MINERALS AND WEATHERING

 

A.  INTRODUCTION

        The risks of chronic exposure associated with our natural, physical
        environment are largely the result of inhaling or ingesting elements
        derived from geologic materials (rocks and minerals).  Therefore, to
        understand the risks from chronic exposure, we should understand
        (1) how elements are distributed in the geologic environment, and
        (2) how elements are released into the air, water, and soil. 

 

B.  BASIC GEOCHEMISTRY

        1.  EARTH'S COMPOSITION

             a.  There are 90 naturally occurring elements.

             b.  Eight elements compose 98.7% of the earth's "average" crust:
                  the others occur in "trace" (i.e., very low) concentrations.

       

        2.  MINERALS

             a.  Definition: naturally occurring, inorganic solids characterized
                  by a narrow range in composition.

             b.  There are ~ 3,000 minerals, but only about 20 are common.
                  

 

       3.  ROCKS

             a.  Definition:  solids composed of one or more minerals.

             b.  There are hundreds of rocks, but only about 25 are common.

 

C.  THE ORIGINS OF ROCKS AND MINERALS

        1.  MINERALS

              a.  Silicates:  minerals that crystallize from cooling magma

                   Example:  quartz (SiO2)
 

              b.  Precipitates:  minerals that precipitate from fluids

                   Evaporites: precipitate from water   

                   Example:  halite (NaCl), (i.e., salt, or rock salt)
 

                   Hydrothermal Deposits:  minerals left by magmatic fluids   

                   Example:  galena (PbS), Wisconsin's State Mineral
 

              c.  Secondary Minerals:  form as a result of weathering 

                       Example:  hematite (Fe2O3), a result of oxidation

 

        2.  ROCKS

              a.  Igneous:  rocks that have crystallized from magma

                   Example:  granite, the Wisconsin State Rock
 

              b.  Sedimentary:  rocks formed from deposits of sediments

                   Examples:  mudstone (shale), siltstone, and sandstone
 

              c.  Metamorphic:  rocks changed by heat and pressure

                   Example:  slate is metamorphic shale

D.  WEATHERING

        Rocks weather physically and chemically to produce regolith.
 

        1.  PHYSICAL DISINTEGRATION   (Old Man of the Mountain: article)

              a.  Unloading (exfoliation or sheeting):  Stone Mountain

 

              b.  Ice (Frost) Wedging

 

              c.  Plant Roots

 

              

        2.  CHEMICAL DECOMPOSITION

              a.  Solution

 

              b.  Oxidation:  oxidized soil

 

              c.  Hydrolysis

 

 

        3.  RATES OF WEATHERING

              a.  Depends on rock composition:  Appalachian Mtns.

 

              b.  Depends on climate:  Cleopatra's Needle

  

 

          

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