Page 126 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
6 Soil Minerals

6.1 INTRODUCTION

Minerals vary from the aristocratic brilliance of diamonds to the common jumble
of quartz grains and clay minerals. All minerals have one property in common:
they consist of regularly repeating arrays of atoms. The tendency for crystals to
exhibit flat faces led to the first speculation that they are made up of a regular
arrangement of atoms as early as the fifth century B.C.E. A simple analogy is to the
repeated pattern of wallpaper, but in three dimensions. Glass, even a natural glass
such as obsidian, is not a mineral because it does not have a regular array of
atoms, and glass fractures along curved instead of planar surfaces. Glass is not
crystalline; it is amorphous. Not all gemstones are minerals: opal is amorphous
silica, and amber is amorphous and petrified tree sap. Ice is a mineral; liquid water
is not.

The crystalline nature of minerals affects the way in which they refract light, and
the difference often is directional. For example, a crystal of calcite, which is the
most common calcium carbonate mineral, has different refractive indices that
depend on the polarization of the light waves penetrating through the crystal.
A black dot observed through a crystal of calcite appears as two dots because the
light rays become polarized, the same as in polarizing sunglasses but in two
directions. The two rays transmit differently. This can be confirmed by viewing
the dots through a polarizing filter and rotating the filter, in which case the two
dots will take turns appearing and disappearing. This property of minerals is used
in a polarizing microscope to help identify minerals.

Clay particles are too fine for their shapes to be resolved by a light microscope, so
clay mineralogy was mainly a matter of guesswork based on chemical analyses
until the introduction of X-ray diffraction in the early 1900s. Direct observation
of clay particles later became possible with the higher resolving power of the

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