Chapter 2 Inorganic Solids in Soil continued. .
-
date post
20-Dec-2015 -
Category
Documents
-
view
222 -
download
2
Transcript of Chapter 2 Inorganic Solids in Soil continued. .
Chapter 2Inorganic Solids in Soil
continued
http://www.uwgb.edu/dutchs/graphic0/rockmin/ion-cov.gif
http://www.webchem.net/images/bonds/covale2.gif
Nuclei repel, but are attracted by the pair of negative electrons.
What holds a covalent bond together?
http://www.gly.fsu.edu/~salters/GLY1000/6_Minerals/6_Minerals_index.html
Pauling’s Rules predict configuration of atoms into a crystal structure
Soil clay minerals Silica Tetrahedrons – one building
block of soil minerals
Crystal pictures are from Bob Harter at Univ. of New Hampshire http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
Various linkages of the tetrahedra create classes of silicates
www.indiana.edu/~geol116/week2/sillmin.jpg
www.winona.edu/geology/MRW/minrx.htm
socrates.berkeley.edu/~eps2/wisc/Lect4.html
Figure 1: Single silica tetrahedron (shaded) and the sheet structure of silica tetrahedrons arranged in a hexagonal network.
http://www.britannica.com/ebc/article-80127
clay mineral: hexagonal tetrahedral sheets
http://www.britannica.com/ebc/article-80127
Aluminum Octahedrons – another building block or layer in minerals
Single octahedron (shaded) and the sheet structure of octahedral units.
http://www.britannica.com/ebc/article-80127
Primary Soil Minerals
• Not chemically altered or significantly weathered • Inherited from original crystallization or
deposition• Found mostly in the sand and silt fractions• Ex: Quartz, Feldspars/Plagioclases, Amphiboles,
Pyroxenes, etc. (Sparks, Table 2.2 p. 46)• Source of Na, Mg, K, Ca, Mn, and Fe ions as
they weather/decompose.• Also source of trace elements and heavy metals
in soils.
Photograph taken by Klaus-Peter Kelber
http://www.mineralogie.uni-wuerzburg.de/gallery/Seiten/quartz2.htm
Secondary Soil Minerals
• Low-temperature weathering product of primary minerals– Structural alteration of primary minerals– Precipitation out of solution– Inherited from sedimentary rocks
• Predominant in clay fraction (<2µm)• Very reactive chemically and physically• Source of readily available nutrient ions (Ca, Mg,
K, NH4, S, Fe…)• Ex: Phyllosilicates (kaolins, smectites, illites);
oxides and hydroxides, carbonates, sulfates, etc.
Phyllosilicates or “layer” silicates
• Most abundant in the clay-sized fraction (hence referred to as “clay minerals”
• Very high surface area + unsatisfied charges = very reactive
(both physically and chemically)
• Composed of sheets of SiO4 tetrahedra + Al or Mg octahedra
• Strong ionic/covalent internal bonding and weaker H or van der Waals bonding between the layers
Ionic radius (size) determines the coordination with ligands (O or OH)
-face- -edge-
http://intro.chem.okstate.edu/1314F97/Chapter8/Ionic%20Radii3.Gif
Isomorphous Substitution
Substitution, during formation, of one ion for another of similar SIZE (but not necessarily the same charge) in an ionic solid without changing the structure (shape, morphology) of the crystal.
Isomorphic = “same shape”
Schematic representation of 2:1 clay mineral such as montmorillonite, indicating locations of substitution sites on tetrahedral and octahedral layers and the hydrated interlayer cations.
www.sandia.gov/geobio/randy.html
Layer charge
Results from isomorphic substitution with ions of different charge:
Al+3 for Si+4 in tetrahedra = -1
Mg+2 for Al+3 in octahedra = -1
Fe+2 for Al+3 “ “ = -1
Li+ for Mg+2 or Al+3 “ = -1 or -2
Negative charge must be neutralized by cations adsorbed on the mineral surface or in the interlayer (between the sheets) region