The Biogeochemistry of Soils: Soils from Stars

1
The Biogeochemistry of Soils Soils from Stars

• Composition of soils on earth is arguably
  unexpected
• Soils, and Earth, not reflective of chemistry of
  Universe
• Soils reflect chemical fractionation processes
  since beginning of universe
• Big Bang
• Subsequent star formation/collapse
• Chemical differentiation during formation of
  solar system
• Chemical differentiation during formation of
  Earth
• Late cometary additions to Earth

2
Chemistry of Solar System

• Exponential decline in abundance w/ atomic number
  (number of protons)
• Sawtooth pattern
• Elements from Fe have passed through stars
• Solar system is dominantly H and He

3
Crust vs. Solar System

• Depleted in volatiles (as other inner planets)
• Noble gases (group VIIIA)
• H, C, N
• Core formation depleted crust in siderophile
  elements (group VIIIB..)
• Crust also reflects late stage cometary additions
  of light elements, etc. including water

4
Soil vs. Crust

• Soil enriched in biochemically impt elements (C,
  N, S, Se)
• Soil depleted in alkali and alkine earths, Si, .
• Date normalized to a relatively immobile element
  (Zr)

5
Methods of (reasons for) Normalization to Index
Element
6
Weathering Losses of Elements from Soils

• As might be expected, water enriched relative to
  crust via chemical reactions
• Relative concentration related to chemical nature
  of elements and their reactivity in water and
  type of bonds they form in crust

7
Plant Composition and Soil Chemistry

• Plants reflect water chemistry (with some
  selectivity) and photosynthesis/N fixation

8
Soil Biogeochemistry Highlights

• Biological group
• Alkali/alkaline earths
• Halogens
• Rare earths
• Ti group
• Si, Al, Fe, P

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Soil Mineralogy Primary Minerals

• Minerals are associations of elements
• Mineralogical composition a function of elemental
  behavior and abundances
• O 474,000 mg/kg
• Si 277,000
• Al 82,000
• Fe 41,000
• Ca 41,000
• Na 23,000
• Mg 23,000
• K 21,000
• Relative abundance and behavior leads to reality
  that soils are dominated by aluminosilicates
  (O,Si, Al).

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Structure of Silicates

• Silica tetrahedron
• Net charge
• Role of Al
• Covalent bonds (Si-O, Al-O) vs. ionic bonds
  (cations-O)
• Bond type based on electronegativity differences
  and tendency to attract electrons
• Big differences lead to ionic bonds
• Similar electronegativities lead to covalent
  bonds
• Linage of tetrahedra dictate classes of silicates
  and their chemical behavior
• Nesosilicates
• Inosilicates
• Phyllosilicates
• Tectosilicates

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Electronegativities of the Elements

• Electonegativities dictated by position on table
  elements with outer shells almost filled highly
  electonegative, those just starting new shell
  not.
• Si-O form mainly covalent bond

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The Silica Tetrahedron

• 1 Si, 4 O -4 net charge
• Tetrahedra can be linked by sharing O, thereby
  reducing net negative charge.
• Class of silicate is determined by number of
  shared O, and need for cations to neutralize net
  negative charge

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Nesosilicates Singe Tetrahedra Linked with
Cations

• Foresterite
• Single tetra linked with Mg2
• Other minerals in group have all Fe2
• Highly susceptable to chemical weathering via
  ejection of cations by acid (H)
• Products then form secondary silicates and oxides

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Inosilicates Chains

• Diopside
• Single chains

• Tremolite
• Double chains

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Phyllosilicates Sheets

• Phlogopite
• trioctahedral w/ Mg2
• K strongly adsorbed in cavities

• Muscovite
• dioctahedral w/ Al3

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Tectosilicates Framework

• Anorthite (Ca)
• 50 Al for Si substition

• Albite (Na)
• 25 Al substition

• Quartz
• No substition/O charge

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Primary Silicate Summary
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Mineralogical Composition of Igneous Rocks
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Stability of Primary Minerals in Soils

• Increasing Si/O ratio increases stability
• More covalent bonds
• Fewer ionic bonds
• Less susceptable to acids
• Decreasing Si/Al ratio reduces stability
• Al creates charge imbalance and need for cations
• Presence of Fe2 reduces stability
• Fe2 oxidizes to 3
• Size and charge altered and Fe is expelled