Chemistry Reference
In-Depth Information
by an increase in volume. Calcium may come from the alteration of clinopy-
roxene - and/or possibly from a carbonatization.
Ultramafic rocks of ophiolite complexes are generally more or less com-
pletely serpentinized. Serpentinization is attributed to hydrothermal meta-
morphism of the rocks of the oceanic crust and of the associated rocks of
the upper mantle. Metamorphism of greenschist or blueschist facies may
be superimposed on the hydrothermal metamorphism contemporaneous
with the formation of these rocks. Ultramafic rocks of greenstone belts of
Archean and Proterozoic age are also largely serpentinized. Serpentiniza-
tion is probably less frequent and intense in the layered mafic-ultramafic
complexes.
3.3 ALUMINOUS MINERALS
These minerals are mainly metamorphic minerals of metapelites (and paleo-
alterites) initially more or less rich in clay (and not calcareous). They also
occur in peraluminous igneous rocks: granodiorites, granites, aplites, pegm-
atites, etc. and in some metasomatic rocks whose peraluminous character is
due to the leaching of alkalis and calcium (greisens, “secondary hydrother-
mal quartzites”, etc.).
Other peraluminous minerals, muscovite, biotite and chlorite, have been
previously presented. Spinel ((Fe, Mg)Al
2
O
4
) and tourmaline are dealt with
accessory minerals. Aluminous minerals are treated here:
alumina silicates: andalusite, sillimanite, kyanite;
most commonly ferriferous aluminous silicates: garnet (almandine),
staurolite, chloritoid;
mostly magnesian aluminous silicates: cordierite magnesiocarpholite,
sapphirine;
topaz, corundum and beryl, minerals whose mode of occurrence is some-
what different and that could be classified with accessory minerals;
pyrophyllite, diaspore and aluminum hydroxides, gibbsite and boeh-
mite; the latter two are minerals of sedimentary rocks and are not deter-
mined under the petrographic microscope, and therefore not treated
here.
3.3.1
Structure and chemical composition
3.3.1.1 Alumina silicates: andalusite, sillimanite,
kyanite
There are three natural polymorphs of formula Al
2
SiO
5
: sillimanite poly-
morph of high temperature, kyanite of high pressure and relatively low
