Chemistry Reference
In-Depth Information
Regional metamorphism of metapelites
Muscovite, chlorite, biotite, andalusite, kyanite, sillimanite, garnet, stauro-
lite, cordierite, chloritoid (and more rarely magnesiocarpholite) as well as
plagioclase, are common metamorphic minerals of the metapelitic rocks.
Isograds, defined by the appearance (or disappearance) of such minerals,
and the paragenesis (mineral association in equilibrium) in which they are
involved, are used to accurately characterize the regional metamorphism.
The appearance of these minerals is controlled by physical conditions
(Figure 1.9). Chlorite, chloritoid and magnesiocarpholite are low temper-
ature minerals; andalusite, kyanite, garnet, staurolite, cordierite, biotite,
muscovite are characteristic of a medium-grade metamorphism; garnet,
cordierite, sillimanite, sapphirine are stable at high or very high tempera-
tures. Cordierite is a mineral of low pressure. Garnet is rare at low pres-
sure, besides in the iron- and manganese-rich rocks. Staurolite is unlikely to
appear at very low pressure. Magnesiocarpholite and kyanite are minerals
of medium to high pressure. Chloritoid is more common in areas of medium
to high pressure than in areas of low pressure.
The chemical composition of these minerals, and particularly the ration
between iron and magnesium) allows the following classification:
minerals without iron nor magnesium: alumina silicates, muscovite.
Strictly speaking, muscovite should not be classified in this group, as
it is generally more or less phengitic, but it can be considered as a first
approximation a purely aluminous muscovite;
mostly iron-bearing minerals: almandine garnet, staurolite, chloritoid;
mostly magnesian minerals: cordierite, (magnesiocarpholite);
(ferromagnesian) minerals with large variation of the iron-magnesium
ratio biotite, chlorite, phengite.
At a given grade of metamorphism (that is at given physical conditions),
the presence in a rock of any of such mineral, depends essentially on the
proportions of iron, magnesium, aluminum and potassium.
Plagioclase is generally present; but, the balance between Fe, Mg, Al and
K, only consider the aluminum that is not fixed in the plagioclase, which
can be expressed as Al-Na-2Ca. Ilmenite is often present in metapelites and
strictly speaking, this balance should take account of the iron fixed in ilmen-
ite (FeTiO
3
). It may be difficult to assess the proportion of iron contained in
ilmenite: either one considers that all the titanium is contained in ilmenite and
the parameter to be considered is Fe-Ti (but there is some titanium fixed in the
biotite); another way is to consider that the amount of iron fixed in ilmenite is
negligible and the parameter to be considered in this balance is the total Fe.
A low to medium grade of metamorphism, muscovite is almost always
present in metapelites. It is therefore most times considered that muscovite is
in excess: when studying the formation of minerals the balance between Al,
