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. 

 

 

TABLE 1:  CHEMICAL WEATHERING OF COMMON MINERALS

 

 

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)	< 0.1 %	6.5 %	5.0 %
All other elements	0.5 %	1.4 %	0.6 %

 

¹ Percentage of element or compound on a weight basis for "average" continental crust, including

   igneous, metamorphic and sedimentary rocks.

² Percentage of element or compound on a weight basis for the total dissolved solids in rivers or in

   groundwater.

³ Note that in crustal rocks, this compound appears as carbonate (CO₃)

 

 

1.  Does there appear to be any correlation between the resistance of a mineral to chemical

weathering and the process by which it is weathered?

 

 

 

 

 

2.  Hypothesize a reason for the differences in the average crustal rock composition and the
     average composition of groundwater.

 

 

 

3.  What auxiliary assumption(s) is (are) needed for your hypothesis?

 

 

 

 

 

 

 

 

 

 

4.  Caves are typically found in limestone, rock salt, rock gypsum, dolomite and sometimes
     marble.  Hypothesis a reason for this.  How might climate affect the formation of caves?

 

 

 

 

 

 

 

 

 

 

 

5.  Trace elements that occur in  silicate  minerals (i.e., minerals containing both Si and O)
     usually do not end up in groundwater at high enough concentrations to pose a threat to
     human health; but if those same trace elements occur in a sulfur (S) mineral, they often
     do exceed human health standards in groundwater.  Hypothesis a reason for this.

 

 

 

 

 

 

 

 

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