FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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i)
Mineralogical composition
The mineral composition in igneous rocks provides
clue concerning the nature of the parent magma. The minerals occurring in
igneous rocks may be grouped as essential and accessory. The essential minerals
are the major constituents of the rock while the accessory minerals constitute
to the minor constituents. Both the essential and accessory minerals are
primary minerals resulting from magmatic crystallization. The essential
minerals control the name of the rock as such are employed in mineralogical
classification. Generally consideration is based on the present or absence of
quartz, composition of feldspars, and proportion and kinds of ferromagnesian
minerals.
a) Granite –
Rhyolite family
This family has the following mineral composition as
per Hamblin and Howard (2005):
Quartz 10 – 40%
Potassium
feldspar 15 – 30%
Plagioclase
feldspar 0 – 33 %
Biotite and amphibole 8 – 15%
The igneous rocks under this family are sometimes
referred to as felsic rocks because contain high percentage of feldspars
and silica (quartz). This family represents light coloured rocks.
Representative examples:
Granite: Commonly coarse to very
coarse grained with porphyritic or phaneritic texture. Many types of granite
are grey in colour, but if K-feldspar dominates, the colour becomes pink or
red. The granites are composed of potassium feldspar and quartz some with
biotite and in small amount.
Rhyolite: Usually light coloured, some
have pinkish and grey colour. Rhyolite is fine to very fine. It has aphanitic
texture. It is microcrystalline
extrusive equivalent of granite formed at or near the earth's surface.
Granodiorite: Resembles granite except
that it has abundance of plagioclase feldspar. Grey colouration is dominant.
The rock has the same texture as that of granite. It is plutonic igneous rocks.
b) Gabbro -
Basalt family.
This family is sometimes referred to as mafic igneous rocks. The family has the
following general composition:
Plagioclase
(mostly calcic)
25 - 70%
Ferromagnesian minerals (olivine, pyroxene and
amphibole) 22 - 75%.
The rocks crystallize from magma rich relatively in
Fe, Mg and Ca with deficient in silica (basaltic magma). These rocks are usually dark - green or black in colour.
Examples:
Basalt: It is black, dense and
massive. Individual crystals cannot be
seen with naked eyes (fine-grained extrusive rock). Half of the total volume of
basalt consists of small holes called vesicles. The vesicles developed as a
result of the escape of trapped gases on cooling of lava. The highly vesicular basalt is referred to as
scoria. The presence of olivine
phenocrysts gives basalt greenish colouration. The basalt is composed of
plagioclase and pyroxene with or without olivine.
Phonolite: It is dark green to grey. Fine-grained and compact
with sometimes few isolated rectangular phenocrysts of feldspars or
nepheline. Often the rock has platy
structure that makes it to break into flat slabs. The rock crystallizes from
lava flows, sills and dykes (Hamilton et al, 1992).
Gabbro: It is coarse to medium
grained intrusive rock composed of pyroxene, hornblende, biotite and feldspar
(calcic plagioclase). Also contains olivine with no quartz. Most gabbros are dark in colour.
c) Diorite - Andesite family (intermediate group)
It is intermediate in composition between the
families of granite - rhyolite and gabbro - basalt (felsic and mafic rocks).
It is
characterized by the following composition:
Plagioclase 55 - 70%
Amphibole
and biotite 15 - 40%
The K - feldspar and quartz are present only in
minor quantity. The family is therefore characterized by grey colour.
Examples:
Andesite: Generally dark, grey,
green or red in colour. Upon weathering
may become dark brown or reddish brown.
Porphyritic andesite is the most common variety with phenocrysts of
mainly plagioclase, amphibole or biotite set in a matrix of aphanitic
plagioclase and some glass.
Diorite: Has the same texture as
that of granite. The two rocks differ only in composition. Diorite is composed predominantly of
plagioclase and ferromagnesian minerals.
Ferromagnesian minerals make up a greater proportion of the rock and
give it a dark colour. Quartz occurs in
less than 5% of the total volume.
Sometimes quartz and potassium feldspar are completely missing. Diorite
occurs in large intrusive such as batholiths. Also found in dyke, sills and
laccoliths
d) Peridotite
family
Sometimes called Ultramafic igneous rocks. The
family has the following composition:
Olivine
65 – 100%
Pyroxene
0 - 25%
Ca
– Plagioclase 0 – 5%
Ore minerals –
magnetite, ilmenite, chromite, etc 0-10%
Typical examples:
Peridotite: Composed of almost entirely
of the mineral olivine. Crystals of olivine are light green and have a glassy
texture. They are medium to coarse grained.
Peridotite rock has phaneritic texture. The rock originates far below
the surface. It is believed to be the
major constituent of the upper mantle. There is no extrusive equivalent of
peridotite.
Kimberlite: The rock is coarse-grained with a porphyritic
texture. Kimberlite is composed principally of sepentinized olivine, phlogopite,
and pyrope. Accessory minerals are
diamond, ilmenite and chromite. Kimberlite is usually bluish or greenish in
colour. Kimberlite forms as an oval pipe.
e)
Volcanic glass and
pyroclastics
Pumice
It is light and porous volcanic rock. It is white to yellowish in colour. Escape of
gases when cooling makes it to have vesicles.
Tuff
It includes consolidated ash, dust, and fragments
with diameters up to 64mm. The rock usually is layered like sedimentary rocks.
Agglomerate: Contains angular to
sub-rounded fragments more than 64 mm in a fine – grained matrix. The rock resembles
conglomerate.
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