Page 137 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
132 Geotechnical Engineering

6.3.9 ABA

The three-layer structure shown in the lower part of Fig. 6.5 is thicker than the AB
˚
þ
by one oxygen ion, about 10 A. In mica a potassium ion, K , has about the same
size as an oxygen and fits into hexagonal openings in the faces of stacked A layers.
These holes are shown at the upper right in Fig. 6.4. Potassium ions, being
positive while the net charge on the layers is negative, hold the layers together, so
mica can occur as large but relatively weak crystals. Mica is used commercially for
heat-resistant insulating sheets in electrical appliances. A black variety of mica
called biotite is common in granite.

þ
Question: What is the coordination of the interlayer K ion in mica?
Answer: It’s not 6.

6.3.10 Take Away the K

Weathering strips away K þ from mica so the crystal becomes weaker and
less well organized. This is the illlite family of minerals, named for the state of
þ
Illinois. Illite is abundant in marine shales that tend to pick up K ions from sea
water.

þ
When all of the K is gone, the layers separate and the crystal structure takes in
water and expands like an accordion. Even a change in relative humidity can
cause expansion or contraction due to gain or loss of interlayer water. These are
the ubiquitous and damaging expansive clays.

6.3.11 Influence from Interlayer Ions
þ
In most naturally occurring expansive clays the place of K is taken by
2þ
calcium ions, Ca , plus variable amounts of water. Calcium exerts
some control on expansion, which when the clay is wet normally gives a
˚
˚
maximum interlayer spacing of 19.6 A. Of that about 10 A is the mineral
ABA layer and the rest is water molecules clustered around calcium ions
and having an ice-like structure. Air-drying removes water and the spacing
˚
goes to about 15.4 A. Heating to over 1008C is required to drive off all of the
water.

Question: What volume expansion can be attributed to the change in d 001 of
calcium expansive clay from air-dry to saturation?

Answer: 100(19.6 – 15.4)/15.4 ¼ 27% and the floor is slowly rising.

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