Download 8Hb(8) Minerals in igneous rocks

8Hb(8)
Minerals in igneous rocks
There are many kinds of igneous rock, because igneous rocks can contain different combinations
of minerals and can have different crystal sizes. Many of the names of rocks you have examined are
names of groups of rocks, rather than describing just one combination of minerals.
For example, if you examine
a rock closely and it has a lot
of plagioclase feldspar and
pyroxene, but no quartz,
it would be either gabbro
or basalt. You would need
to look at the sizes of the
crystals to decide which of
these rocks it was.
Mineral content (as a % of the volume)
Geologists use a diagram like the one below to summarise the different combinations of minerals
that are found in igneous rocks. Extrusive rocks are rocks formed when extruded lava cooled.
Intrusive rocks formed
100
when magma cooled
potassium
feldspar
down inside the Earth. The
80
shaded minerals produce
dark-coloured crystals, the
quartz
plagioclase
60
unshaded ones produce
feldspar
light-coloured minerals.
40
20
muscovite
mica
biotite mica
pyroxene
olivine
amphibole
0
intrusive
granite
diorite
gabbro
extrusive
rhyolite
andesite
basalt
pale-coloured minerals
8 8
H H
b c
peridotite
dark-coloured minerals
1 a What mineral would gabbro contain in addition to pyroxene and plagioclase feldspar?
b If a rock was gabbro, would it have large or small crystals? Explain your answer.
2 Which types of intrusive rock can contain the following minerals? (There may be more than one
type of rock for each answer.)
a amphibole b pyroxene
c muscovite mica
3 Which types of extrusive rock do not contain the following minerals?
a olivine
b biotite mica c quartz
4 a Why are granites and rhyolites usually fairly
pale in appearance?
b Copy and complete the table. The first two
rows have been done for you.
5 Suggest why geologists use mineral
diagrams rather than just a table showing the
compositions of various rocks.
6 Find out where peridotite is found.
I CAN...
Composition of granite
Mineral
Minimum
percentage
Maximum
percentage
amphibole
0
8
biotite mica
4
12
muscovite mica
plagioclase feldspar
quartz
potassium feldspar
• use information from data in a chart
• use information about a rock’s mineral content to identify it.
Exploring Science
M08_EXSC_CM_Y8GBR_2506_UH.indd 373
edition
373
© Pearson Education Limited 2008
21/8/08 14:55:30