Chemistry Reference
In-Depth Information
possibly cordierite and/or garnet. Micas in such rocks are rich in alumina:
Al
2
O
3
contents of biotite are about 19-21 wt%.
The contents of Na
2
O and TiO
2
increase with metamorphic grade: for
example, in Montagne Noire, TiO
2
content of biotite increases from 1.6 wt%
at the biotite isograd to 2.7-2.9 wt% in the sillimanite
muscovite zone (the
content of muscovites increases from 0.3-0.7 wt%). In the same interval the
contents of Na
2
O in muscovites grow from 0.4-0.5 to 1-1.1 wt% (0.10-0.15
to 0.25-0.30% in biotites); this is without any paragonite in the paragenesis.
Iron-magnesium ratios vary according to more complex laws and depend
on the parageneses, the metamorphic grade and the type of metamorphism.
This last point will be developed in the chapter on aluminous minerals.
The composition of the biotite in orthogneiss of intermediate to acidic
reflects the compositions of biotites of the initial rocks and are clearly distin-
guishable from biotites from metapelites particularly by their contents of alu-
minum and titanium. It is the same for the biotite ortho-amphibolites derived
from basic igneous rocks; the common paragenesis of these rocks is plagi-
oclase - amphibole (hornblende in amphibolite facies) - biotite (± garnet).
Phlogopite is a common constituent of impure marbles and calc-silicate-
gneisses. Ordinary biotite may exist in association with the amphibole and
plagioclase in some (para-) calc-silicate-gneisses - poor in calcium. But if the
calcium content increases biotite is replaced by amphibole.
Biotite is not stable in high pressure
metamorphisme. It is then
replaced
by phengite
+
chlorite and/or stilpnomelane. Phengite also replaces potas-
sium feldspar, particularly in orthogneisses.
+
Metasomatic rocks
Muscovite is a secondary mineral developing on potassium feldspar by
the leaching of potassium: its habit is large mottled crystals (“skelettal” or
“spongy” muscovite). Plagioclase is altered to sericite by Na-Ca leaching
and the addition of potassium. Alumina silicates and aluminum silicates
(cordierite, staurolite) are also altered into muscovite, especially during
stages of retrograde metamorphism.
Further leaching of alkali led to
greisen
, rocks formed mainly of quartz
muscovite. Greisens often contain beryl, topaz, tourmaline, fluorite, cassiterite,
wolframite, etc. The greisens form mainly on granitoids and pegmatites. They
can also develop on more varied rocks as far as they are aluminous enough.
Muscovitites are related rocks, in which quartz is completely leached.
Potassic alteration transforms amphibole into biotite. Some mafic and
ultramafic rocks can be transformed into biotitites in contact with granitic
magmas and pegmatites. Biotitites can also form in halos in hydrothermal
ore deposits: in the gold Salsigne deposit (Aude, France), biotitites develop
from silts and pelites around the orebodies on decimeter to meter thick-
nesses (Demange et al. 2006).
+
