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

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

3. A rock is a solid combination of minerals or mineral materials.
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 are the building blocks of rocks.
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. Granite is made up of quartz, feldspar, mica, and hornblende.
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. The Properties of Minerals
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.

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 Tables Which mineral has the lowest density? The highest density?

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

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

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

24. Density of Minerals
Formulating Hypotheses Olivine 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 Hypotheses Olivine 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 Conclusions How is a mineral’s density related to its silicon and oxygen content?

27. Density of Minerals
Drawing Conclusions How 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 fracture of 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
size ANS: D