Chapter 2 continued

2:1 phyllosilicates

Vermiculite, Mica, and Illite

2 Tetrahedral sheets

http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS

+ 1 octahedral sheet

Form 2:1 minerals

www.geoclassroom.com/mineralogy/silicatelayer.gif

2:1 Layer silicates with medium layer charge

Vermiculite x = 1.2 – 1.8 • Dioctahedral vermiculite

Mx,H2O [Si,Al]8[Al,Mg]4O20(OH)4

• Trioctahedral vermiculite

Mx,H2O [Si,Al]8[Mg]6O20(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

http://grunwald.ifas.ufl.edu/Nat_resources/silicates/silicates.htm

http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/verstru.jpg

Medium-charge layer silicate properties

• Variable layer charge • CEC = 10 - 200 cmol/kg (higher for tri- than

dioctahedral)• High S.A. = 600-800 m2/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 mineralsMica x = 2

Dioctahedral:

• Muscovite Kx[Si,Al]8 [Al4]O20(OH, F)4

• Paragonite Nax[Si,Al]8 [Al4]O20(OH, F)4

Trioctahedral:

• Biotite Kx[Si,Al]8 [Mg, Fe, Al]6O20(OH, F)4

• Phlogopite Kx[Si,Al]8 [Mg6]O20(OH, F)4

• Lepidolite Kx[Si,Al]8 [Li,Al]6O20(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 m2/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

(100) Plane) (001) Plane

http://www.geoclassroom.com/mineralogy/phyllosilicates.html

Mica structure

http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpg

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 m2/g

Fixed K+• Term goes in and out of favor; used more by

geologists & engineers than soil scientists

http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/illstruc.jpg

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.

http://grunwald.ifas.ufl.edu/Nat_resources/silicates/illite.gif

http://faculty.plattsburgh.edu/robert.fuller/370%20Files/Week6Mineralogy/Illite.htm