Chapter 2 continued
2:1 phyllosilicates
(Chlorites)
Non-silicates
2 Tetrahedral sheets
+ 1 octahedral sheet
+ 1 octahedral sheet
(in the interlayer)
http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
Form “2:1:1” minerals
1:1 mineral
0.7 nm
1.4 nm
interlayer
Oct.
Chlorite
structure
2:1 interlayer
Tet.
2:1 layer
Oct.
Tet.
http://pubpages.unh.edu/~harter/crystal.htm
2:1 Interlayer or Chlorite Minerals
(formerly called 2:1:1 or 2:2 Layer Silicates)
2:1 layer silicate with positively charged Al(OH)x
(Dioctahedral) or Mg(OH)x (Trioctahedral) in the
interlayer (x < 3) e.g., Al(OH)2+1
Properties:
S.A. = 70-150 m2/g
c-spacing = 1.4 nm
Non-expansive
CEC = 10-40 cmol/kg
High pH-dependent charge
http://www.geoclassroom.com/mineralogy/chlorite.GIF
Crystal structure of chlorite
http://www.gly.uga.edu/schroeder/geol6550/chlorite.gif
http://www.soils.wisc.edu/courses/SS325/chlorite.gif
http://www.uwm.edu/Course/422-100/Mineral_Rocks/chlorite1.jpg
Chlorite hand specimen
http://www.casdn.neu.edu/~geology/department/courses/minerals/
photogallery/index.html
Non-Layer Silicates
Allophane
• Amorphous (non-detectable by X-Ray Diffraction)
• Derived from volcanic ash in relatively young soils the rapid cooling of lava ejected from volcanoes
leads to deposition of much of the ash as
amorphous glasses. Amorphous gels of silica,
alumina, and silica-alumina polymers are formed
during decomposition of the ash.
•
(Al2O3) (Fe2O3) SiO2
•
Intermediate product of weathering
•
Highly pH dependent CEC = 5 - 350 cmol/kg
•
High S.A. = 70-300 m2/g
Al, Fe, Mn, Ti Oxides,
Hydroxides, and Oxyhydroxides
• Weathering products of other minerals; Found in
highly weathered soils
Fe oxides impart soil color (brown, yellow,
red, orange, black)
• Often present as coatings on soil particles, in
cracks, or as nodules
High surface area
Fe and Mn oxides indicate redox conditions
pH dependent charge
• See Table 2.5 Sparks p. 60 for list of common
oxide minerals in soils
Oxisol – high in Fe and Al oxides
http://soils.usda.gov/technical/classification/orders/images/oxisol.jpg
Microscopic view of an Inceptisol, showing small crystallites of
carbonate minerals (around the central black void), quartz sand
grains (white), and iron oxides and organic matter (dark brown).
http://www.britannica.com/ebc/art/print?id=19571&articleTypeId=0
csd.unl.edu
http://fr.wikipedia.org/wiki/Structures_cristallines_des_oxydes,_oxyhydroxydes_et_oxydes_de_fer
Carbonates and Sulfate Minerals
Calcite = CaCO3
Dolomite = CaMg(CO3)2
Siderite = FeCO3
Gypsum = CaSO4.2H2O
Soluble minerals (compared to silicates)
Present in arid zones (high evapotranspiration)
Formed from precipitation out of solution as
water evaporates
Rosettes of gypsum
Surface Area
• Specific Surface = the surface area per
unit weight of material (m2/kg)
• Affects water retention and CEC
• Total S.A. = External surfaces + Internal
(interlayer) surfaces
http://bio1151.nicerweb.com/doc/class/bio1151/Locked/media/ch06/06_07SurfaceVolumeRatio_L.jpg
http://www.gly.uga.edu/railsback/1121WeatheringArea.jpeg