Of the almost 4000 known minerals, only about 30 are common. The most common are quartz,feldspar,mica, and calcite.
These minerals make up most of the rocks found in the Earth’s crust.
How do geologists classify minerals? Identified about 3,800 minerals Each has characteristic properties that can be used to identify it What do you predict some of those characteristic properties might include?
Video clip on Rocks and Minerals Why do we “care” about minerals? How do they impact our daily lives? http://www.youtube.com/watch?v=9cHUbyaid1k
COLOR Easily observed physical property Often too little information to make identification Can only be used to identify minerals that always have own characteristic color Examples: - Gold, Pyrite, and Chalcopyrite all have gold color, so you need additional information to identify these minerals - Malachite is always green
Action! Arrange your minerals by color How did your group arrange the minerals? Was arranging them by color easy or difficult? What considerations did you need to make? Were there any minerals that you found difficult to place by color? Why?
Streak of a mineral is the color of its powder when rubbed on an unglazed white tile.
The streak is often not the same color as the mineral. A minerals color may vary, but the streak rarely will!
STREAK To do this test, rub the mineral across a piece of unglazed porcelain tile and see what color the powder is Examples: - Pyrite has a gold color but a greenish black streak - Gold has a gold color and a golden yellow streak
Action! Streak Test Use the unglazed tile and try the streak test on your minerals. Which mineral left a streak that was the same color as your mineral? Which mineral left a streak that was a different color than the mineral? Was there anything about running this test that your group had difficulty doing?
Luster refers to the way a mineral shines in reflected light. Notice the difference between these two minerals?
The mineral on the left has a metallic luster, the one on the right, a nonmetallic luster.
There are several terms used to describe nonmetallic luster. Examples could be vitreous, like the quartz on the left, or pearly, like the gypsum on the right.
Other terms that might be used include greasy, dull, and earthy. Can you tell which of these has an earthy luster and which has a vitreous luster? Earthy Vitreous
LUSTER Examples: - Galena is an ore of lead, and has a bright, metallic luster - Quartz has a glassy luster
Action! Identify the luster of your minerals. Look on page 117 (red) or 123 (orange) in Inside Earth textbook. Identify the luster for each of your minerals. - How did your group classify mineral 1? - How did your group classify mineral 2? - How did your group classify mineral 3? - How did your group classify mineral 4? - How did your group classify mineral 5? Did your group have trouble identifying the luster for any of these minerals?
DENSITY Each mineral has a characteristic density. Density is the mass in a given space, or mass per unit volume. You can “heft” or feel a mineral’ weight by picking two mineral samples up and comparing their weight. How do you think geologists could precisely measure the mass of a mineral sample?
Measuring Density Geologists measure density by using a balance to determine the mineral sample’s mass, and then by placing the mineral in water and determining how much water was displaced. The volume of water displaced equals the volume of the sample. Dividing the sample’s mass by its volume gives the density of the mineral. Density = mass/ volume
Density Problem If a sample of Olivine has a mass 237 grams and a volume of 72 ml (cm 3 ), then the density will be - 237 g/ 72 cm 3 = 3.3 g/cm 3 Now your turn! A sample of Calcite has a mass of 324 grams and a volume of 120 ml (cm 3 ). What is its density?
HARDNESS One of the best clues when identifying minerals In 1812, Friedrich Mohs developed the “Mohs hardess scale” to describe the hardness of minerals Ranks ten minerals from softest to hardest Can be determined by a “scratch test” A mineral can scratch any mineral softer than itself, and can be scratched by a mineral that is harder. Which of these minerals do you think is the softest? Quartz, Diamond, or Talc
Talc = 1 The softest know mineral Talc flakes when scratched by a fingernail Used as a powder on people’s skin
Gypsum = 2 A fingernail can easily scratch it! Used in plaster, shampoo, hair products, and foot creams
Calcite = 3 A fingernail can’t scratch it, but a penny can! One of the most common elements on Earth Primary mineral in cave formations Also most sea shells are composed of calcite Pulls carbon dioxide out of sea and thus functions as a filter for Earth Will fix and dissolve when in an acidic solution Used in construction: limestone, marble Also in paint, animal feed, and as a cleaner
Fluorite = 4 A steel knife can easily scratch this mineral. Is used in aluminum, on dishes that can go in the oven, in telescopes and lenses, and for ornamental uses
Apatite = 5 A steel knife can scratch this mineral as well, though not as easily as Fluorite. Used commonly in fertilizers
Feldspar = 6 It can’t be scratched by a steel knife, but it can scratch window glass. Used in ceramics and cleaners Most abundant mineral found in Earth’s crust
Quartz = 7 It can easily scratch steel and hard glass. Second most abundant mineral found in Earth’s continental crust making of sandpaper, optics, glass, circuit boards, computer components, cement, mortar, and jewelry. Time can be measured from the vibrations of the quartz crystals so quartz crystals are often used in clocks.
Topaz = 8 It can scratch quartz. Most common use is as a gemstone in jewelry.
Diamond = 10 Mostly use as gemstones but also used in semiconductors, cutting, grinding, and drilling Hardest mineral
Action! Run hardness tests on your minerals. Use the penny and steel nail to arrange your minerals from softest to hardest. Look on p. 122 (orange) or 118 and 119 (red) to determine where each mineral might fall on the Mohs hardness scale. Where did you classify mineral 1? Where did you classify mineral 2 Where did you classify mineral 3? Where did you classify mineral 4? Where did you classify mineral 5?
Crystal shape can be a useful property to identify minerals if the minerals have had the time and space to form crystals. Most mineral grains that are found in rocks, lack the room to grow.
Crystal Systems - Crystals of each mineral grow atom by atom to form that mineral’s crystal structure - Geologists classify minerals into six groups based on the number and angle of the crystal faces. Look in Inside Earth on p. 124 (orange) or pp. 120 - 121 (red). What is the crystal system of quartz called? What is its density? What is the crystal system of Magnetite? What is its density?
What is the crystal system of quartz called? Hexagonal What is its density? 2.6 g/cm 3 What is the crystal system of Magnetite? Cubic What is its density? 5.2 g/cm 3
The cleavage of a mineral is its tendency to split easily or to separate along flat surfaces. Cleavage can even be observed on tiny mineral grains making it a very useful property!
Cleavage This is determined by how the atoms in its crystal are arranged. This arrangement causes the mineral to break apart more easily in one direction than in another.
Example: Mica is probably the best example as it splits into thin sheets. It is said to have one perfect cleavage.
Fracture “Fracture” describes how a mineral looks when it is broken apart in an irregular way. Geologists use many terms to describe this characteristic; including - “shell-shaped” when it breaks and leaves a surface that looks like a seashell - “hackly” when pure metals, such as copper and iron break, and form jagged points. - “earthy” when soft minerals crumble like clay
Special Properties Look at p. 126 (orange)and p. 122 (red) to find examples of each of these special properties- Magnetism: has properties of magnets Magnetite or Lodestone Fluorescence: glows under ultraviolet light Scheelite Optical properties: bends light Calcite Reactivity: reacts chemically Calcite, Aragonite
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