Page 140 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 135

The most direct approach is to measure the expansion in a compression
device that allows wetting of the soil. This method is discussed in Chapter 16
in this text.

6.3.16 Particle Size Analysis by X-ray Diffraction

Soil particle sizes ordinarily are measured by sieving or by monitoring the
settlement rate in water, as described in the next chapter. However, the thickness
of thin crystals can be measured from its effect on the sharpness of a diffraction
peak, referred to as ‘‘line broadening.’’ As a crystalline particle becomes thinner
its orientation can be more variable with respect to the X-ray beam without
causing annulment of waves. This is shown on the diffraction diagram in Fig. 6.6,
where quartz shows very sharp diffraction peaks, illite and kaolinite are less sharp,
and smectite peaks are quite broad. Even glass or liquids diffract a broad peak or
halo, which indicates an average spacing of molecules even though the spacing is
random.

While many other factors such as diffractometer alignment and mineral
crystallinity are involved in peak broadening, a reasonable estimate of an
equivalent crystallite thickness for a platy mineral can be made with the following
formula, which applies for copper K radiation:
150
D ¼ ð6:2Þ
cos
Example 6.2
The diffraction peak width at half-intensity for the illite peak in Fig. 6.6 is approximately
0.38 and for the smectite peak 1.08 2 , respectively. What are the estimated average
crystallite thicknesses, and how many unit cell layers do they represent?
˚
Answer: For the illite, D ¼ 150/[0.3 cos (8.8/2)] ¼ 500 A, which represents 500/10 ¼ 50
˚
layers. The solution for the smectite gives 150 A, or only about 8.5 layers.
6.3.17 Non-Clay Layer Silicates

Two classes of layer silicates have ABA-B crystal structures and a d 001 spacing
˚
of 14.4 A, with the interlayer ‘‘B’’ representing a separate sheet that does not share
ions with the other sheets. The layers are held together with hydrogen bonds.

Chlorite minerals are common in shale, and the additional B has OH corner
ions. Vermiculite, which occurs in soils, has water in octahedral corner positions
such that upon heating, the water is released explosively and each grain puffs up
like popcorn. Expanded vermiculite is used as a fireproof building insulation, but
some can contain asbestos, which if breathed can cause lung cancer. The mineral
name comes from the shape of the expanded particles, vermes being Latin for
worms.

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