Minerals and Rocks

1. Emeralds are a form of the mineral beryl. These gems form deep beneath Earth’s surface and are found in relatively few locations.

2. What is a mineral? A mineral is a naturally occurring, inorganic solid with a crystal structure and a characteristic chemical composition. Minerals and Rocks

3. Minerals and Rocks A rock is a solid combination of minerals or mineral materials. • Minerals are inorganic, meaning that living things did not produce them and they occur naturally. • Geologists don’t classify coal as a mineral because coal was created from plant remains. • Materials like brick and concrete are not considered minerals either.

4. Minerals and Rocks Each mineral is a unique substance with its own chemical composition and crystal structure. • For most minerals, the proportion of elements can vary slightly while the mineral still retains a similar set of characteristics. • Within each mineral crystal, the chemical composition is nearly constant.

5. Minerals and Rocks Minerals are the building blocks of rocks. • Only a fraction of the nearly 4000 known minerals are common. These common minerals are called the rock-forming minerals. • Each particle within a rock is a separate mineral crystal. • A crystal is a solid in which atoms are arranged in a regular repeating pattern.

6. Minerals and Rocks This portrait of Abraham Lincoln on Mt. Rushmore in South Dakota was sculpted in a granite cliff.

7. Minerals and Rocks Granite is made up of quartz, feldspar, mica, and hornblende. A magnified view reveals the individual crystals of the minerals that make up granite. Feldspar Mica Quartz Hornblende

8. What are some important properties of minerals? The properties by which minerals can be identified include their crystal structure, color, streak, luster, density, hardness, fracture, and cleavage. The Properties of Minerals

9. The Properties of Minerals Crystal Structure In each type of mineral, the atoms are arranged in a particular geometric shape, or crystal structure. • Each mineral always has the same crystal structure. • The size of a mineral’s crystals can vary.

10. The Properties of Minerals Many crystals form long prisms with a specific number of sides. Other minerals have crystals shaped like cubes, sheets, needles, or threads. Halite, the mineral form of table salt, forms cubic crystals. Garnet, a red, semiprecious gem, often forms 12-sided crystals.

11. The Properties of Minerals Quartz that is pure silicon dioxide is clear or white. Slight impurities produce a range of colors, including the violet quartz (amethyst) specimen shown here.

12. The Properties of Minerals Color Some minerals can be identified by a characteristic color. • Pyrite, also known as fool’s gold, is always golden in color. • Crystals of pure sulfur are always yellow. • But a mineral’s color can often be deceptive, because slight changes in composition can cause significant changes in a mineral’s color.

13. The Properties of Minerals Streak The color of a mineral’s powder is known as its streak. • A mineral’s streak can be found by scraping the mineral on a piece of unglazed porcelain called a streak plate. • The color of a mineral’s streak is not always the same as the color of the mineral itself.

14. The Properties of Minerals Although this piece of hematite is silver-gray, it can be identified by its red-brown streak.

15. The Properties of Minerals Luster Luster is the way in which a mineral’s surface reflects light. • A rough, crumbly surface is said to be an earthy luster. Other terms that describe luster include silky, pearly, and vitreous (glassy.) • Galena and pyrite have a metallic luster. • Sulfur has a resinous-to-greasy luster.

16. The Properties of Minerals Density The density of a mineral depends on its chemical composition. In general, minerals made up of elements with higher atomic masses have higher densities than minerals made up of atoms with lower atomic masses.

17. The Properties of Minerals Galena contains much lead, which has a relatively high atomic mass of 207. Galena’s density is about 7.5 grams per cubic centimeter. Quartz is made up of silicon and oxygen, which have relatively low atomic masses of 28 and 16 respectively. Quartz’s density is only about 2.6 grams per cubic centimeter.

18. The Properties of Minerals Galena Pyrite Sulfur

19. Density of Minerals The density of minerals varies, depending on what elements the minerals contain. Samples of the minerals in the data table were analyzed for density, silicon and oxygen content, and the presence of metals. Study the data table and then answer the questions.

20. Density of Minerals • Using TablesWhich mineral has the lowest density? The highest density?

21. Density of Minerals • Using TablesWhich mineral has the lowest density? The highest density? Answer: quartz; olivine

22. Density of Minerals • Using TablesWhich minerals have the lowest percentage of silicon and oxygen? Which has the highest?

23. Density of Minerals • Using TablesWhich minerals have the lowest percentage of silicon and oxygen? Which has the highest? Answer: olivine and augite; quartz

24. Density of Minerals • Formulating HypothesesOlivine and augite are abundant in oceanic crust and in the mantle. Quartz, muscovite, and hornblende are abundant in continental crust. Formulate a hypothesis to explain why continental crust floats higher on the mantle than oceanic crust.

25. Density of Minerals • Formulating HypothesesOlivine and augite are abundant in oceanic crust and in the mantle. Quartz, muscovite, and hornblende are abundant in continental crust. Formulate a hypothesis to explain why continental crust floats higher on the mantle than oceanic crust. Answer: Continental crust is composed of less dense minerals than oceanic crust. Therefore, continental crust is more buoyant and floats higher on the mantle.

26. Density of Minerals • Drawing ConclusionsHow is a mineral’s density related to its silicon and oxygen content?

27. Density of Minerals • Drawing ConclusionsHow is a mineral’s density related to its silicon and oxygen content? Answer: In general, the higher the silicon and oxygen content of a mineral, the lower its density.

28. The Properties of Minerals Hardness The atoms of minerals are held together by chemical bonds of different kinds and strengths. Hardness is the resistance of a mineral to scratching.

29. The Properties of Minerals Hardness A hard mineral can scratch a softer mineral. The hardness of minerals is ranked on a scale from 1 to 10, called Mohs hardness scale. • Diamond is the hardest mineral, with a hardness of 10. • Talc is one of the softest minerals, with a hardness of 1.

30. The Properties of Minerals Geologists use scratch tests to determine the hardness of mineral specimens. • A glass plate has a hardness of 5.5. A mineral that scratches the plate has a hardness greater than 5.5. • A copper penny, which has a hardness of about 3.5, can scratch talc, but it cannot scratch quartz. • A fingernail, with a hardness of 2.5, and a streak plate, with a hardness of 6.5, can also be used to test hardness.

31. The Properties of Minerals Fracture and Cleavage The fractureof a mineral is how the mineral breaks. Fracture is determined by the crystalline structure of the mineral and the bonds between the atoms in the crystals.

32. The Properties of Minerals Cleavage is a type of fracture in which the mineral tends to split along regular, well-defined planes where the bonds are weakest. • Mica and graphite form sheets. Each sheet contains chemical bonds that are very strong. • The sheets are held together with weak bonds, so they can easily be peeled apart from each other.

33. The Properties of Minerals Mica forms in thin, flat sheets that can be easily peeled apart. When halite is broken apart, it forms small cubes that show its crystal structure.

34. The Properties of Minerals Other Properties Some minerals have unusual properties. • Acids dissolve calcite easily. • Magnetite is strongly attracted by a magnet. • Fluorescent minerals like fluorite give off visible light when they are held under an ultraviolet light.

35. The Properties of Minerals Some minerals have unusual electrical properties. Quartz and tourmaline, for example, become electrically charged when heated and cooled or subjected to pressure. Quartz’s electrical properties have applications in electronics equipment.

36. Assessment Questions • Which of the following properties would not be used to identify an unknown mineral? • crystal shape • density • hardness • size

37. Assessment Questions • Which of the following properties would not be used to identify an unknown mineral? • crystal shape • density • hardness • sizeANS: D