EXERCISE #6: ROCK CHEMISTRY AND WEATHERING
To complete this exercise,
you will need to refer to the discussion of minerals and rocks in your text
(pages 26 to 38). It will also be
helpful to read the section on chemical weathering (pages 172 to 175). The
questions are based on these readings, your lecture notes, and the three tables
provided with this assignment.
Table 1 lists
13 of the common rock-forming minerals in order of increasing resistance to chemical weathering. For each mineral, the chemical process or
processes that cause it to decompose is also identified.
|
MINERAL NAME |
CHEMICAL FORMULA |
RELATIVE RESISTANCE |
CHEMICAL WEATHERING PROCESS(ES) |
|
Halite |
NaCl |
Very low |
Dissolution |
|
Gypsum |
CaSO4 2H2O |
Very low |
Dissolution |
|
Pyrite |
FeS2 |
Low |
Dissolution, Oxidation |
|
Calcite |
CaCO3 |
Low |
Dissolution |
|
Dolomite |
CaMg(CO3)2 |
Low |
Dissolution |
|
Olivine |
(Fe, Mg)SiO4 |
Moderately low |
Oxidation, Hydroloysis |
|
Pyroxene |
Ca, Mg,
Fe, Al Silicates |
Moderate |
Oxidation, Hydrolysis |
|
Plagioclase |
CaAl2Si2O8 NaAlSi3O8 |
Moderate |
Hydrolysis |
|
Hornblende (Amphibole) |
Na, Ca,
Mg, Fe, Al Silicates |
Moderate |
Oxidation, Hydrolysis |
|
Biotite |
K, Fe, Mg, Al Silicate |
Moderate |
Oxidation, Hydrolysis |
|
K-Feldspar |
KAlSi3O8 |
Moderately high |
Hydrolysis |
|
Muscovite |
K, Al Silicate |
High |
Hydrolysis |
|
Quartz |
SiO2 |
Very high |
Dissolves only very slowly |
Table 2 lists common rock
types and their
mineralogic composition and relative resistance to chemical
weathering. Note how the resistance of a
rock to chemical weathering is related to its mineral composition and the rate
at which those minerals weather (refer to Table 1).
TABLE 2: CHEMICAL WEATHERING OF COMMON ROCKS
|
TYPE OF ROCK |
DOMINANT MINERALS PRESENT
IN ROCK |
RESISTANCE TO CHEMICAL WEATHERING |
|
Granite |
Quartz and K-feldspar |
High |
|
Basalt |
Olivine, Pyroxene |
Moderate |
|
Quartz-cemented Sandstone |
Quartz and K-feldspar |
High |
|
Calcite-cemented Sandstone |
Quartz, K-feldspar and Calcite |
Low |
|
Siltstone |
Quartz, K-feldspar, and Clays |
High |
|
Shale |
Quartz and Clays |
High |
|
Limestone |
Calcite |
Low |
|
Rock Salt |
Halite |
Low |
|
Rock Gypsum |
Gypsum |
Low |
|
Slate |
Quartz, Biotite and/or Muscovite |
High |
|
Phyllite, Schist |
Quartz, K-feldspar, Biotite and/or
Muscovite, and Amphibole |
High |
|
Gneiss |
Quartz, K-feldspar,
Plagioclase, Biotite, and Amphibole |
High |
|
Marble |
Calcite |
Moderate |
|
Quartzite |
Quartz |
Very High |
TABLE 3:
AVERAGE CHEMICAL COMPOSITION OF THE EARTH'S CONTINENTAL CRUST, RIVERS
AND GROUNDWATER
|
ELEMENT or COMPOUND |
CONTINENTAL CRUST1 |
RIVERS2 |
GROUNDWATER2 |
|
Silica (SiO2) |
74.3 % |
10.9 % |
3.0 % |
|
Aluminum (Al) |
8.1 % |
< 0.1 % |
< 0.1 % |
|
Iron (Fe) |
5.1 % |
0.1 % |
< 0.1 % |
|
Calcium (Ca) |
3.6 % |
12.5 % |
11.2 % |
|
Sodium (Na) |
2.8 % |
5.3 % |
10.0 % |
|
Potassium (K) |
2.5 % |
1.9 % |
0.7 % |
|
Magnesium (Mg) |
2.1 % |
3.4 % |
2.5 % |
|
Bicarbonate3 (HCO3) |
0.5 % |
48.7 % |
62.5 % |
|
Sulfate (SO4) |
< 0.1 % |
9.3 % |
10.0 % |
|
Chloride (Cl) |
|