Geology Reference
In-Depth Information
Box 8.3 Silicate melts
at the atomic level a silicate melt has much in common
with silicate crystals: it consists of covalently bonded sil-
icate structural anions such as (SiO
4
)
4
−
and (Si
2
O
7
)
n
6
n
-
glued together electrostatically by cations like Mg
2+
and
Fe
3+
. Over short distances there is the semblance of order
(Box 1.2), but a silicate melt, like any liquid, lacks the
long-
range order
characteristic of the crystalline state.
another important difference is that, whereas nearly all
silicate crystal structures are each constructed around a
single type of silicate skeleton (chains in the case of pyrox-
enes, for example), the more open structure of a melt
allows several types of silicate structural unit to coexist
and intermingle. In
basic melts
, those rich in basic oxides
and relatively poor in silica, a range of relatively simple
polymers predominates (SiO
4
4
−
, Si
2
O
7
6
−
, Si
3
O
10
8
−
, (SiO
3
)
n
2
n
−
,
etc.). this relatively unpolymerized structure favours the
crystallization of minerals like olivine and pyroxene,
because the appropriate polymers, at least in embryo
form, are already present in the melt. an
acidic melt
has a
more polymerized structure conducive to the crystalliza-
tion of minerals like mica (if water is present), feldspars
and quartz.
the degree of polymerization of molten silicates is most
clearly expressed in their viscosity. Viscosity increases
with the degree of polymerization, because large polymeric
units get tangled up with each other more than small ones
do, and this inhibits the flow of the liquid. hence rhyolites
(acid lavas) are much more viscous than basalts
(Figure 3.8), even when allowance is made for temper-
ature differences.
Dissolved water has a dramatic effect on magma viscos-
ity, through its ability to sever polymers and therefore
'depolymerize' the melt as shown in Figure 8.3.1. Magma
rich in dissolved water is therefore more fluid and conducive
to diffusion than a dry magma of corresponding composi-
tion. In this instance water is behaving as a basic oxide.
a high dissolved water content is only stable when con-
fined under high pressure, however. Close to the surface,
ascending water-rich melt will become supersaturated and
vesiculate to form bubbles of h
2
O vapour. Viscosity then
rises dramatically as the loss of water allows the magma
to polymerize, and the presence of bubbles further
enhances viscous behaviour (as stiffly whipped egg white
illustrates).
Si
Si
Si
+
H
2
O
Si
Si
+
Si
O
O
O
O
O
O
O
H
HO
O
Long polymer
Shorter polymer
Shorter polymer
Figure 8.3.1
Depolymerization of a silicate polymer by a water molecule. the h and Oh derived from the h
2
O molecule
are shown in bold for clarity.
oxygen atoms. Si is known to occupy tetrahedral sites
between these atoms. We can work out how the other
ions such as Mg
2+
and K
+
fit in by considering their
radius ratios
(Chapter 7). These are shown in Table 8.2,
calculated from the ionic radii given in Box 7.2.
Two radii are given for Al
3+
in Box 7.2 because, as the
radius ratios suggest, its size permits it to fit into both
tetrahedral and octahedral sites in silicates. (X-ray
crystallography shows there is a slight difference in
Al-O bond length in 4- and 6-fold co-ordination, so
that a different Al
3+
radius is associated with each site.)
A glance at the formulae in Table 8.1 supports this con-
clusion. The formula of feldspar only makes sense if
we include Al in Z, and X-ray investigations confirm
that Al is tetrahedrally co-ordinated in feldspars.
In beryl, on the other hand, it is octahedrally co-
ordinated. In many minerals, as in mica, it is found in
both types of site.
The radius ratios of titanium (4+), iron (3+ and 2+),
magnesium and manganese point to occupation of the
octahedral ('Y') sites available in all
ferromagnesian
silicates. Ca
2+
and Na
+
are somewhat larger, and require
8-fold co-ordination. Such ('X') sites do not exist in
olivine, from which these elements are excluded (only
traces of Ca are found), but they are available in pyrox-
ene and amphibole (Box 8.5). K
+
clearly requires a still
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