Geoscience Reference
In-Depth Information
decay, p. 47, and Soundness, impurities, and undesirable
constituents of aggregates, p. 66, and a weathering
classification scheme is shown in
Table 14
, p. 67.
Most granites used for building are coarsely crystalline
and even textured. Some exhibit porphyritic texture with
large feldspar crystals set in a finer groundmass. An
extreme and attractive example of this is Rapakivi
texture, where large oval orthoclase feldspars occur with
a mantle of sodium-rich plagioclase feldspar (
23
).
Rhyolite lavas are little used as building stone. They
consist of phenocrysts of quartz and alkali feldspar in a
finely crystalline or glassy groundmass. When acid lavas
cool very rapidly they do not crystallize and instead,
solidify to form a natural glass, obsidian (
24
). This was
historically worked for use as a gemstone and for tool
making, and is important archaeologically.
Tuffs of acid and intermediate composition are used
for building, especially for hard landscaping. Figure
25
shows a tuff of rhyolitic composition that is marketed as
'porphyry' for use as paving.
I
NTERMEDIATE IGNEOUS ROCKS
Although they are less used for building than acid or basic
rocks, intermediate igneous rocks include some significant
and attractive stones. Syenite is a coarsely crystalline
plutonic intermediate rock consisting chiefly of alkali
feldspar with less than 5% quartz and/or feldspathoid.
Clinopyroxene, hornblende, biotite mica, or olivine may
be present in minor proportions. Larvikite, a syenite from
Norway, is a popular decorative stone because of the
moonstone schiller shown by its alkali feldspars.
23
23
'Baltic Brown' Rapakivi textured granite (Finland)
showing a section through the edge of a large feldspar
orb consisting of orthoclase feldspar (right) mantled
with sodic plagioclase feldspar (centre). The coarsely
crystalline groundmass is seen (left) including quartz
(grey/white), biotite mica (brown/green), and
hornblende (brown); XPT, ×35.
24
25
24
Obsidian (Lipari, Italy) consisting of isotropic glass
that exhibits flow banding, air bubbles, and
spherulites (brown); PPT, ×35.
25
'Porfido Rosso' (Italy). A rhyolitic tuff comprising
phenocrysts of quartz and feldspar set in a
groundmass of cryptocrystalline devirtified volcanic
glass (black); PPT, ×35.
