Chemistry Reference
In-Depth Information
a mineral sub-saturated in aluminum compared to anorthite (CaAl
2
Si
2
O
8
);
it is found in some rare alkali granites (Corsica) or some nepheline syenites
(Alnö). Peralkaline rocks only represent 1 to 2% of the igneous rocks.
The concepts of saturation in silica and in alumina are independent:
some granites (silica-saturated rocks) are peralkaline (and contain Na-
amphibole); nepheline syenite (rock under-saturated in silica) are saturated
in alumina (miaskitic syenites); other nepheline syenites are under-saturated
in both silica and alumina (agpaitic syenites).
These notions of silica and alumina saturation were originally defined
for the igneous rocks. They can be extended to the other rocks.
4
Metamorphic rocks
result from the endogenous transformation of other
rocks: sedimentary rocks (para-derived metamorphic rocks), igneous
rocks (ortho-derived metamorphic rocks) or other metamorphic rocks. As
metamorphism is nothing but a transformation due to changes of physi-
cal conditions, without any change in chemical composition other than
the content of volatile elements, the mineralogy of metamorphic rocks
reflects the chemical composition of rocks from which they derive.
Among the clastic sedimentary rocks, the pelitic rocks are often rich in
clays, and thus peraluminous. Metamorphism develops in such rocks alu-
minous minerals: alumina silicates, muscovite, garnet, cordierite, stauro-
lite, chloritoid .... The more or less impure sandstones are generally much
less rich in alumina, while remaining mostly peraluminous: the latter alu-
minous minerals appear less frequently or, at least, in smaller quantities.
The rocks that initially contained carbonate (calcite and/or dolomite),
present a varied mineralogy depending on the relative proportion of the
initial carbonates and the other minerals, that may contain alumina (clay,
feldspar) or not (quartz).
if the proportion of carbonate is low, the rock will be made of quartz,
feldspar, mica, aluminous silicates; calcium is only expressed in garnet
or plagioclase by a by a more or less large proportion of grossular mol-
ecule or anorthite;
a higher proportion of calcium is expressed by specific mineral, cal-
cic and calcic and magnesian silicates (amphibole, diopside, grossular-
richer garnet, anorthite-richer plagioclase): such metamorphic rocks
are calc-silicates-gneiss (or calc-silicate rocks). In this family of rocks,
one can distinguish members richer in aluminous minerals (calcic pla-
gioclase, epidote), which contained before metamorphism more or less
clay minerals (such rocks were originally marls) and terms poorer in
aluminous minerals, which were originally calcareous sandstone (“calc-
silicate-quartzites”). In such rocks there is no free primary carbonates -
or they are in a very small proportion;
