Chapter 2 continued

2:1 phyllosilicates Vermiculite, Mica, and Illite

2 Tetrahedral sheets + 1 octahedral sheet http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS

Form 2:1 minerals

www.geoclassroom.com/mineralogy/silicatelayer.gif

2:1 Layer silicates with medium layer charge

Vermiculite

• Dioctahedral vermiculite

x = 1.2

M x ,H 2 O [Si,Al] 8 [Al,Mg] 4 O 20 (OH) 4

– 1.8

• 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.

Vermiculite

– common packing material and soil amendment http://www.duralite.com.au/images/Vermiculite_ore.jpg

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

dioctahedral) (higher for tri- than • 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

Mica x = 2

• •

Dioctahedral: Muscovite K x [Si,Al] Paragonite Na x 8 [Si,Al] [Al 8 4 ]O [Al 4 20 ]O (OH, F) 20 4 (OH, F) 4

• • •

Trioctahedral: Biotite K x [Si,Al] Phlogopite K Lepidolite K x x 8 [Mg, Fe, Al] [Si,Al] [Si,Al] 8 8 [Mg 6 ]O [Li,Al] 6 6 20 O O 20 20 (OH, F) (OH, F) 4 (OH, F) 4 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

Mica structure

(100) Plane) (001) Plane

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

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 m 2 /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 + limit expansion on wetting. to • The K + micas. content of hydrous mica is less than that of • Charges not neutralized by K + hydrated cations.

are countered by • 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