2:1 Layer silicates with medium layer charge Vermiculitex = 1.2 – 1.8 Dioctahedral vermiculite M x,H 2 O [Si,Al] 8 [Al,Mg] 4 O 20 (OH) 4 Trioctahedral vermiculite M x,H 2 O [Si,Al] 8 [Mg] 6 O 20 (OH) 4 The name vermiculite was created from the Latin word for worm, vermiculus. This is a reference to the fact that when vermiculite is heated, it expands into wormlike shapes.
http://www.duralite.com.au/images/Vermiculite_ore.jpg Vermiculite – common packing material and soil amendment http://www.epa.gov/region8/sf/libby/abcasbestos.html
Medium-charge layer silicate properties Variable layer charge CEC = 10 - 200 cmol/kg (higher for tri- than dioctahedral) High S.A. = 600-800 m 2 /g Moderately Expansive (not as bad as smectites) Non-plastic, non-sticky C-spacing = 1.0-15 nm depending on treatment K + fixation
Mica (L. micare, to shine) hand samples Oven windows, cosmetics, paints, …
High-charge 2:1 minerals Micax = 2 Dioctahedral: MuscoviteK x [Si,Al] 8 [Al 4 ]O 20 (OH, F) 4 Paragonite Na x [Si,Al] 8 [Al 4 ]O 20 (OH, F) 4 Trioctahedral: Biotite K x [Si,Al] 8 [Mg, Fe, Al] 6 O 20 (OH, F) 4 Phlogopite K x [Si,Al] 8 [Mg 6 ]O 20 (OH, F) 4 LepidoliteK x [Si,Al] 8 [Li,Al] 6 O 20 (OH, F) 4
High-charge 2:1 mineral properties Source of K + in soils as they weather Non-expansive, non-sticky, non-plastic S.A. = 70-120 m 2 /g (mostly external) CEC = 10 - 40 cmol/kg c-spacing = 1.0 nm Interlayer held tightly together by K + fit in ditrigonal cavities of tetrahedral sheet
Illite “Hydrous mica”, “micaceous clay”, weathered mica Intermediate in formula and properties between mica and vermiculite or smectite CEC = 20-40 cmol/kg S.A. = 70-120 m 2 /g Fixed K+ Term goes in and out of favor; used more by geologists & engineers than soil scientists
Hydrous Micas (Illites) Illite is essentially a group name for non-expanding, clay- sized, dioctahedral, micaceous minerals. Structurally similar to muscovite in that its basic unit is a layer composed of two inward-pointing silica tetragonal sheets with a central octahedral sheet. 2 : 1 type minerals containing sufficient interlayer K + to limit expansion on wetting. The K + content of hydrous mica is less than that of micas. Charges not neutralized by K + are countered by hydrated cations. Hydrous micas are widespread in soils. The layer thickness of hydrous micas are about 1.0 nm.