Mineral Identification Name Notes Page Objectives Explain which mineral properties are most important in identification. Explain how to identify minerals by their properties. Classify some common minerals according to their hardness by doing an experiment. Observe the properties of minerals. The BIG Idea Minerals are basic building blocks of earth. Key Concept A mineral is identified by its properties. A mineral s appearance helps identify it To identify a mineral, you need to observe its properties. Properties are characteristic features that identify a mineral. You might begin by looking at the mineral s color. However, many minerals occur in more than one color, so you would need to examine other properties as well. There are several properties by which a mineral can be identified: o Color and Streak o Luster o Cleavage and Fracture o Density o Hardness o And other special properties such as fluorescence and acid reaction tests. We will be looking at each of these properties more in depth and exploring them through a variety of labs. To begin, let s look at color and streak. 1
o Some minerals can almost be any color, but most minerals have a more limited color range. For example, quartz can be a variety of colors, unlike azurite, which is always blue. o Three main factors cause minerals to vary in color. 1. A mineral may get its color from tiny amounts of an element that is not part of its normal chemical makeup. For example, a sample of pure quartz is clear and colorless, but tiny amounts of iron (Fe) can give quartz a violet color. 2. A mineral s color can change when it is at or near Earth s surface and is in contact with the atmosphere or water. 3. Mineral crystals can have defects in their crystal structure that change their color. o Some minerals have a different color when they are ground into a fine powder than when they are left whole. o A mineral s streak is the color of the powder left behind when the mineral is scraped across a surface. o Often times a geologist will use a tile of unglazed porcelain, called a streak plate, as a tool to identify minerals by their streaks. o Streak is a better clue to a mineral s identity than surface color is. A mineral s streak will always be the same color regardless of the surface color of the mineral. Why do you need to look at properties other than color to identify a mineral? The color of a mineral can be shared by other minerals. So, the color alone cannot identify a mineral. What is streak? The color of the powder of the mineral. What is the difference between color and streak? The color of the mineral can change. The streak of a mineral will almost always be the same. 2
Moving on, we will now learn about luster. o A mineral s luster is the way in which light reflects from its surface. o Two major types of luster are metallic and nonmetallic. o The mineral pyrite has a metallic luster. This means it looks as though it were made of metal. o A mineral with a nonmetallic luster can be shiny, but it does not appear to be made of metal. An example of this would be quartz. o There are several subcategories of nonmetallic luster. We are going to learn about five.! Greasy: the surface of the mineral has the appearance of being coated with an oil or grease.! Pearly: the mineral looks white.! Silky: the sheen fibrous material-like silk; it appears as though threads are running through the mineral.! Earthy: rock-like, little reflection of light, dull (think of a clump of dirt.)! Glassy: transparent or translucent, appears to be coated with glass (think of a piece of glass or something translucent.) o Like a mineral s color, its luster may vary from sample to sample. o If a mineral has been exposed to the atmosphere or to water, its surface luster can become dull. o However, if the mineral is broken to reveal a fresh surface, its characteristic luster can be seen. What are the two main types of luster? Metallic and non-metallic Are all lusters bright and shiny? No. Earthy is dull. 3
The way a mineral breaks helps identify it If you hit a piece of calcite with a hammer, the calcite will break into tilted blocks. You can peel off layers of mica because it splits into thin, flat sheets. Each mineral always breaks the same way, and this property can help identify a mineral. In fact, the way a mineral breaks is a better clue to its identity than are its color and luster. There are two ways to describe how a mineral breaks: cleavage and fracture. Let s look at the first way. o Cleavage is the tendency of a mineral to break along flat surfaces. o The way in which a mineral breaks depends on how its atoms are bonded, or joined together. o In a mineral that displays cleavage, the bonds of the crystal structure are weaker in the directions in which the mineral breaks. o When geologists describe the cleavage of a mineral, they consider both the directions in which the mineral breaks and the smoothness of the broken surfaces. o Mica has cleavage in one direction therefore it breaks into sheets. o Calcite has cleavage in three directions, and breaks into titled blocks. o Both mica and calcite leave very smooth surfaces when the break. Because of this, they are said to have perfect cleavage. What happens when the mineral does not break evenly? o Then we have fracture. o Fracture is the tendency of a mineral to break into irregular pieces. o Some minerals, such as quartz, break into pieces with curved surfaces. Others break into many small, sharp-edged pieces. 4
o If a mineral displays fracture, the bonds that join the atoms are fairly equal in all directions. Which better identifies a mineral, its luster or the way it breaks? How it breaks because it will always break the same way. What determines how a mineral breaks? The bonds between the atoms in the mineral. How does the strength of the bonds between atoms determine whether a mineral displays cleavage or fracture? Bonds that are strong in all directions will cause a mineral to display fracture. Bonds that are strong in some directions and not others will create a smooth, clean break where the weak bonds are. This causes the mineral to display cleavage. A mineral s density and hardness help identify it If you hold a tennis ball and a baseball, you will notice the feel different. You would be able to tell the two apart even with your eyes closed by how heavy and hard they feel. You can identify minerals in a similar way. Even though a baseball and a tennis ball are about the same size, the baseball has more mass and so it is more dense. o A substance s density is the amount of mass in a given volume of the substance. o For example, one cubic centimeter of copper has a mass of 5.1 grams. So, coppers density if 5.1 g/cm 3 (grams per cubic centimeter) o Density is very helpful in identifying minerals. o For example, gold and pyrite look very similar. You can tell the two apart by comparing their densities. 5
o A mineral s density is determined by the kinds of atoms that make up the mineral, as well as how closely the atoms are joined together. o So, to find the density of a mineral, you have to find the mass and the volume. Divide the mass by the volume and you have your density! o How do we find the mass of a mineral if we do not already know it?! Finding the mass is very simple with today s technology.! But, before we start, always remember that weight and mass are completely different!! Mass is a measurement of the amount of matter in an object.! Weight is the measurement of the force of gravity on an object.! No matter where you go in the universe, the mass of an object will remain the same. Its weight, however, will change as the force of gravity changes from place to place.! Okay, so to find the mass of an object, in our classroom we will use a digital scale! (We ll all get a chance to do this.) All you have to do is turn the scale on, zero it out, make sure it is in the correct unit, and then measure the mineral. Within a few seconds the screen will tell you the mass of the mineral. o How do we find the volume of a mineral if we do not already know it?! Finding the volume of an object can be easy if it is a perfect square, rectangle, sphere, etc. However, many minerals are oddly shaped. 6
! To find the volume of a mineral, we will be using some scientific equipment: a spill can, a large beaker filled with water, a small beaker, and a graduated cylinder.! We ll talk about the exact procedure when we do the lab on this. Hardness is another way to determine the identity of a mineral. o A mineral s hardness is its resistance to being scratched. o Like a mineral s cleavage, a mineral s hardness is determined by its crystal structure and the strength of the bonds between its atoms. o Harder minerals have stronger bonds. o A scale known as the Mohs scale is often used to describe a mineral s hardness. This scale is based on the fact that a harder mineral will scratch a softer one. o The scale ranges from one to ten. The softest mineral is talc and the hardest mineral is diamond. o A mineral can only be scratched by minerals that have the same hardness or are harder. o The table above shows us the hardness of several minerals. What minerals can scratch topaz? You should have three minerals: diamond, corundum, and topaz. o There are also everyday objects that we can use to help use determine the hardness of minerals. For example, you can use your fingernail (2.2-2.5), a copper penny (~3), or a steel nail 7
(~6.5) to test for hardness. For example, if you have a mineral that can scratch your fingernail, but not a copper penny, then you know the mineral s hardness is between 2.5 and 3. How is density used to identify minerals? Two minerals that appear to be similar may be identified by testing their density. How do you use the Mohs scale to rank the hardness of an unknown mineral? You can test (scratch) it against other minerals in which the hardness is known. Which ever it does not scratch, it will be weaker than. According to the scale on the previous page, what minerals can calcite scratch? Calcite, gypsum, and talc. Some minerals have special properties Minerals in the carbonate group, such as calcite, react with acid. Chalk is a familiar item that is made up of carbonate minerals. If the acid reacts with the mineral, carbon dioxide gas will form and bubble out of the acid. This shows that the mineral is a carbonate. Some minerals have a property known as fluorescence. Fluorescent minerals glow when they are exposed to ultraviolet light. Fluorite, calcite, and willemite are fluorescent minerals. Although fluorescence is fascinating, it has limited value in mineral identification. Different samples of the same mineral may or may not display fluorescence. They may also glow in different colors. A few minerals respond to magnets. A magnet is pulled toward these minerals. The mineral magnetite strongly attracts magnets, and some other minerals weakly attract magnets. 8
Some rare minerals have a property known as radioactivity. They contain unstable elements that change into other elements over time. As this happens, they release energy. Geologists can measure this energy and use it to identify minerals that contain unstable elements. 9