Mineral Weathering and Secondary Mineral Formation weathering: chemical alteration of minerals in so

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Mineral Weathering and Secondary Mineral
Formationweathering chemical alteration of
minerals (in soils involves water gases acids
etc).

• Parent material soil
• Desilication
  via weathering
• Parent Materialprimary silicates formed from
  igneous/metamorphic processes
• Soil secondary silicates oxides carbonates
  etc.formed from weathering processes

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Behavior of Elements During Chemical Weathering

• Soils are depleted in elements relative to parent
  material
• Element loss/depletion is determined by elements
  position on periodic table (which column or group
  of columns) AND the elements ionic potential
• Z/R ionic potential zcharge rradius
• Classes
• Z/R 0-3 ion surrounded by H2O shell soluble in
  H2O (Na Ca etc)
• Z/R3-9.5 ion so strongly attracts H2O that
  insoluble oxides/hydroxides form (Al Fe)
• Z/Rgt9.5 soluble oxyanions form (S C etc.)

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Ionic potential of important elements

• Red arrow indicates decreasing attaction to H2O
  within a group of elements
• Decreasing attraction is reflected in weathering
  losses..

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Element loss varies with ionic potential
Ti group
Alkali metals and alkaline earths
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Mineral Particle Size and Mineralogy

• Gravel gt 2mm (primary)
• Sand gt 0.05 to 2.0 (primary)
• Silt lt0.05 to 0.002 (primary secondary)
• Clay lt 0.002 (secondary)
• Most secondary mineral are silicates and most
  secondary silicates are phyllosilicates.

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11 phyllosilicates kaolinite

• One layer of Si tetrahedra
• One layer of Al octahedra
• Individual minerals are held to another via H
  bonds

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21 Phyllosilicates di and trioctahedral

• Dioctahedral (smectites)
• Substitution of 2 for 3 in octahedral layer
  (called isomorphous substitution)
• Creates a net negative charge (and property of
  cation exchange capacity)
• Results in expandable layers
• Trioctahedral (vermiculite)
• Substitution of 3 for 4 in tetrahedral layer
• Also has CEC but little or no expansion

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Other secondary mineral groups oxides

• Al oxides (gibbsite)
• Results of vigorous chemical weathering
  (desilication)

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Non-silicate secondary minerals oxides

• Fe oxides
• Geothite
• Yellowish brown
• Acidic OM-rich envir.
• 2. Hematite
• Bright red
• Warm dry environments

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Non-silicate secondary minerals carbonates

• Calcite
• Ca is released from some weathering source
• Forms in arid to semi-arid environments when soil
  solution becomes saturated
• Presence in upper 1m related to MAP
• Depth of carbonate layer related to MAP

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Geographical distribution related to climate

• Greater than 100cm/yr removes carbonate
• Below 100cm depthMAP

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Non-silicates sulfates (gypsum)

• Presence of sulfates in soils usually occurs in
  hyperarid climates (or sites with high water
  table and evaporative enrichment of salts)

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Secondary Minerals in California Soils Sierra
Nevada
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Soil Mineralogy vs. Elevation