Environmental Engineering Reference
In-Depth Information
Petrographic Microscope
Polarized light is used in the petrographic microscope for studying thin sections of min-
eral or rock specimens. To prepare a thin section, a sample about 25 mm in diameter is
ground down to a uniform thickness of about 0.03 mm by a sequence of abrasives. At this
thickness it is usually translucent. The specimen is enveloped in balsam and examined in
polarized light. The minerals are identified by their optical properties.
Other Methods
Minerals are also identified by a table microscope, the electron microscope, blow-pipe
analysis, or X-ray diffraction.
5.2.3
Igneous Rocks
Origin and Occurrence
Molten rock charged with gases (magma) rises from deep within the Earth. Near the surface
a volcanic vent is formed, the pressures decrease, the gases are liberated, and the magma
cools and solidifies. Igneous rocks occur in two general forms (see also
Section 6.2.2).
Intrusive
The magma is cooled and solidified beneath the surface, forming large bodies (plutons) that
generally consist of coarser-grained rocks; or small bodies such as dikes and sills, and vol-
canic necks, which generally consist of finer-grained rocks because of more rapid cooling.
Extrusive
Associated with volcanic activity, extrusive rocks originate either as lava, quiet out-
wellings of fluid magma flowing onto the Earth's surface and solidifying into an extrusive
sheet, or as pyroclastic rocks, magma ejected into the air by the violent eruption of gases,
which then falls as numerous fragments.
Classification
Igneous rocks are classified primarily according to mineral content and texture as pre-
sented in
Table 5.6.
Mineral Composition and the Major Groups
The important minerals are quartz, feldspar, and the ferromagnesians, as given in
Figure
Verhoogan, 1960).
Sialic rocks
(acid rocks) are light-colored, composed chiefly of quartz (silica) and
feldspar (silica and alumina, Al
2
O
3
) with silica
●
66%.
Intermediate group
rocks have a silica content between 52 and 66%.
●
Mafic rocks
(basic rocks) are the ferromagnesian group containing the dark-col-
ored minerals (biotite mica, pyroxine, hornblende, olivine, and the iron ores),
with a silica content between 45 and 52%.
●
Ultramafic rocks
have a silica content
45%.
●
Alkaline rocks
contain a high percentage of K
2
O and Na
2
O compared with the
content of SiO
2
or Al
2
O
3
.
●
