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.

9 a) What is the cleavage angle of muscovite? Hint - think about the way muscovite breaks. b) Based on the cleavage angle you observe, make a hypothesis about the crystalline structure of muscovite. How might the atoms be arranged to result in flat sheets? (Please include a drawing in the space below.) Halite, calcite and muscovite are all said to have perfect cleavage . However, not all minerals have cleavage. Examine the quartz (SiO 2 ) sample. Quartz does not have cleavage but displays what is known as conchoidal fracture – which means it breaks in smoothly curved surfaces like plate glass. Minerals with good cleavage have planes of atomic weakness. Typically, these planes contain no atoms. Q6 What can you say about the atomic structure of quartz? Mohs hardness scale Cleavage and fracture are properties of minerals used in mineral identification that fall into the class of mechanical cohesion . The other primary tool in this class is hardness , the ease with which a mineral is scratched. The hardness of a mineral is another property directly related to its atomic structure and chemical composition. We can determine a mineral's hardness by attempting to scratch the mineral with various objects, using the Mohs hardness scale to help. This is a qualitative scale that can be easily used in the field. For example, if a mineral can be scratched by a US penny but scratches your fingernail, it would have a hardness between 2 and 3. If a mineral scratches a streak plate, its hardness is greater than 7. When answering these questions, providing a (small) range is acceptable. In this exercise, there are two unnamed minerals (mineral 62 and mineral 64). Keep careful note of any observations you make of these minerals, as you will need to identify them later.