Chapter 2 continued 2:1 phyllosilicates (Chlorites) Non-silicates

2 Tetrahedral sheets octahedral sheet (in the interlayer)

Form “2:1:1” minerals1:1 mineral 1.4 nm 0.7 nm

Chlorite structure 2:1 interlayer Tet. Oct. Tet. Oct. 2:1 layer interlayer

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. = m 2 /g c-spacing = 1.4 nm Non-expansive CEC = cmol/kg High pH-dependent charge

Crystal structure of chlorite

Chlorite hand specimen 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. (Al 2 O 3 ) (Fe 2 O 3 ) SiO 2 Intermediate product of weathering Highly pH dependent CEC = cmol/kg High S.A. = m 2 /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

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).

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hydroxydes_et_oxydes_de_fer

Carbonates and Sulfate Minerals Calcite = CaCO 3 Dolomite = CaMg(CO 3 ) 2 Siderite = FeCO 3 Gypsum = CaSO 4.2H 2 O 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 (m 2 /kg) Affects water retention and CEC Total S.A. = External surfaces + Internal (interlayer) surfaces