Geoscience Reference
In-Depth Information
A
CID IGNEOUS ROCK
Acid igneous rocks occur mainly as the coarsely crystalline
plutonic variety known as granite and to a much lesser
extent, finely crystalline rhyolite lavas. Rhyolite lavas are
often associated with pyroclastic rock such as tuff and
ignimbrite, which may also have an acid composition.
Granite is a very important building stone that is
popular for its pleasant appearance, strength, and
durability. Granite is composed essentially of feldspar
(40-80%, mainly alkali feldspar with some plagioclase),
quartz (20-60%), and dark-coloured ferromagnesian
minerals (>20%, typically biotite mica or hornblende).
Accessory minerals (which are present in such small
quantities that they are not considered for rock
classification purposes) may include augite, muscovite
mica, apatite, zircon, and magnetite.
Unweathered granite is available in a range of light
colours including white, grey, pink, and red. The dominant
colour of fresh granite is nearly always determined by that
of the feldspars, which are the most abundant constituent.
Figure
20
shows a red hornblende granite that has been
quarried at Aswan, Egypt since 4000
BC
and was used
extensively in Pharaonic architecture (Arnold, 1991).
Figure
21
shows a white granite that is currently being
produced in China. Discolouration caused by weathering
results in brownish and yellowish coloured granites, which
are also produced for sale. It is important to establish the
weathering grade of the granite as this could potentially
affect its strength and durability. Figure
22
shows a yellow
coloured granite that has been slightly discoloured and
slightly weakened by weathering (grade II). Rock
weathering is discussed in Petrography of stone defects and
20
20
Aswan granite (Egypt) consisting chiefly of
plagioclase feldspar (lower right), orthoclase feldspar,
and quartz (upper centre), with minor hornblende
(brown, centre left) and biotite mica (upper right).
From the casing of Menkaure's pyramid at Giza; XPT,
×35.
21
22
21
'Royal White' granite (China) consisting chiefly of
alkali feldspar (lower left), plagioclase (upper left), and
quartz (upper right), with minor biotite mica
(green/brown, centre); XPT, ×35.
22
'Yellow Rock' granite (China) consisting chiefly of
alkali feldspars that exhibit perthitic intergrowth
(centre) and quartz (lower right), with minor biotite
mica (pink, lower centre). This stone has been slightly
weathered giving it a brown discolouration from iron
compounds (best viewed in PPT); XPT, ×35.
