identification of minerals

mohs hardness scale

As you can see, diamond is a 10 on the Mohs Hardness Scale. Diamond is the hardest mineral; no other mineral can scratch a diamond. Quartz is a 7. It can be scratched by topaz, corundum, and diamond. Quartz will scratch minerals that have a lower number on the scale. Fluorite is one. Suppose you had a piece of pure gold. You find that calcite scratches the gold. Gypsum does not. Gypsum has a hardness of 2 and calcite is a 3. That means the hardness of gold is between gypsum and calcite. So the hardness of gold is about 2.5 on the scale. A hardness of 2.5 means that gold is a relatively soft mineral. It is only about as hard as your fingernail. Hardness 1 Mineral Talc

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hardness

Hardness is a minerals ability to resist being scratched. Minerals that are not easily scratched are hard. You test the hardness of a mineral by scratching its surface with a mineral of a known hardness. Mineralogists use the Mohs Hardness Scale, shown in Table 3.2, as a reference for mineral hardness. The scale lists common minerals in order of their relative hardness. You can use the minerals in the scale to test the hardness of an unknown mineral.

cleavage

Cleavage is the tendency of a mineral to break along certain planes. When a mineral breaks along a plane it makes a smooth surface. Minerals with different crystal structures will break or cleave in different ways, as in Figure 3.14. Halite tends to form cubes with smooth surfaces. Mica tends to form sheets. Fluorite can form octahedrons. Minerals can form various shapes. Polygons are shown in Figure 3.15. The shapes form as the minerals are broken along their cleavage planes. Cleavage planes determine how the crystals can be cut to make smooth surfaces. People who cut gemstones follow cleavage planes. Diamonds and emeralds can be cut to make beautiful gemstones.

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cleavage and fracture

Different types of minerals break apart in their own way. Remember that all minerals are crystals. This means that the atoms in a mineral are arranged in a repeating pattern. This pattern determines how a mineral will break. When you break a mineral, you break chemical bonds. Because of the way the atoms are arranged, some bonds are weaker than other bonds. A mineral is more likely to break where the bonds between the atoms are weaker.

other identifying characteristics

Minerals have other properties that can be used for identification. For example, a minerals shape may indicate its crystal structure. Sometimes crystals are too small to see. Then a mineralogist may use a special instrument to find the crystal structure. Some minerals have unique properties. These can be used to the minerals. Some of these properties are listed in Table 3.3. An example of a mineral that has each property is also listed. Property Fluorescence Magnetism Radioactivity Reactivity Smell Description Mineral glows under ultraviolet light Mineral is attracted to a magnet Mineral gives off radiation that can be measured with Geiger counter Bubbles form when mineral is ex- posed to a weak acid Some minerals have a distinctive smell Example of Mineral Fluorite Magnetite Uraninite Calcite Sulfur (smells like rotten eggs)

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fracture

Fracture describes how a mineral breaks without any pattern. A fracture is uneven. The surface is not smooth and flat. You can learn about a mineral from the way it fractures. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, fracture to form smooth, curved surfaces. A mineral that broke forming a smooth, curved surface is shown in Figure 3.16.

density

You are going to visit a friend. You fill one backpack with books so you can study later. You stuff your pillow into another backpack that is the same size. Which backpack will be easier to carry? Even though the backpacks are the same size, the bag that contains your books is going to be much heavier. It has a greater density than the backpack with your pillow. Density describes how much matter is in a certain amount of space. Substances that have more matter packed into a given space have higher densities. The water in a drinking glass has the same density as the water in a bathtub or swimming pool. All substances have characteristic densities, which does not depend on how much of a substance you have. Mass is a measure of the amount of matter in an object. The amount of space an object takes up is described by its volume. The density of an object depends on its mass and its volume. Density can be calculated using the following equation: Density = Mass/Volume Samples that are the same size, but have different densities, will have different masses. Gold has a density of about 19 g/cm3 . Pyrite has a density of only about 5 g/cm3 . Quartz is even less dense than pyrite, and has a density of 2.7 g/cm3 . If you picked up a piece of pyrite and a piece of quartz that were the same size, the pyrite would seem almost twice as heavy as the quartz.

luster

Luster describes the way light reflects off of the surface of the mineral. You might describe diamonds as sparkly or pyrite as shiny. But mineralogists have special terms to describe luster. They first divide minerals into metallic and non-metallic luster. Minerals that are opaque and shiny, like pyrite, are said to have a metallic luster. Minerals with a non-metallic luster do not look like metals. There are many types of non-metallic luster. Six are described in Table 3.1. Non-Metallic Luster Adamantine Earthy Pearly Resinous Silky Vitreous Appearance Sparkly Dull, clay-like Pearl-like Like resins, such as tree sap Soft-looking with long fibers Glassy Can you match the minerals in Figure 3.13 with the correct luster from Table 3.1 without looking at the caption?

how are minerals identified

Imagine you were given a mineral sample similar to the one shown in Figure 3.10. How would you try to identify your mineral? You can observe some properties by looking at the mineral. For example, you can see that its color is beige. The mineral has a rose-like structure. But you cant see all mineral properties. You need to do simple tests to determine some properties. One common one is how hard the mineral is. You can use a minerals properties to identify it. The minerals physical properties are determined by its chemical composition and crystal structure.

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color streak and luster

Diamonds have many valuable properties. Diamonds are extremely hard and are used for industrial purposes. The most valuable diamonds are large, well-shaped and sparkly. Turquoise is another mineral that is used in jewelry because of its striking greenish-blue color. Many minerals have interesting appearances. Specific terms are used to describe the appearance of minerals.

color

Color is probably the easiest property to observe. Unfortunately, you can rarely identify a mineral only by its color. Sometimes, different minerals are the same color. For example, you might find a mineral that is a gold color, and so think it is gold. But it might actually be pyrite, or fools gold, which is made of iron and sulfide. It contains no gold atoms. A certain mineral may form in different colors. Figure 3.11 shows four samples of quartz, including one that is colorless and one that is purple. The purple color comes from a tiny amount of iron. The iron in quartz is a chemical impurity. Iron is not normally found in quartz. Many minerals are colored by chemical impurities. Other factors can also affect a minerals color. Weathering changes the surface of a mineral. Because color alone is unreliable, geologists rarely identify a mineral just on its color. To identify most minerals, they use several properties.

streak

Streak is the color of the powder of a mineral. To do a streak test, you scrape the mineral across an unglazed porcelain plate. The plate is harder than many minerals, causing the minerals to leave a streak of powder on the plate. The color of the streak often differs from the color of the larger mineral sample, as Figure 3.12 shows. Streak is more reliable than color to identify minerals. The color of a mineral may vary. Streak does not vary. Also, different minerals may be the same color, but they may have a different color streak. For example, samples of hematite and galena can both be dark gray. They can be told apart because hematite has a red streak and galena has a gray streak.

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instructional diagrams

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questions

amount of mass per unit volume of a substance

a. cleavage

b. fluorescence

-->  c. density

d. fracture

e. hardness

f. luster

g. streak

If the volume of a mineral is 6 and the mass is 3, what is its density?

a. 6

b. 3

c. 2

-->  d. .5

how a mineral breaks when it does not break along a plane

a. cleavage

b. fluorescence

c. density

-->  d. fracture

e. hardness

f. luster

g. streak

The streak of a mineral is

a. the same as the color of the mineral

b. never the same color of the mineral

-->  c. the same even when the same mineral is found in various colors

d. either black or white

color of the powder of a mineral

a. cleavage

b. fluorescence

c. density

d. fracture

e. hardness

f. luster

-->  g. streak

Which of the following is NOT a property used to identify a mineral?

a. radioactivity

b. cleavage

c. reactivity

-->  d. number of electrons

Mass is

a. how much space an object takes up

-->  b. the amount of matter in an object

c. how much matter takes up a certain amount of space

d. the weight of an object

tendency of a mineral to break along certain planes

-->  a. cleavage

b. fluorescence

c. density

d. fracture

e. hardness

f. luster

g. streak

ability of a mineral to resist being scratched

a. cleavage

b. fluorescence

c. density

d. fracture

-->  e. hardness

f. luster

g. streak

What mineral is number 1 on the Mohs Scale?

-->  a. talc

b. diamond

c. topaz

d. calcite

ability of a mineral to glow under ultraviolet light

a. cleavage

-->  b. fluorescence

c. density

d. fracture

e. hardness

f. luster

g. streak

how light reflects off the surface of a mineral

a. cleavage

b. fluorescence

c. density

d. fracture

e. hardness

-->  f. luster

g. streak

A minerals physical properties are determined by its

a. vitreous luster.

b. crystal structure.

c. chemical composition.

-->  d. two of the above

Factors that may affect a minerals color include

a. mass.

b. streak.

c. cleavage.

-->  d. weathering.

To do a streak test, you scrape a mineral across a

a. diamond crystal.

b. piece of talc.

-->  c. porcelain plate.

d. cleavage plane.

The color of a mineral is a more reliable test of its identity than its streak.

a. true

-->  b. false

What is the least reliable property for identifying minerals?

a. streak

b. hardness

-->  c. color

d. luster

An object with the density of 2 is denser than an object with the mass of 4 and the volume of 1.

a. true

-->  b. false

A mineral with which type of luster is soft looking with long fibers?

a. resinous

b. earthy

-->  c. silky

d. pearly

Which of the following minerals has the greatest density?

-->  a. gold

b. pyrite

c. quartz

d. fools gold

A crystal with six sides that are all the same size has a octahedral structure.

a. true

-->  b. false

Which sequence shows minerals in the correct order from softer to harder?

-->  a. gypsum, apatite, corundum

b. apatite, gypsum, corundum

c. apatite, corundum, gypsum

d. corundum, apatite, gypsum

Topaz is a harder mineral than calcite.

-->  a. true

b. false

The luster of a mineral its color under ultraviolet light.

a. true

-->  b. false

You can usually identify a mineral by its color alone.

a. true

-->  b. false

The color of a mineral is always same as the color of its powder.

a. true

-->  b. false

The streak of a given mineral does not vary.

-->  a. true

b. false

A mineral with a vitreous luster appears glassy.

-->  a. true

b. false

The mineral pyrite has a non-metallic luster.

a. true

-->  b. false

The Mohs hardness scale ranges from 1 to 100.

a. true

-->  b. false

The cleavage of a mineral depends on its crystal structure.

-->  a. true

b. false

Mica tends to form cubes when it cleaves.

a. true

-->  b. false

Some minerals have a distinctive smell.

-->  a. true

b. false

Certain minerals are attracted to a magnet.

-->  a. true

b. false

diagram questions

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