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IDENTIFICATION OF SEDIMENTARY ROCKS
Sedimentary rocks form from material which has accumulated on the surface of the Earth
under conditions we can readily observe. They are the easiest rocks to understand. We will be
looking at 9 representative samples of the major varieties. There are hundreds even thousands of
variations on the basic classification theme below.
A. Clastic or Detrital Sedimentary Rocks
These are formed from the broken bits and pieces of previously existing rocks. Quartz is
the most common mineral in these sediments because quartz is a hard, tough mineral and is
stable chemically at the surface of the Earth. Feldspars, the most common minerals in igneous
and metamorphic rocks decompose chemically into clay minerals and dissolved materials. Clay
minerals are a major component of shale, the most common sedimentary rock.
Composed of a variety of rounded grains ranging in size from
less than 1/16 mm (0.06 mm) to rocks as big as your head.
Most commonly think of a rock made up of gravel. Quartz
Pebble is the most common kind of conglomerate.
CONGLOMERATE
Similar to the above in grain size only the fragments or "clasts"
are angular. Many breccias have unique and fascinating
stories to tell.
BRECCIA
Composed of quartz grains from 2 mm down to 1/16 mm
(0.06 mm). You can test for quartz by scratching glass.
There are many other varieties of sand stone based on the
mineral composition of the sand composing them.
QUARTZ
SANDSTONE
Composed of clay minerals and clay sized (less than 1/256 mm
0.004 mm). A true shale is fissile - it splits into thin layers.
You can scratch it easily with a knife or the corner of the
glass plate. Shale can vary in color from black to gray
to red and greenish.
SHALE
B. Chemical and Biochemical Rocks
These are rocks which resulted primarily from chemical or biochemical precipitation of
material from water, usually sea water.
Composed primarily of the mineral calcite which will fizz easily
with dilute HCl (hydrochloric acid) can be scratched easily
with the corner of the glass plate. The vast majority of
limestone is the result of biological activity. This latter
fact may only revealed by microscopic study.
LIMESTONE
Similar to the above only containing abundant fossils. Sometimes the rock is entirely composed of fossil fragments.
FOSSILIFEROUS
LIMESTONE
Composed of the mineral dolomite this nondescript rock is
distinguished by being softer than glass and only the powder
fizzes in HCl. Dolostone is the result of limestone being
"dolomatized" Mg++ + 2CaCO3 = CaMg (CO3) + Ca++.
How this happens is largely a mystery.
DOLOSTONE
Hard (scratches glass), white to gray to black, breaks in large
FLINT when black
smooth curved surfaces. Commonly has a soft candle wax luster. or CHERT when white
Composed of microcrystalline quartz, Native Americans used
this rock for arrow heads, scrapers and knives because it could
be flaked into sharp edges.
Composed entirely of the mineral halite. Usually has a peculiar
ROCK SALT
translucent look. Unfortunately the only sure test is taste.
Yucck! This rock is the result of the evaporation of some large body of
water, usually sea water.
Black! Obvious low specific gravity, soft and soils hands.
The result of terrestrial (on land) accumulation of biological
material. The evidence for the environment of accumulation
usually suggests a swamp.
COAL
C. There are many variations on these basic themes.
shaley limestone
limey shale
sandy shale
shaley sandstone
dolomitic limestone cherty limestone
These are just a few variations based on combining basic forms. There is a world of
sedimentary rocks when the simple mineralogy of the rocks discussed is varied.
D. The Exercise:
1. Write the number of the specimens in numerical order.
2. Describe grain size and/or texture
microscopic
fine - less than .5 mm ( mm scale on reference chart)
coarse - .5 mm to 2 mm
very coarse - larger than 2 mm
smooth
3. Color/hardness
As well as color, will it scratch glass or glass scratch it?
4. Mineral composition and other properties.
Minerals detected
Quartz - hardness & luster
Halite - look and taste
Calcite - fizzes in HCl
Dolomite - only powder will fizz
Properties observed
Fizzes in acid
Splits into thin layers
Smooth fracture pattern
Fossils present
Tastes salty
5. Name the rock.
E. Structures in Sedimentary Rocks
Explain what the following structures might tell you about the environment when the
sediment was being deposited. The structures listed will tell you whether the deposit of
sediments was on land or under water. They may also hint of climate and even whether it was
fresh water or sea water.
e.g. Mud Cracks
Ripple marks and "fossil"
It might have been a tidal flat. When the tide went out,
the mud dried in the sun and cracked.
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dune structures in a sandstone
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Fossil corals and sponges
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Clam fossils in a sandstone
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Other Rocks
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Observations
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