Page 198 - Laboratory Manual in Physical Geology

B. Place a charcoal briquette into a plastic sandwich bag and take it to the hammering station in your lab. Lightly hammer the
bag enough to break apart the briquette. Return to your table with the bag of charcoal.

1. View the broken pieces of charcoal with a hand lens. Describe what kinds of grains you see and their texture.

2. Charcoal is made by allowing wood to smolder just enough that an impure mass of carbon remains. In the presence of

oxygen, the charcoal briquette will naturally combine with oxygen to make carbon dioxide. Over a period of many years,
it will all react with oxygen and chemically weather to carbon dioxide. When you burn charcoal in your grill, you are
simply speeding up the process. However, if plant fragments are buried beneath layers of sediment that keep oxygen away
from them, then they can slowly convert to a charcoal-like rock (peat, lignite, or coal) and remain so for millions of years.
Obtain a piece of coal and compare it to your charcoal. How is it different? Why?

C. REFLECT & DISCUSS Based on your observations in this activity, write a definition of biochemical sedimentary rock in
your own words.

D. Bedrock can remain buried underground for millions to billions of years. However, when it is exposed to water and air at

Earth’s surface it weathers chemically and physically. For example, acidic water reacts with potassium and plagioclase feldspars
to make clay minerals plus water containing dissolved silica (hydrosilicic acid) and metallic ions (K, Na, Ca). This is one
of the main sources of clay found in soil and worn away into rivers and the ocean. The metals in many minerals oxidize
(combine with oxygen) to form metal oxides like limonite (“rusty” iron) and hematite. Obtain and observe samples of both.

1. What is the color and chemical formula for hematite? (Refer to Minerals Database, page 95 )

2. What is the color and chemical formula for limonite? (Refer to Minerals Database, page 96 )

3. As iron oxides form, they act like glue to cement together grains of sediment, like the
“sandstone” above. Which iron oxide mineral has cemented together this sandstone? How sandstone 1
can you tell?

4. Powder some limonite in a mortar and pestle, and note its true streak color (yellow-brown). Put on safety goggles. In a
fume hood or behind a glass shield, heat some of the powder in the Pyrex test tube over the Bunsen burner. Be sure to
point the test tube at an angle, away from people. After about a minute of heating, pour the hot limonite powder onto the
foil on the table. What happened to the yellow-brown limonite? Why?

5. REFLECT & DISCUSS The rapid chemical change that you observed above can occur quickly only at temperatures like
those above the Bunsen burner. However, some modern desert soils do contain hematite and appear red. How can that be?

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