Download Lab 2

Geology 103
Name:
Lab 2: The rocks
Outcome: The continents and seafloors are made of rocks, which are in term made of
minerals. The properties of the rocks will influence the process of plate tectonics. In
this lab, you will discover some of the properties of different minerals and rocks.
Igneous rocks
All of these elements occur in the crystal structure of eight common igneous rockforming minerals. These minerals in turn make up 95% of the volume of all igneous
rocks. The names and chemical formulae for these minerals:
Olivine
Plagioclase
Pyroxene
Amphibole
Biotite
Orthoclase
Muscovite
Quartz
(Mg,Fe)2SiO4
NaAlSi3O8 (sodium-rich) CaAl2Si2O8 (calcium-rich)
Complex Ca-Mg-Fe-Al silicates
Complex water-bearing Na-Ca-Mg-Fe-Al silicates
K(Mg,Fe)3AlSi3O10(OH)2
KAlSi3O8
KAl2(AlSi3O10)(F,OH)2
SiO2
The list of minerals is in decreasing order of freezing (melting) temperature of the
mineral; in other words, olivine freezes from a magma first as the magma cools, and
quartz last.
1. Using the mineral ID charts in the rock handbook, identify the eight common
igneous rock-forming minerals. I don’t care how you do it, but do list the features of the
mineral that most distinguished it to you.
Mineral #
M1
M7
M9
M10
M11
Mineral name
Distinguishing characteristic(s)
M12
M13
M14
Finally, those minerals made mostly of elements from the first column of elements on the
previous page are called felsic, and those minerals made mostly of elements from the
second column of elements on the previous page are called mafic (look at the first initials
of the top three element symbols in the second column).
2. One way to separate minerals is according to elemental composition, which, fortunately
for us, is roughly associated with color (as seen in the mineral ID charts): darker minerals
tend to be mafic, and lighter minerals tend to be felsic. The exceptions will be calcium-rich
plagioclase, which is considered to be mafic, and biotite, which is considered to be felsic.
Color
Minerals
Average specific
gravity
(density)
Diagnostic
chemistry
Darkcolored
(mafic)
3.3
Magnesium,
aluminum and
iron-rich
Lightcolored
(felsic)
2.8
Silica-rich
3. Look at rock samples R4 and R6. Identify the minerals that are present in each
sample (a hint is given as to the color of each mineral), then determine if the rock
overall is mafic, felsic or intermediate, based on the table in question 2.
Sample
Minerals present
Mafic, felsic or
intermediate?
Pink =
R4
White =
Translucent =
Dull black =
R6
Shiny black =
Dull green =
4. So what is the difference between a mineral and a rock?
5. An igneous rock may be identified by either its texture or its characteristic
mineralogy and grain size. Mineralogy means “are the minerals in the rock generally
felsic, generally mafic, or even amounts of both (intermediate)?”. Grain size means
“what is the predominant size of the minerals in this rock? Sand-size? Silt-size? Claysize?” Texture means “overall, what sort of distinguishing irregularities occur over the
whole rock? Bubbles (vesicles)? Shattered bits of other rock?” Using Tables B-1a (grain
size and mineralogy) and B-1b (texture), identify the following rocks:
(BIG hint: use Table B-1a for R4 – R9 and use Table B-1b for R12 – R16)
Rock #
R4
R5
R6
R7
R8
R9
Felsic, mafic or
intermediate?
Predominant
grain size
Rock name
Rock #
Texture
Rock name
R12
R13
R14
R15
R16
The viscosity of a fluid is its ability to resist flow; higher viscosity means that the fluid
doesn't flow very fast. You will easily be able to outrun a viscous lava flow; you will not
be able to outrun a non-viscous (runny) lava flow. The reason for this lies in the
chemical composition of the lava: the silicate ion is not as mobile (in other words,
tends to stick to other silicate ions) as metal (e.g., iron, magnesium, aluminum) ions.
6. Answer the following questions with mafic, felsic, intermediate or any:
a. Which magma contains more silica?
b. Which magma will be erupted at an initially lower temperature?
c. Which magma will have higher viscosity?
d. Which magma will have higher volatile (gas) content?
e. Which magma will likely erupt explosively?
f. Therefore, which magma will likely end up as a widespread ash?
Table B-1a. Igneous rocks which are identified by their
composition/grain size
Felsic
Intermediate
Mafic
quartz
orthoclase
biotite
Na-plagioclase
Na-plagioclase
amphibole
pyroxene
biotite
Ca-plagioclase
olivine
pyroxene
Coarse-grained (mostly
visible grains)
GRANITE
DIORITE
GABBRO
Fine-grained (mostly
invisible grains)
RHYOLITE
ANDESITE
BASALT
COMPOSITION
(Minerals present)
TEXTURE
Table B-1b. Igneous rocks which are identified by their texture
1. Is the rock glassy on any fresh surface?
Yes — Obsidian
No — Go to #2.
2. Is the rock vesicular (containing gas bubbles)?
Yes — Go to #3
No — Go to #4
3. Is the rock dark-colored on a fresh surface? Yes — Scoria
No — Pumice
4. Is the rock composed of large pieces of broken rocks?
Yes — Breccia
No — Go to #5
5. Is the rock well-cemented (doesn't crumble easily)?
Yes — Welded tuff
No — Tuff
Tuff and pumice are sometimes difficult to tell apart; however, a simple test
distinguishes them: pumice floats on water and tuff sinks.
Sedimentary rocks
Sedimentary rocks are deposited. Using the agents of transport (wind, water or ice),
the loose material (called sediment) is compacted (compressed so that the pore space is
removed) and cemented (what pore space remaining is filled with a different chemical)
into a sedimentary rock.
Sediment is made by the erosion of weathered rocks. Weathering is the process that
either chemically or physically breaks up fresh rock material; an example of chem-ical
weathering is a rock reacting with acid rain, while an example of physical wea-thering is
when water freezes with a crack in the rock and makes the crack bigger.
Though most sediment deposits on the Earth’s surface (which includes the bottom of the
ocean), sediment must be buried to achieve proper lithification. During lithification, the
sediment may be heated but nothing close to melting. Instead, the heat may cause
chemical reactions that alter some of the minerals; for instance, calcium carbonate in the
mineral aragonite may repack into calcium carbonate in the mineral calcite. These kinds of
changes are called diagenesis.
Sedimentary rocks fall into two categories: clastic sedimentary rocks, which are those that
are made of weathering products of pre-existing rocks, and chemical or biochemical
sedimentary rocks, which are the result of either evaporation or precipitation of
mineral-rich water or the accumulation of the remains of biological organisms. To sum
up: “Some sedimentary rocks are born, not made, but all are deposited”.
Many of the same elements that occur in the igneous rock-forming minerals also occur
in sedimentary minerals. However, since weathering reactions occur, the elements
will rearrange and form new crystal patterns, which will have different mineral names.
In addition, the surface environment contains compounds such as water (H2O) and
carbon dioxide (CO2) that will react and become part of the sedimentary minerals.
Sedimentary minerals are the most common minerals found on the earth’s surface
because sedimentary rocks make up most of the earth’s surface. The names and
chemical formulae for these minerals:
Quartz
Calcite
Gypsum
Halite
Pyrite
Magnetite
SiO2
CaCO3
CaSO4•2H2O
NaCl
FeS2
Fe3O4
Some minerals you will not see in lab, because they are clay minerals and thus
difficult to identify without more sophisticated equipment:
Kaolinite
Al2Si2O5(OH)4
Illite
Smectite
Complex hydrated potassium and aluminum sheet silicates
Complex hydrated sodium and aluminum sheet silicates
7. Again using the mineral ID charts and any means possible, identify the six common
sedimentary rock-forming minerals (i.e., the list given above, except for the clay
minerals). Note that one of these will be the same as one you saw in the igneous
minerals portion of this lab. Give a distinguishing characteristic of each mienral.
Mineral #
Mineral name
Distinguishing characteristic(s)
M1
M2
M8
M15
M16
M17
Properly speaking, magnetite is also formed as an igneous mineral, but not a major one.
Note the abundance of elements like oxygen (O) and sulfur (S) in sedimentary minerals;
these elements help stabilize metal ions at the Earth’s surface and will play a large role
in the preservation of fossils.
Sediment and the process of making sedimentary rocks
Find the drawer with the weathering samples. Samples W1 and W2 are igneous rocks
from the first part of the lab. Sample W3 is a partially weathered igneous rock. Sample
W4 is beach sediment.
8. Identify W1 and W2 (hint: you’ve seen these before in the previous section).
9. Did sample W3 weather from W1 or from W2?
10. Look at sample W4. Did this sediment weather from W1 or from W2?
11. Obtain rock specimens R4, R18, R19 and R29. When sample W4 lithifies, which
rock will it become? What’s a generic name for this type of rock? Is it the same rock as
the rock you believe W4 originated?
12. As mentioned above, after compacting sediment, in order to make a sedimentary
rock, the sediment needs to be cemented by another chemical agent. If there are only the
following three cement materials available, how would you identify each cement (think
of a test for each)?
calcite:
iron oxide:
silica:
13. Look at rock samples R18 and R19. What is the cement that holds each rock’s grains
together?
R18 =
R19 =
14. How would you be able to tell apart a sedimentary rock made of calcite from one that
was made of quartz grains cemented by calcite? (Hint: is the acid test useful here? What
other test might be useful?)
15. Sedimentary rocks will be identified, in large part, to the materials they are made of.
Fill in the table with the most common mineral of each rock. Note that this question is
not asking you about the cement of each rock!
Sample
R23
R25
R27
Most common mineral
Identifying sedimentary rocks
16. Fill in the following table using the clastic Sedimentary Rock ID flow chart. Note there
are separate tables for clastic and chemical/biochemical sedimentary rocks.
For clastic rocks:
• Determine the most common grain size
• Determine the degree of grain rounding (choose from: angular, subangular,
subrounded or rounded)
• Determine the degree of sorting of the grains that occurs in the rock (choose from:
well-sorted, moderately-sorted or poorly-sorted)
• Then, based on your observations and the ID table, identify the rock.
• Finally, determine whether the sediment that makes up the rock was deposited in a
low-energy, medium-energy or high-energy depositional environment.
Flow chart for identifying clastic sedimentary rocks — If the rock is made of grains or
other materials which have been deposited by wind, water or ice, it's a sedimentary rock.
For any sedimentary rock, if it contains any fossils, use the adjective fossiliferous in
front of the rock name.
Consider the most common grain size in the rock from the following list.
cobble or
pebble
sand
silt
clay
> 2 mm
0.062 — 2 mm
0.005 — 0.062
mm
< 0.005 mm
easily visible to naked eye; "grains" may
contain identifiable minerals
visible to naked eye;
not visible but can be felt between fingers or
across teeth
not visible; cannot be felt between fingers or
across teeth
If the most common grain size is cobble or pebble (it may contain other grain
sizes in its matrix) → conglomerate
If the most common grain size is sand → sandstone (don’t stop here)
If the most common mineral is quartz → arenite
If the most common mineral is feldspar (orthoclase or plagioclase — rock
may look gray or pink) → arkose
If there are many dark rock fragments and a significant volume of the rock
is silt or clay size→ graywacke
If the most common grain size is silt → siltstone
If the most common grain size is clay → claystone
If there is both silt and clay in the rock → shale
Clastic sedimentary rocks
Sample #
Grain
size
Grain
rounding
Grain
sorting
R18
Medium
Well to subrounded
Well-sorted
Rock name
Energy of
depositional
environment
Medium-energy
R19
R20
R21
R22
R29
17. Return to R18 and R19 and circle the correct answers:
a. Which rock contains the most stable mineral clasts?
(= lowest freezing point of the minerals in the rock)
R18
R19
b. Which rock is composed of rounder grains?
R18
R19
c. Which rock is more well-sorted?
R18
R19
d. Based on a-c, which sample was deposited furthest
from its source?
R18
R19
e. Therefore, which is the more mature rock?
R18
R19
18. The energy of the system (how much force is behind the medium of transport
(air or water)) can be characterized by the size of the particles the system can carry. For
instance, high-energy systems can carry large grains; low-energy systems can carry
small grains. Examine and rank rocks R19, R20 and R21 in order from highest energy
to lowest energy depositional system.
19. For chemical or biochemical sedimentary rocks:
• Write the name of the greatest percentage mineral component
• Determine if the sample was chemical in origin or biological in origin
• Determine whether the sediment that makes up the rock was deposited in a lowenergy, medium-energy or high-energy depositional environment.
• Then identify the rock, using your observations and the ID table. Remember, if there
are fossils, add the adjective fossiliferous before the rock name!
Flow chart for identifying chemical or biochemical sedimentary rocks — Identify the
most common mineral in the specimen
If the most common mineral is quartz → chert
If the most common mineral is halite → rock salt
If the most common mineral is gypsum → rock gypsum
If it is black-colored, not very dense and flaky → coal
(also look for plant fibers)
If it fizzes, the most common substance is calcium carbonate, usually in the form of
the mineral calcite (be careful you are not fizzing the cement, or mistaking the fizzing
for the fluid entering holes in the rock)
If the rock is not very dense, powdery and pure white → chalk
If the rock is made of small, round “clasts” → oolitic limestone
If the rock is made of almost exclusively broken-up shells → coquina
If the rock is banded and/or breaks into sharp-edged pieces →
crystalline limestone
If the rock is buff colored, does not fizz readily, but the powdered rock
fizzes readily → dolostone
Else → micrite
Note: all of these rocks that fizz are sometimes generically called limestone.
Chemical and biochemical sedimentary rocks
Sample #
Mineral
composition
Biological or
chemical?
R23
Calcite
Biological
(clam shells)
R25
R26
R27
Rock name
Energy of
depositional
environment
High-energy
R28
R30
R31
R32
R33
20. R31 is deposited under high-energy conditions, usually a tropical beach. How do the
individual “clasts” (called ooids) in R31 form? Write a paragraph explaining the
formation of these ooids. Hint: At the center of the ooid is typically a grain of sand or a
fecal pellet. Also, tropical seawater is saturated in dissolved calcium carbonate.