Environmental Engineering Reference
In-Depth Information
heat of molten rock injected into adjoining rock, which causes chemical changes and pro-
duces new minerals.
5.2.2
Petrographic Identification
Significance
Rocks are described and classified by their petrographic characteristics of mineral content,
texture, and fabric.
A knowledge of the mineral constituents of a given rock type is also useful in predict-
ing the engineering characteristics of the residue from chemical decomposition in a par-
ticular climatic environment. Residual soils are commonly clayey materials, and the
”activity” of the formation is very much related to the original rock minerals.
Rock Composition
Minerals
Rock minerals are commonly formed of two or more elements, although some rocks con-
sist of only one element, such as carbon, sulfur, or a metal.
Elements
Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium comprise
98% of the Earth's crust. Of these, oxygen and silicon represent 75% of the elements. These
elements combine to form the basic rock minerals.
Groups
The mineral groups are silicates, oxides, hydrous silicates, carbonates, and sulfates.
Silicates and oxides are the most important. The groups, mineral constituents, and chem-
ical compositions are summarized in
Table 5.2.
Chemical composition is particularly
important as it relates to the characteristics of materials resulting from chemical weather-
ing and decomposition.
Texture
Texture refers to the size of grains or discrete particles in a specimen and is generally clas-
sified as given in
Table 5.3.
Fabric
Fabric refers to grain orientation, which can be described in geologic or in engineering ter-
minology.
Geologic Terminology
Equigranular: grains essentially of equal size
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Porphyritic: mixed coarse and fine grains
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Amorphous: without definite crystalline form
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Platy: schistose or foliate
●
Engineering Terminology
Isotropic: the mineral grains have a random orientation and the mechanical
properties are the same in all directions.
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