Page 146 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 141

Mineral dusts vary in toxicity depending on their composition and age. Silicosis
develops from inhaling silica (quartz) dust, but not all silica dust is equally toxic.
Quartz has no natural cleavage, so grinding exposes highly active unsatisfied

þ
chemical bonds that take OH ions from water and leave a surplus of H ions
with instantly acidic conditions. Breathing freshly pulverized quartz dust therefore
is like putting a quick shot of acid into the lungs. As mineral surfaces age they are
deactivated by adsorbing water and gas molecules.

Asbestos defines a group of toxic minerals in which silica tetrahedra are shared in
only one direction so that particles are shaped like needles. Asbestos once was
widely used in fireproof insulation, fiberboard, and roofing materials, and is
present in many buildings constructed prior to the 1970s. About 95 percent of the
commercial asbestos is composed of the mineral chrysotile, in which each fiber is a
˚
rolled-up tube with a diameter of the order of 250 A and a length of up to
0.25 mm. When inhaled, the tiny, rigid, spike-like particles remain as a permanent
irritant that eventually can cause cancer. The cancer risk is greatly increased by
smoking, and the disease can develop many years after the exposure to asbestos
dust. Although dust-related respiratory diseases cause several thousand deaths per
year in the U.S., many times more fatalities are linked to tobacco smoking.

Problems

6.1. What is a hydrogen bond? How does it help to explain the properties of
water?
6.2. Kaolinite sometimes is called a 1:1 clay mineral, and smectite a 2:1. Why?
6.3. Which is more expansive, Na-smectite or Ca-smectite? How might one
change smectite from the sodium to the calcium form?
6.4. Which is more dangerous to breathe, freshly ground quartz in a mine or
pulverized limestone in road dust? Why?
6.5. Coal miners sometimes make a choice between the use of face masks and
early retirement. Explain.
˚
6.6. Based on an X-ray basal spacing of Ca 2þ smectite of 15.4 A and a
˚
mineral ABA layer thickness of 10 A, what is the percent water held in the
interlayer? Assume that the specific gravity of ABA is 2.65 and of the
interlayer water 1.0.
Answer: The ABA mass is 10 2.65 g ¼ 26.5, and of water 5.4 1.0 ¼ 5.4 g.
This gives 5.4/26.5 ¼ 20% water on an oven-dry weight basis.
6.7. Which is more effective to stabilize expansive clay: (a) drilling holes and
filling them with quicklime; (b) drilling holes and filling them with hydrated
lime; (c) drilling holes and mixing hydrated lime with the soil, referred to as
‘‘lime columns’’?
6.8. Do you expect that agricultural lime, which is calcium carbonate, would be
effective to stabilize expansive clay? Why (not)?

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