SPLM #2 (Copy 2) Mineral & Petrology
SPLM #2 (Copy 2) Mineral & Petrology
SPLM #2 (Copy 2) Mineral & Petrology
The study of rocks in all their aspects including their mineralogies, textures, structures (systematic
description of rocks in hand specimen and thin sections); origin and their relationships to other rocks.
This helps in knowing the strength, durability, colour, appearance, workability, etc.
These properties are very important for CE to know because different rocks are suitable for
different purposes and no rock is ideal or best suited for all purpose.
Petrology is very important from civil engineering point of view, as it provides a proper concept and
logical basis for interpreting physical properties of rocks. The study of texture, structure, mineral
composition, chemical composition etc., gives all necessary details regarding the strength, durabiliy,
colour, appeareance, workability, etc.,
These inherent characters of rocks are of chief concern for a civil engineer to judiciously assess the
suitability occuring at project site for required purpose.
ROCKS
In Geology
Rock or Stone - is a naturally occurring solid aggregate of one or more minerals or mineraloids.
For Examples
1. The common rock granite is a combination of the quarts, feldspar and biotite minerals.
2. The Earth's outer solid layer, the lithosphere, is made of rock.
Rocks used by mankind throughout history.
From the Stone Age, rocks have been used for tools.
The minerals and metals found in rocks have been essential to human civilization.
These physical properties are the end result of the processes that formed the rocks. Over the
course of time, rocks can transform from one type into another, as described by the geological
model called the rock cycle.
EXTRUSIVE ROCKS
Extrusive, or volcanic, igneous rock is produced when magma exits and cools outside of, or very
near the Earth's surface.
These are the rocks that form at erupting volcanoes. The magma, called lava when molten
rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the
relatively cool temperature of the atmosphere.
Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.
Pumice, obsidian, and basalt are all extrusive igneous rocks.
ROCK TEXTURE
Igneous rock textures are used by geologists in determining the mode of origin igneous rocks
and are used in rock classification. There are six main types of textures;
1) Phaneritic,
2) Aphanitic,
3) Porphyritic,
4) Glassy,
5) Pyroclastic and
6) Pegmatitic
Aphanitic
(not visible) rocks in contrast to Phaneritic rocks, typically form from lava which crystallize
rapidly on or near the Earth‘s surface. Because they make contact with the atmosphere they cool
quickly, so the minerals do not have time to form large crystals. The individual crystals in an
aphanitic igneous rock are not distinguishable to the naked eye.
Examples of aphanitic
Igneous rock; include
basalt,
andesite, and
rhyolite.
Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so
rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no
crystals.
Examples include obsidian and pumice.
Phaneritic
(phaner = visible) Textures are typical of intrusive igneous rocks, these rocks crystallized slowly
below the Earth's surface. As a magma cools slowly the minerals have time to grow and form
large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each
individual cyrstal with the naked eye.
Examples of phaneritic igneous rocks are gabbro, diorite and granite.
Pegmatitic texture
Occurs during magma cooling when some minerals may grow so large that they become massive
(the size ranges from a few cm to several metres).
Porphyritic textures
As large crystals, develop when conditions during cooling of a magma change relatively quickly.
The earlier formed minerals will have formed slowly and remain whereas, sudden cooling causes
the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix.
The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its
matrix.
Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before
completing crystallization thus forcing the remaining lava to crystallize more rapidly with much
smaller crystals.
Pyroclastic (pyro=igneous, clastic = fragment) textures occur when explosive eruptions blast the
lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash,
lapilli, and volcanic bombs.
SUMMARY (Igneous Rock)