You now know that there are many materials that can be mistaken

Transcription

1 15 Mineral Properties R EA D I N G You now know that there are many materials that can be mistaken for diamonds. Yet diamonds are usually the most expensive. What makes diamonds special? Find out more about diamonds in the reading below. CHALLENGE What makes diamonds a valuable natural resource? MATERIALS For each student 1 completed Student Sheet 13.1, Data on the Earring Material 1 Student Sheet 15.1, Three-Level Reading Guide: Mineral Properties READING Use Student Sheet 15.1, Three-Level Reading Guide: Mineral Properties to guide you as you complete the following reading. Diamonds are Minerals Gemstones such as diamonds, rubies, and emeralds are highly valued because they are rare, beautiful, and last a long time. All of these gemstones are minerals (MIN-er-uls). Minerals are non-living materials that are found naturally on earth. Most minerals are solids and each one has a particular internal structure, because each one is made of a specific set of atoms that are arranged in a repeating pattern, called a crystalline (KRIS-ta-lin) structure. In the right environment, minerals form large crystals because of their crystalline structure. B-14

2 Mineral Properties Activity 15 Figure 1: Diamond Crystal Diamonds are made of carbon atoms arranged in the crystalline structure shown in the diagram on the left.this structure results in the octahedral crystal shape of many rough diamonds, above at right. Fluorite is a non-living solid found naturally on earth. It is made of calcium and fluorine atoms arranged in a crystalline structure similar to that of diamonds (see Figure 1). For these reasons, fluorite is a mineral. Acrylic and glass are not minerals. Acrylic is a plastic that is made by people, and glass does not have an internal crystalline structure. The table below lists some common minerals. Which of these minerals do you recognize? Which of these minerals do you use? Some Common Minerals Mineral May also be called Some Common Uses corundum rubies, sapphires jewelry fluorite fluoride, fluorspar toothpaste, industrial processes galena lead ceramics, electronics, ammunition halite salt, rock salt table salt, melt snow on roads native copper copper, pure copper, wiring, plumbing (past) elemental copper mineral specimens (today) quartz quartz crystal watches, radios, jewelry silver jewelry, electronics, photography There are more than 2,000 different minerals, and most of them are not as expensive as diamonds. This is because minerals are found in something that you see everyday: rocks. Rocks are made of minerals. Some of these minerals are very common, while other minerals are harder to find. Large pieces of certain minerals, like diamond, are the B-15

3 Activity 15 Mineral Properties hardest to find and are the most valuable. The minerals in most rocks are small, as you can see in Figure 2, below. Figure 2: Minerals in Rocks Most rocks, like those shown in the photo, are made up of more than one mineral. It is easier to see the different minerals in some rocks than it is in others. Identifying Minerals With so many different minerals, it is important to be able to tell one mineral from another. In Activity 13, Diamond Dilemma, you made many observations of a mineral. Observations like the ones you made can be used to identify different minerals, and are called properties. Minerals can be classified according to their properties the characteristics that make them unique. Color is one property that people sometimes use to identify minerals. Many minerals that are used to make jewelry are known for their beautiful colors. Diamonds are usually colorless, but sometimes they are gray, yellow, or even pink. Because a mineral like a diamond may be found in different colors, color alone cannot be the only property used to identify a mineral. Some minerals are soft, while others are hard. Hardness is another property used to identify minerals. In 1822, the German scientist Friedrich Mohs created a scale to rank the hardness of a mineral from 1 to 10. He scratched one mineral with another. The mineral that was scratched was softer than the other.talc, a soft mineral used in talcum powder, has a hardness of 1. Diamond, the hardest mineral on earth, has a hardness of 10. You can see the Mohs Hardness Scale in Figure 3. Today, scientists have created other hardness scales, but the Mohs Scale is still the easiest to use. On the Mohs Scale, minerals with higher numbers can scratch minerals with lower numbers. Diamonds can be used to scratch any other mineral, but no other mineral except another diamond can scratch a diamond. This doesn t mean that a diamond cannot break. If hit hard enough, a diamond will break into smaller pieces. B-16

4 Mineral Properties Activity 15 Figure 3: Mohs Hardness Scale Compare the hardness of the minerals to some everyday materials. What material(s) could scratch fluorite? What material(s) could scratch corundum? EVERYDAY MATERIALS HARDEST 10 Diamond MINERALS 9 Corundum 8 Topaz Steel 7 Quartz 6 Orthoclase Glass 5 Aptite Platinum 4 Fluorite Fingernail 3 Calcite Native copper Silver 2 Gypsum 1 SOFTEST Talc Graphite (pencil lead) B-17

5 Activity 15 Mineral Properties There are many other properties that can be used to identify minerals. Geologists (gee-all-oh-jists) are people who study rocks, minerals, and other non-living parts of the earth. Geologists also use properties, such as the color a mineral makes when rubbed on a white ceramic plate (called streak color) and the way a mineral usually breaks (called cleavage), to accurately identify a sample. Since each mineral has a unique set of properties, geologists can tell when a yellow stone is a diamond and not another yellow mineral, like topaz. Finding Minerals Many valuable minerals were formed in the earth over thousands of years. Most of the easy-to-find, valuable minerals have been collected already. To gather minerals today, people usually have to dig into the surface of the earth. They remove large pieces of rock and break them into smaller pieces to find particular minerals. This process is known as mining. The world s deepest diamond mine is in South Africa, and it is already over 3.5 kilometers (2.1 miles) deep. Because not all minerals are found everywhere on earth, people mine for particular minerals in certain parts of the world. For example, diamonds are commonly mined in the countries of Canada, Botswana, South Africa, Russia, and Australia. (See Figure 4.) Canada Russia Botswana Australia South Africa Figure 4: Some Countries that Produce Diamonds Countries that produce a lot of diamonds are shown in orange. B-18

6 Mineral Properties Activity 15 Today, not all minerals are mined from the earth. Some can now be created in laboratories and are called synthetic (sin-thet-ik). Even though synthetic mineral substitutes are made by humans, they are not fake. They have the same structure and other properties as the natural mineral. But synthetics are not defined as minerals since they are not found naturally on earth. ANALYSIS 1. a. Create a concept map using the following 10 words: rocks minerals diamonds fluorite geologists properties color hardness Mohs Hardness Scale crystalline structure b. Add at least five more words related to minerals to your concept map. Choose words that show what you now know about minerals. 2. What makes diamonds a valuable natural resource? 3. Does the size of a mineral affect its properties? 4. Look at Student Sheet 13.1, Data on the Earring Material, from Activity 13, Diamond Dilemma. Besides color and hardness, what are some other properties that can be used to identify a mineral? 5. Look at Student Sheet 13.1 again. Compare your observations to the Mohs Hardness Scale. Is the hardness of the earring material most likely to be 1, 4, or 8? Explain your reasoning. B-19

7 Activity 15 Mineral Properties EXTENSION 1 Do you want to see more minerals? Bring in any minerals you may have collected to share with your class or go the Issues and Earth Science page of the SEPUP website to link to sites with photos of valuable gemstones and other minerals. EXTENSION 2 Minerals like diamonds get their shape when they are formed in the earth when molten rock cools. Minerals that can dissolve in water form crystals as the water evaporates. To try this yourself, dissolve as much salt or epsomite (epsom salts) as you can in some warm water. Pour the solution into a large open jar. Next, tie a piece of string around the middle of a pencil and place the pencil across the mouth of the jar. Let a small length of the string dip into the solution. Keep the jar as still as possible as you allow the water to evaporate (the more water you use, the longer it will take to evaporate). Examine the crystals that form on the string, either with a magnifying lens or a microscope. Will all of the crystals from a single mineral have the same crystal shape? What is your prediction? B-20