure. 2. Learn how to identify hand samples of common rock-forming minerals. What is a mineral, and why do we care? Minerals are defined by meeting the following five criteria: Naturally occurring – not synthetic/man-made Inorganic – not produced by life (e.g., bones are not minerals) Solid Crystalline – show an orderly arrangement of atoms in a mineral-specific crystalline structure Set chemical composition, homogeneous – a predictable composition A compound must meet all of these criteria to be considered a mineral. Rocks are not minerals. They are physical aggregates of minerals. There are many reasons to care about minerals! Minerals are widely utilized for a variety of products. For example, rutile and muscovite are commonly used in cosmetics. Talc is used to produce talcum powder, and Borax soap is made from ulexite. Iron-rich minerals like hematite are used as iron ore and to fortify breakfast cereals. Uraninite, a uranium oxide mineral, is mined to produce nuclear fuel. Plaster is made from gypsum. We eat halite (salt - NaCl) at almost every meal. Glass is made from quartz. We use minerals containing rare earth elements in modern technologies like phones. Naturally occurring water ice is a mineral. The list could go on for a while! Geologists have even more reason to care about minerals: Rocks are aggregates of minerals, and the specific minerals found in a rock can tell us a lot about the rock's history. We'll learn more about this in

8 later labs, but specific minerals are typically found together in rocks. Mineral assemblages reflect the chemical make-up of a rock as well as the temperature and pressure it experienced. Rocks are identified by their mineral assemblages. For example, granite contains biotite, hornblende, orthoclase, and quartz. These minerals are all unique chemicals with specific crystallization temperatures, so their presence together can tell us how hot the rock was when it was a fully molten magma. Also, since we know the compositions of these minerals, we can estimate the composition of the rock, which is the same as the magma from which it solidified. There are many more ways that minerals can help geologists understand rocks, but first we need to be able to identify the minerals. Today, we focus on different characteristics of minerals that allow geologists to identify them in the field. In subsequent labs, we will use the presence of different mineral assemblages to help us identify rocks. Cleavage Planes and Habit A crystal’s habit is the term used for the shape of a crystal. Cleavage planes are planes of weakness along which minerals break. The angle between two faces of a mineral after breaking is known as a cleavage angle . Q1 Look at the ball-and-stick model of halite (NaCl) provided. Halite is otherwise known as table salt. Examine the model and make a hypothesis about the habit and cleavage angle of halite. Q2 Now look at a mineral sample of halite. a. Describe the crystal habit of halite. b. Does it support or refute your hypothesis? Q3 Using the contact goniometer measure the cleavage angle of halite. a. What is this angle? b. Does this support or refute your hypothesis? Q4 Look at a mineral sample of calcite (CaCO 3 ) and measure the cleavage angles. a. What are the angles? b. What would you predict the angles are at the atomic scale? Q5 Examine the sample of muscovite [KAl 2 (AlSi 3 O 10 )(OH,F,Cl) 2 ], a common mineral found in igneous and metamorphic rocks. Muscovite has a special property--it breaks off in flat sheets. Observe the specimen of muscovite.

10 Mohs hardness scale: Hardness Mineral 10 Diamond (hardest mineral on Earth) 9 Corundum (rubies and sapphires) 8 Topaz 7 Quartz 6 Orthoclase 5 Apatite (this is in your teeth!) 4 Fluorite 3 Calcite 2 Gypsum 1 Talc Hardness of common materials: Hardness Material ~7 Ceramic streak plate 6.5 Hardened steel file 5.5 Window/bottle glass 5-6 Steel nail ~4 Wire nail ~3 US penny ~2 Fingernail Q7 Using the streak plate, nail, penny and your fingernails, estimate the hardness of your eight mineral samples, this may be a range. Make sure to try and scratch the streak plate with the mineral instead of the other way around. Record the hardness of the minerals below:

11 calcite __________ halite __________ quartz __________ muscovite __________ pyrite __________ hematite __________ orthoclase __________ magnetite __________ Q8 Based on your findings, which mineral likely has the strongest atomic bonds and which has the weakest? Strongest _________________________ Weakest _________________________ Color, luster, streak, and special properties Color and luster Some light that strikes a mineral surface is reflected, and some is transmitted. A mineral's color is entirely dependent on the absorption of some wavelengths in the visible spectrum. A mineral's luster is associated with the mineral's ability to transmit and reflect light. The two basic categories of luster are metallic and non-metallic . Within the non-metallic category there are several lusters, such as glassy (vitreous) , earthy (dull) , pearly , silky , or resinous . When a mineral appears almost metallic but appears duller than the bright, reflective sheen of polished metal, this luster is termed sub-metallic. Q9 Examine pyrite (FeS 2 ). What is its color and luster? Q10 Now examine orthoclase (KAl 3 Si 3 O 8 ), a common mineral found in granites. Describe its color and luster. Q11 Some minerals are opaque and some are transparent. Examine quartz and calcite. a. Are they opaque or transparent? b. Based solely on color and luster, could you easily tell these minerals apart?

13 The calcite sample bubbles and fizzes, termed effervescence. The bubbles are CO 2 gas. There are many types of carbonate minerals, and they all effervesce in the presence of acid. Carbonate minerals can be strikingly beautiful (such as azurite, malachite, and rhodochrosite) Q14 Drop a few drops of dilute acid onto quartz. Describe what happens. Q15 Now, drop some dilute acid on calcite. What happens this time? Q16 Just for kicks, try it on three more samples and describe what you find. Q17 Calcite also has the property known as double refraction . a. Place the calcite crystal on this page and describe what you see. b. Why do you think this happens? Hint: light interacts with the crystal lattice (tell us more). Q18 Some minerals are magnetic. Which of the minerals available to you is magnetic? Lastly, geologists often like to taste rocks. Please don’t do this here because somebody in a previous lab may have been dropping hydrochloric acid on all the samples. But what mineral do you think would have a very distinctive taste?

14 Identification Time Eight different samples of the minerals you just investigated have been set out for you to identify. Identify each one, using two diagnostic properties to identify the mineral (do NOT use color). 1.) Property 1: Property 2: 2.) Property 1: Property 2: 3.) Property 1: Property 2: 4.) Property 1: Property 2: 5.) Property 1: Property 2: 6.) Property 1: Property 2: 7.) Property 1: Property 2: