Chemistry Reference
In-Depth Information
biotites in the common calc-alkaline series (granites and Saint-Arnac and
Ansignan granites) and iron-rich series (Orgaõs) have medium Al
2
O
3
content 14 to 18 wt%; this content increases greatly in differentiated
terms in the Saint-Arnac and Ansignan series. The Laouzas peraluminous
biotite granite have Al
2
O
3
contents of about 19-22%. For comparison,
the Al
2
O
3
content of biotite in nepheline syenites are about 12-13%.
In charnockites that are granitoids that crystallized in the condition of
the granulite facies, biotites appear only in late phases (crystallized at higher
water pressures) or as a secondary mineral that develops on the (ortho)
pyroxenes.
Biotite/phlogopite is less common in basic rocks. Indeed the limit of
stability of phlogopite is close to the temperature of crystallization of basalt.
However biotites are stabilized in such rocks by higher water pressure, the
presence of fluorine, alkaline and especially the potassic character of the
magma: so biotite is present in basanites and alkali gabbros (theralites, etc.).
Biotite is also a major constituent of lamprophyres and kimberlites.
The limit of stability of muscovite is close to the temperature of crystal-
lization of granite. It is much less common in igneous rocks than biotite: a
comparison of stability curves of muscovite and the melting of granite (Fig-
ure 3.11) shows that muscovite can crystallize only under water pressures of
at least 1.5 kb. This stability range is increased by the presence of fluorine.
Muscovite therefore only appears in evolved plutonic rocks, granites and
pegmatites. Muscovite as a primary magmatic mineral, is an indication of
the peraluminous character of these rocks. But in the plutonic rocks, musco-
vite is often a secondary mineral formed by alteration of feldspar (leaching
of alkalis) or of aluminous minerals such as cordierite.
Metamorphic rocks
The field of stability of muscovite/phengite covers metamorphic facies of
low temperature until the middle part of the amphibolite facies. That of
biotite covers the upper greenschist facies and amphibolite facies. Biotite
disappears in the granulite facies.
Metamorphism of low to medium pressure (and contact metamorphism)
transforms the more or less clay-rich sedimentary rocks into schists rich in
biotite and muscovite. In low-grade metamorphism (epizone), sedimentary
illites recrystallize into sericite and muscovite. The biotite isograd marks
the medium grade metamorphism (mesozone). Aluminum-rich rocks (meta-
pelites, meta-shales) contain more aluminous minerals (silicates of alumina,
chloritoid, staurolite, cordierite, garnet, etc.). The isograd of the disappear-
ance of muscovite can be taken as the limit of high grade metamorphism
(catazone). In high grade metamorphism, metapelites are transformed into
aluminous gneisses with biotite, potassium feldspar, plagioclase, sillimanite,
