Chemistry Reference
In-Depth Information
The rocks of the last two groups are rich in Ca, Fe, Mg, Ti. Their miner-
als will be more or less calcic plagioclase, amphibole, pyroxene, olivine. Let us
recall the importance of water in the crystallization of amphibole or pyroxene.
1.3.2.3 Silica saturation of igneous rocks
After oxygen, silicon is the most abundant element in most igneous rocks -
which are mainly formed of silicates. Minerals of igneous rocks fall into two
groups:
-
silica saturated minerals that may be in equilibrium with quartz;
-
silica undersaturated minerals, which react with quartz to form a silica
saturated. Among the latter, feldspathoids (feldspathoid
+
quartz
=
feld-
spar) and magnesian olivine (olivine
orthopyrox-
ene) are particularly important Another example is the perovskite
(perovskite
+
quartz
=
+
quartz
=
titanite).
So the rocks may be classified according to this the silica saturation:
quartz present in an amount greater than 5%: silica over-saturated rocks;
less than 5% of quartz or feldspathoid: silica-saturated rocks;
feldspathoid present in an amount greater than 5%: silica under-satu-
rated rocks.
1.3.2.4 Alumina saturation of igneous rocks
After oxygen and silicon, aluminum is the second most important element
in most igneous rocks. Feldspars and feldspathoids are most often the major
aluminum bearing minerals. Given the composition of feldspar (KAlSi
3
O
8
,
NaAlSi
3
O
8
, CaAl
2
Si
2
O
8
) and feldspathoids (KAlSi
2
O
6
, NaAlSiO
4
), there may
be relative excess or deficit of aluminum to alkalis and calcium; this relative
excess of aluminum or alkalis will be expressed by specific minerals:
Al
K:
peraluminous rocks
; this excess of aluminum is
reflected by the presence of aluminous minerals other than feldspars: mus-
covite, garnet, cordierite, alumina silicates ...;
2Ca
>
2Ca
+
Na
+
K:
meta-aluminous
rocks; such rocks contain
feldspar, possibly biotite and hornblende, but no peraluminous minerals nor
alkali mineral;
Al
+
Na
+
K
>
Al
>
Na
+
K:
(per)alkaline rocks
contain alkali ferromagnesian minerals
(sodic amphibole and/or pyroxene, sometimes sodic zircono-silicates and: or
titano-silicates). Most of the alkaline rocks are sodic. There are also potassic
(per-) alkaline rocks. Particular mention should be made for potassic rocks
rich in mafic minerals (kimberlites, some lamprophyres). Rarer are the rocks
that show a relative deficit of aluminum to calcium: wollastonite (CaSiO
3
) is
<
Na
+
