# Physical Geology

101

1 2

3 4

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How many crystals do you see in this sample? _____ List the number of different minerals in the sample and give a description of each one.

How many crystals do you see in this sample? _____ List the number of different minerals in the sample and give a description of each one:

How many crystals do you see in this sample? _____ List the number of different minerals in the sample and give a description of each one:

How many crystals do you see in this sample? _____ List the number of different minerals in the sample and give a description of each one:

B. Which of these samples seems to have crystals of a valuable chemical element? _______ What element? _______

C. rEFLECT & DISCUSS Based on your observations in this activity—what is a rock, and how are rocks related to minerals and crystals?

A c t i v i t Y 3.1 Mineral and rock Inquiry Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. All of the samples below are rocks from Earth’s crust. Record how many crystals you see in each sample (Write 1, 2, 3, or many). Then make a numbered list of how many different kinds of minerals are in the sample and describe each one in your own words. Complete parts B and C.

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102

A. Indicate whether the luster of each of the following materials looks metallic (M) or nonmetallic (NM):

1. a mirror: __________ 2. butter: __________ 3. ice: __________ 4. a rusty nail: __________

B. What is the streak color (i.e., color in powdered form) of each of the following substances?

1. salt: ________________ 2. wheat: _________________ 3. pencil lead: ____________________

C. What is the crystal form (FIGURE 3.4) of the:

1. quartz in FIGURE 3.1B? __________________ 2. native copper in FIGURE 3.6? __________________

D. Look up quartz in the Mineral Database (FIGURE 3.21, page 93) to find a list of the varieties (var.) of quartz. Then identify each quartz variety below, and write its name beneath the image.

var. _________________ var. _________________ var. _________________ var. _________________

E. A mineral can be scratched by a masonry nail or knife blade but not by a wire (iron) nail (FIGURE 3.9).

1. Is this mineral hard or soft? _______________

2. What is the hardness number of this mineral on Mohs Scale? __________

3. What mineral on Mohs Scale has such a hardness? ____________________

F. A mineral can scratch calcite, and it can be scratched by a wire (iron) nail.

1. What is the hardness number of this mineral on Mohs Scale? __________

2. Which mineral on Mohs Scale has this hardness? ____________________

G. The brassy, opaque, metallic mineral in FIGURE 3.7A is the same as the mineral in FIGURE 3.8. What is this mineral’s hardness, and how can you tell?

H. Analyze the mineral samples and figure caption in FIGURE 3.16.

1. What is this mineral’s hardness (give a number or range of numbers)? _____________________________

2. Very carefully cut out the cleavage goniometer from GeoTools Sheet 1 at the back of this manual. Be sure to cut the angles as exactly as possible. Sketch the characteristic shape that this mineral breaks into. Using the cleavage goniometer, measure the angles between flat flat cleavage surfaces of this mineral in FIGURE 3.16, and record the angles here:

What is the name of this kind of cleavage?

I. A mineral sample weighs 27 grams and takes up 10.4 cubic centimeters of space. What is the SG (specific gravity) of this mineral? Show your work.

A c t i v i t Y 3.2 Mineral Properties and Crystals Name: ______________________________________ Course/Section: ______________________ Date: ___________

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103

0.1 mm

J. Analyze these two photomicrographs of ice crystals (snowflakes) by William Bentley.

1. Based on FIGURE 3.4, what is the crystal form of the top crystal?

2. Notice that the crystals are symmetrical, but not exactly. Imperfections are common in crystals, but their underlying crystal form can still be detected. To what crystal system in FIGURE 3.5 do ice crystals belong? How can you tell?

3. rEFLECT & DISCUSS The habit of snowflakes (crystals of water ice) includes a variety of different crystal forms. Why don’t all snowflakes have the same crystal form?

K. Analyze each crystalline household material pictured below and identify which crystal system it belongs to. (Use a hand lens or microscope to observe actual samples of the materials if they are available.)

1 cm

Sucrose Epsomite Halite

1 cm

1. Sucrose (table sugar) belongs to the _________________________________________ crystal system. How can you tell?

2. Epsomite (epsom salt) belongs to the _________________________________________ crystal system. How can you tell?

3. Halite (table salt) belongs to the __________________________________________ crystal system. How can you tell?

4. rEFLECT & DISCUSS Which of these crystalline household materials (sucrose, epsomite, or halite) cannot be a mineral? Why not?

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2.1

2.6–2.7

3.0–3.3

3.5–4.3

4.4–4.6

4.9–5.2

7.4–7.6

8.8–9.0

10.5

19.3

Sulfur

Quartz

Fluorite

Garnet

Barite

Pyrite

Galena

Native copper

Native silver

Native gold

Sample Mass in Grams (g)

Volume in Cubic cm

(cm3)

Specific Gravity

(SG)

Mineral Name

SG OF SOME MINERALS

104

A c t i v i t Y 3.3 Determining Specific Gravity (SG) Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Imagine that you want to buy a box of breakfast cereal and get the most cereal for your money. You have narrowed your search to two brands of cereal that are sold in boxes of the exact same size and price. The boxes are made of opaque cardboard and have no labeling of weight. Without opening them, how can you tell which box contains the most cereal?

B. Like the cereal boxes above, equal-sized samples of different minerals often have different weights. If you hold a mineral sample in one hand and an equal-sized sample of a different mineral in the other hand, then it is possible to act like a human balance and detect that one may be heavier than the other. This is called hefting, and it is used to estimate the relative densities of two objects. Heft the three mineral samples provided to you, then write sample numbers/letters on the lines below to indicate the sample densities from least dense to most dense.

(Least dense) ______________ _______________ _______________ (Most dense)

C. In more exact terms, density is a measure of an object’s mass (weighed in grams, g) divided by its volume (how much space it takes up in cubic centimeters, cm3). Scientists use the Greek character rho (ρ) to represent density, which is always expressed in g/cm3. What is the density of a box of cereal that is 20 cm by 25 cm by 5 cm and weighs 0.453 kg? Show your work.

D. Mineralogists compare the relative densities of minerals according to their specific gravity (SG): the ratio of the density of a mineral divided by the density of water. Since water has a density of 1 g/cm3, and the units cancel out, specific gravity is the same number as density but without any units. For example, the density of quartz is 2.6 g/cm3, so the specific gravity of quartz is 2.6.

Return to the three mineral samples that you hefted above, and do the following: 1. First (while they are still dry), determine and record the mass (weight) of each sample

in grams. 2. Use the water displacement method to measure and record the volume of each sample

(FIGURE 3.15). Recall that one fluid milliliter (mL or ml on the graduated cylinder) equals one cubic centimeter.

3. Calculate the specific gravity of each sample. 4. Identify each sample based on the list of specific gravities of some common minerals.

D. rEFLECT & DISCUSS Were your data and calculations accurate enough to be useful in identifying the samples? If not, how could they be made more accurate?

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- How many crystals do you see in this sample:
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- 1 a mirror:
- 2 butter:
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- 1 quartz in FIGURE 31B:
- native copper in FIGURE 36:
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- 1 Is this mineral hard or soft:
- 2 What is the hardness number of this mineral on Mohs Scale:
- 3 What mineral on Mohs Scale has such a hardness:
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