Environmental Engineering Reference
In-Depth Information
Fractionation changes other important physicochem-ical properties. Silicate percentage
rises steadily from 45-54 per cent (ultramafic) to 55-64 per cent (intermediate) and 65-
78 per cent in felsic rocks. This causes a corresponding increase in viscosity and
progressively slower flow rates. Feldspar minerals also become less alkaline as the
plagioclase-orthoclase series shows. Early settling of denser, ultrabasic minerals and the
continued rise of lighter fractions combine to create a
layering
effect in igneous rocks.
We have seen this already in layered oceanic crust (Chapter 11) and mineral distinctions
between upper (felsic) and lower (mafic) continental crust but it can also be present
within subsurface
plutons
. The ability of mineral-magma reactions to form new mineral
species, formerly set out in Bowen's Reaction Series, is now recognized in general
fractional crystallization, ionic substitution and solid solution processes.
The cooling rate and silicate content of igneous rocks also create textural as well as
chemical properties. Rapid cooling, especially of high-temperature rocks suddenly
exposed to low temperatures, pre-empts the growth of large crystals and forms fine-
grained rocks such as basalt. Even within a specific magma, some minerals will cool
faster, leaving others to enlarge. Larger crystals in finer matrices create a
porphyritic
texture which becomes
pegmatitic
if they grow very large. Slow cooling gives the
characteristic coarse-grained texture to granite, associated with subsurface emplacement
of low-temperature magmas. Volatiles driven off during cooling leave empty pockets
which create a
vesicular
texture if they survive or
amygdaloidal
texture when infused
with slower-cooling melt products or gases (see Colour Plate 3 between pp. 272 and 273).
