Soils 205- General Soils

Lecture 16- Soil Colloids

I.  Properties of colloids

    Size:

    Surface area:

    Surface charge:

    Adsorption:

    a.  Types of soil colloids

        1)  Layer silicates- layer silicate clays have a crystalline, layerlike structure comparable to the pages of a book.  They are the major group of soil colloids and we will discuss their specific structures in great detail.

        2)  Fe and Al oxides- These types of materials are typically dominant in the highly weathered soils found in the tropics and semitropics, but are also found in some temperate regions.  The charge is determined by pH.

                            Fe-O^-  +  H^{+  --}>  Fe-OH  +  H^{+  --}>  Fe-OH₂⁺

                                    ^{higher pH                                                            lower pH}

     examples:

    Gibbsite Al(OH)₃

    Goethite FeOOH

    Hematite Fe₂O₃

       3)  Amorphous materials (short-range ordered minerals)- SROM materials do not have a crystalline structure sufficiently ordered to be detected by x-rays.  These materials are typically found in soils with volcanic parent materials.

    Examples:

    Allophane Al₂O₃^.2SiO₂H₂O

    4)  Organic soil colloids- The colloidal structure of organic material is somewhat similar to clays in that a highly charged micelle is surrounded by cations.  In this type of colloid, most of the charge origniates from the OH groups that are exposed on the surface of the colloid.

II.  Silicate clay minerals

        a.  building blocks

       b.  isomorphic substitution- ions of roughly the same size (ionic radius) may substitute for one another.  

      ISOMORPHIC SUBSTITUTION TAKES PLACE AT THE TIME OF MINERAL FORMATION

        Examples:

        1)

        2)

        Isomorphic substitution is important because it leads to permanent charge in clay minerals

    c.  mineral organization

    Silicate clay minerals are often placed into one of two categories (1:1 or 2:1 minerals) based on their structure.

        i.  1:1 type minerals- general structure

            a.  example of 1:1 type mineral- Kaolinite

            important characteristics of Kaolinite

            1)

            2)

            3)

            4)

    ii. 2:1 type minerals- general structure

        a.  2:1 example- Mica

            important characteristics of Mica

            1)

            2)

            3)

            4)

            5)

        b.  2:1 example- Montmorillonite

         important characteristics of Montmorillonite

        1)

        2)

        3)

        4)

        5)

       iii.  comparison of important properties:

    iv.  Genesis of silicate clays

 different silicate clays forming under different climatic conditions

    -soils in a dry environment tend to have more Montmorillonite from less extreme weathering (Western United States)

    -soils in humid environments tend to have more kaolinite due to more extreme weathering (Southeastern United States)