Page 144 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 139

Angular shapes are preferred for most engineering uses because of improved
interlocking of individual particles.

6.6.3 Minerals in Sand
Sand by definition is finer than gravel and coarser than silt. A natural boundary
exists between sand and gravel because sand grains tend to be composed of single
mineral species instead of being fragments of rocks.

The ultimate source for most sand is disintegration of granite by weathering, so it
initially is about 25 percent quartz and the remainder mainly feldspars with a
sprinkling of mica and iron-rich dark minerals. Transportation by wind or water
then wears away the feldspars and concentrates quartz, so typical river sand
is about 75 percent quartz and the remainder feldspars and other minerals.
Transportation also rounds the grains.

6.6.4 Silt

A natural boundary also exists between sand-size and silt-size particles because
silt grains are small enough that they are carried in suspension instead of impact-
ing one another, so there is little or no impact rounding. A visual distinction can
be made between sand and silt in that individual silt grains are difficult to
discern with the unaided eye, although they readily can be seen with a 10 or
20 lens.

As particle sizes further decrease and surface areas increase, non-clay minerals
become less resistant to weathering so soils are dominated by the clay minerals.

6.6.5 Identifying Non-Clay Minerals

Table 6.1 lists some diagnostic X-rays for clay and non-clay mineral particles.
Non-clay minerals are characterized by color, hardness, and cleavage. Cleavage
refers to a tendency to fracture along discrete atomic planes. Mica, for example,
has excellent cleavage along basal crystallographic planes. Feldspars have only
moderate cleavage but quartz has no cleavage, so crystals of quartz that have flat
faces grew that way. Cleavage can occur in more than one crystallographic
direction, which affects the shapes of individual mineral grains.

Hardness is quantified by determining the scratch order of different minerals. This
order is called the Mohs hardness scale, which has numbers from 1 for the softest
to 10 for the hardest mineral, which is diamond. Gypsum (hardness 2) can be
scratched with a fingernail, calcite (hardness 3) with a penny, and feldspar
(hardness 6) with a hard knife. Quartz has a hardness 7.

The presence of calcite indicates a low degree of weathering and is determined by
touching soil with a few drops of dilute hydrochloric acid (1:10 in water) and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.