minerals are crystals formed from soluble products of primary minerals.
The kind of clay formed depends on the proportion of different ions
in solution and the weathering environment.
clays are crystalline with a definite repeating arrangement of atoms.
they are made up of layers of oxygen atoms bonded together by silicon
basic building blocks of clay minerals are shown below.
of layers each of which consists of 1 silica and 1 alumina sheet.
of approximatley 10 meq/100g. The source of the charge is due to ionizationof
the Al-OH or Si-OH portion of the clay. Loss of the H leaves a temporary
negative charge ( Al-O- or Si-O-
). The extent of ionized H depends on the pH. More ionization occurs
in more basic solutions.
- Low swelling due to the bonding of oxygen and hydrogen between the
sheets. Used for pottery, tile and bricks.
low surface area (7-30 m2/g) and large in size
compared to other clays.
dominant clay in humid, warm, well drained, soils such as the Southeastern
U.S. Much of the silica is removed from the soil due to leaching.
Soils are high in aluminum and have a low pH.
(2:1 type clay)
- A 2:1
clay mineral is composed of layers each of which consists of 1 alumina
and 2 silica sheets.
of approximately 100 meq/100g. The high charge is due to isomorphic
substitution or the substitution of one ion for another of similar
size but lower positive valence. Si4+ and Al3+ cations may be replaced
during clay formation by Fe2+, Fe3+, Mg2+ or Zn2+ resulting in a net
increase in negative charge. More internal surface area between layers
is available for cation exchange.
- high shrink swell capacitybecause waster can go between layers.
surface area (600-800 m2/g)
in soils that have had little or no leaching such as soils of arid
regions, poorly drained soils, soils developed from alkaline parent
to seal ponds for well-drilling muds, and for thickeners in paints
and lipstick. Very plastic and cohesive.
2:1 structure similar to montmorillonite.
of approximately 30 meq/100g
- Interlayer K+ and isomorphic substitution of Al3+ for Si4+ in the
tetrahedral layer forms a strong bond between the three layer units.
surface area (65-120 m2/g).
from weathering of micas
in structure to illite, but K has been lost due to weathering. The
layers are held more weakly by hydrated magnesium.
of approximately 150 meq/100g.
isomorphic substitution of Al3+ for Si4+ than illite results in the
from weathering of micas. More weathered than illite.