Geoscience Reference
In-Depth Information
igneous rocks is silica (SiO
2
). Igneous rocks are usually
grouped according to their silica content as follows:
SiO
2
(%)
(collectively called ferromagnesian minerals). To the four
chemical groups (acid, intermediate, basic, and ultrabasic)
must be added a fifth: that of the alkaline rocks, in which
the silica content varies from basic to acid and the content
of alkalis is higher than the other four groups. The naming
of igneous rocks also depends on rock texture. The main
divisions are drawn between the lavas and associated
rocks, which have fine-grained or glassy textures, the
medium-grained rocks formed in small intrusive bodies,
and the coarse-grained rocks of large or slowly cooled
intrusions. A classification of igneous rock for engineering
purposes is provided in
Table 6
.
Acid
>66
Intermediate
52-66
Basic
45-51
Ultrabasic
<45
Acid igneous rocks have a high content of quartz and
feldspars that are rich in silica, alkalis (sodium and
potassium), and aluminium. Basic rocks are rich in
minerals that contain iron, magnesium, and calcium
Table 6
Geological classification of igneous rock for engineering purposes (a modified form of this table
appears in BS 5930:1999)
Bedded rocks (with
Rocks with massive structure and
at least 50% of grains
Grain size
crystalline texture
of volcanic origin)
(mm)
Grain size
Pegmatite
Fragments of
>20
description
volcanic ejecta in a
finer matrix
Coarse
Granite
Diorite
Gabbro
Pyroxenite
2-20
Syenite
Peridotite
Agglomerate
(alkaline)
Rounded grains
Volcanic breccia
Angular grains
Medium
Micro-
Micro-
Dolerite
Cemented volcanic
0.06-2
granite
diorite
(or
ash
diabase)
Tuff
Fine
Rhyolite
Andesite
Basalt
Fine-grained tuff
0.002-0.06
Dacite
Very fine-
<0.002
Trachyte
grained tuff
(alkaline)
Glassy
Obsidian
Volcanic glass
Amorphous
or crypto-
crystalline
Pale
colour
Dark
Acid
Intermediate
Basic
Ultrabasic
(much
(some quartz) and
(little or
quartz)
Alkaline
no quartz)
(much sodium and
potassium)
