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SEDIMENTARY ROCKS
Lesson 4 - Page 2

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EARTH SCIENCES: SEDIMENTARY ROCKS LABORATORY II

PROBLEM: What would you look for to help you identify sedimentary rocks and the environments where they formed?

HYPOTHESIS:

MATERIALS: Earth Science - Sedimentary Rocks, Swift GH microscope or Hand Lens 

PROCEDURE:

PART I: Answer the following questions. Be sure to examine each specimen using the classification charts to help you.

1. CHERT

Chert is composed of microscopic quartz (SiO2) crystals. The formation of chert is very puzzling because chert rarely can be found forming today. However, some ancient cherts contain fossils of radiolarians (visible only with a microscope), which are a type of one celled protozoa that live in the ocean. When radiolarians die, their shells sink to the deep ocean floor. This suggests that many ancient cherts were formed in deep marine water. Cherts can be red, black, white, or gray in color.

A. Describe the color of this rock

B. Chert is sometimes used to make roads and highways. Why do you think chert is a good material for this purpose?

C. If chert contains radiolarians, in what environment did this rock form?

2. SANDSTONE

A. What shape are the rock and mineral particles that make up this sandstone?

B. Describe the size and sorting of the particles.

C. Was sandstone cemented together or melted together? Explain your answer.

3. MUDSTONE

Mudstone is a fine grained clastic sedimentary rock. Mudstones are composed mainly of clay minerals, quartz, and mica, but the composition has to be determined by a microscope. Many mudstones consist of very thin layers, about as thick as the pages in a book, called laminations. Mudstones range in color from gray to white, brown, red, green to black. Mudstones indicate a quiet water, low energy sedimentary environment, like the deep ocean or a lake bottom.

A. Does your mudstone sample make a thud or a ring when lightly (6 inches from tabletop) dropped?

B. Are the grains in mudstone visible?

C. Does mudstone feel smooth or coarse?

D. Is mudstone layered or banded?

4. DIATOMITE

Diatomite has the same size particles as mudstone. However, the particles arenít clay or quartz, but are the skeletons of a one celled plant called diatoms. Diatoms live near the ocean surface. When they die, their skeletons accumulate to create a mudstone-like deposit of on the sea floor.

A. What is the relative density of this diatomite (light or heavy?) Explain your answer.

B. What is the size of the particles making up this diatomite?

C. Diatoms are one celled plants that either live in marine or fresh water. Radiolarians are one celled protozoa that eat diatoms. Radiolarians only live in marine water. If there are both diatoms and radiolarians in this rock. Where did it form?

D. Put the diatomite sample on the tip of your tongue. What happens?

You have just found out that there is a mineral called kaolinite in this rock. Kaolinite is very effective at absorbing water. What over the counter medicine has "kao" in its name?

What mineral do you think is in this medicine?

5. SHALE

Shales are similar to mudstones in composition and texture, but are composed of slightly larger particles. They are also deposited in quiet water environments.

A. Examine this specimen with a hand lens or microscope. Can you describe the roundness, sorting and size of the particles?

B. What kind of material (hint - what mineral?) may cement the particles in your shale sample together, if any? How can you tell?

C. Does your sample show any other interesting features?

6. LITHOGRAPHIC LIMESTONE

Lithographic limestone is composed of the mineral calcite (CaCO3). The mineral crystals are very small. They are visible as tiny sparkles when you look at a piece of it in bright light. Lithographic limestones are created in two ways. Most are composed of microscopic skeletons of marine plants and animals, which were cemented together after they died. Others formed by mineral precipitation from lake or ocean water.

A. Describe your specimen.

B. Are crystals visible?

C. Lithography is a type of printing, often used for illustrations. Why do you think this type of rock was used for this purpose?

7. FOSSILIFEROUS LIMESTONE

This type of limestone is made from visible fossils that have been cemented together. Most fossiliferous limestones formed in the ocean. They may contain fossils of molluscs like clams and snails, coral, or crinoids (sea lilies), or even fish bones. This type of rock contains important information about the history of life on Earth.

A. Describe your specimen. What color is it? How much of the rock is composed of fossils (give a percentage)?

B. Identify the fossils in the rock, if you can.

8. COAL

Coal is composed of fossil plants which have been buried, heated, and put under pressure. These changes have altered the original plant material into simple hydrocarbon compounds. These burn easily, which is why coal is used as a fuel. There are three types of coal: anthracite, bituminous coal, and lignite. Anthracite is shiny black and hard; bituminous coal is duller and sometimes a black brown color; and lignite is dull brown and soft.

A. Describe your specimen.

B. Which type of coal do you have?

C. Compare your specimen of coal with charcoal. Describe the similarities and differences.

9. QUARTZ is a mineral with the chemical composition SiO2. It is a common cement in sedimentary rocks; it forms the "glue" which holds the grains together. Quartz cements form when water carrying dissolved quartz flows through the spaces between sediment grains. Given the right chemical conditions, quartz crystals will begin to precipitate on the surfaces of the grains. As the crystals grow, they fill up the spaces, making a solid rock.

A. Why is quartz a good cementing agent?

10. GEODE. A geode is formed when an open space in a rock is filled by precipitated minerals, usually calcite or quartz. Like making cements, this happens as water flows through the open space. The crystals begin forming around the edge of the space, and grow inward. To be a geode, there must be some remaining open space in the specimen. Geodes that completely "fill up" are called thunder eggs.

A. How could you determine what mineral makes up this geode?

B. How can you identify if a rock is cemented by quartz?

11. CALCITE (CaCO3) is another common sedimentary rock cement. It forms in a similar fashion to quartz cement, by precipitation of minerals in the spaces between sediment grains, making a solid rock.

A. How can you tell if you have calcite cement? (Hint: what chemical reaction helped you recognize calcite in the minerals lab?)

B. Which cementing agent is harder, calcite or quartz? Explain your answer.

PART II.

Using your answers to the above questions and the identification charts, fill in the table below.

ROCK/SAND

DESCRIBE

ie. color, sorting, crystals

ENVIRONMENT THAT IT WAS FORMED IN

lithographic limestone

fossiliferous limestone

chert

sandstone

mudstone

 

diatomite

siltstone

quartz

calcite

geode

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