Environmental Engineering Reference
In-Depth Information
balanced by potassium (K
1+
), sodium (Na
1+
) or calcium (Ca
2+
) to form orthoclase, albite
or anorthite solid solution feldspars. Quartz and feldspars form over 70 per cent of
continental lithosphere.
Other principal mineral groups are built of simpler oxides and anion complexes (Table
12.1). Oxides and sulphides are important metallic minerals, with sulphur replacing
oxygen as the anion in the latter, and salt cations form halides with fluorine and chlorine.
Oxygen associated with the carbonate, sulphate, phosphate and hydroxyl anion
complexes (CO
3
)
2−
, (SO
4
)
2−
, (PO
4
)
3−
and OH
−
forms carbonates, sulphates, phosphates
and hydroxides respectively. Most of these minerals do not form directly from melts but
are dependent instead on
metamorphic
,
metasomatic
and
sedimentary
processes described
below.
IGNEOUS ROCKS
key processes
How do various mineral combinations come together and develop into distinct igneous
lithologies? Magma derived from partial melting of the asthenosphere or continental crust
is converted from a hot melt to cold, solid
lithified
rock. The rate and location of cooling
determine its mineralogical evolution and eventual rock character. The initial melt, at
temperatures of 900°-1200° C, does not cool uniformly and its homogeneous, minero-
elemental composition changes
en route
by
fractional crystallization
as solid minerals
with successively lower melt temperatures form and settle out through the rising magma.
Fractionation proceeds in several ways. Melts become depleted of higher-temperature
products and therefore enriched in lower-temperature elements. Denser minerals settle
out faster through the viscous melt, although they may still react with it chemically; and
further mineral
speciation
occurs as more subtle changes alter element ratios in solid
solutions (Table 1).
Three classes of magma are recognized. Fractionation of asthenosphere peridotite
proceeds through basaltic → andesitic → granitic stages, although andesite-granite
magmas are also derived from continental crust
wet melts
in subduction zones. Basaltic
magmas crystallize first from asthenosphere peridotite, commencing with olivine,
followed by plagioclase feldspar (anorthite-albite solid solutions) and pyroxene. Their
denser, dark minerals - Mg, Fe-rich and relatively silicate-poor - form
ultrabasic-basic
or
ultramafic-mafic
rocks.
Dry
or
anhydrous melts contain
less than 0·2 per cent water.
Andesitic magmas are
intermediate
in nature with more albite plagioclase, amphibole
(hornblende) and biotite and compositionally close to 'average' continental crust. They
solidify at a temperature range of 900-1000° C. Low-temperature (500-600° C) granitic
or
rhyolitic
magma is dominated by the lighter, less dense minerals orthoclase feldspar
(potassium-rich), quartz and biotite-muscovite to form Fe, Si-rich, acid or
felsic
rocks.
Table 1 Comparative composition of selected magmas and rocks (% by
weight)
Min-
eral
C'tal
crust
Canadian
craton
Upper
mantle
Basaltic
magma
Basalt
Andesitic
magma
Rhyolitic
magma
Granite
LR
tuff
Green
schist
SiO
2
58·0
66·1
45·16
50·3
49·1
62·5
75·1
70·9
75·6
42·1
